Jeff Bowlsby CCS, CCCA
Exterior Wall and Stucco Consultant
Licensed California Architect
Stucco Shrinkage Movement Joint Subassembly (SMJS)
This webpage is dedicated to my ASTM C11 Committee colleagues
To determine which stucco movement joint is appropriate for a given condition, one must understand the anticipated movement at the condition. Shrinkage and thermal movements occur in the lath and stucco membrane. BMJS, PMJS and SMJS each accommodate shrinkage and thermal movements because the lath and stucco composite membrane is discontinuous through and terminate at each side of these subassemblies. A SMJS does not accommodate substrate support movement because the substrate support is continuous at SMJS. BMJS and PMJS accommodate substrate support movement because the substrate support is discontinuous at these subassemblies.
Stucco Movement Joint Selection Matrix
Everyone interested in exterior stucco wall cladding systems has a perspective on “control joints” and those perspectives are not unilaterally held by all. The vast array of information about “control joints” can easily perplex and overwhelm, and the industry is replete with a variety of differing viewpoints and oftentimes conflicting, incomplete, inaccurate or obsolete information. Much of the information being circulated and relied upon today was developed many years ago, and while perhaps based on the best information available at the time, stucco research and technology has advanced since then and now some of that information is obsolete or better quality information is available. Some current resources have innocently parroted other resources, continuing the spread of conflicting, incomplete, inaccurate or obsolete information and base opinions on this lack of correct information which is unfortunate and is one cause of much strife in the industry on this subject. We have the capability and information available now to discern myths from reality.
The term “control joint” is vague and ambiguous and does not describe its primary function initiating endless debates about what it is, what it does and how it does it. The replacement term Shrinkage Movement Joint Subassembly (SMJS) is used on this website which is intended to better convey its essential association with shrinkage movement, and significantly identify its role as a subassembly of a larger stucco wall cladding system. A “control joint” is not just a lath accessory bolted onto a wall. It is a primary purpose of this webpage and website to convey reasonable, credible, rationally-based and defensible information leading to an understanding of the primary purposes of the SMJS subassembly, how it is intended and required to function, and how it actually does function, that it is hoped will resolve any lingering debate about its design, installation and location requirements to be an effective stucco wall cladding subassembly that serves its primary function towards minimizing stucco cracks.
The SMJS subassembly, its intended purpose, function, and installation configuration can be misunderstood amongst building owners, architects and craftsman. If cracking did not occur, stucco would be much more popular, respected and prolifically used as an exterior wall cladding. These are the intended outcomes of these webpages regarding stucco movement joints.
Portland cement-based plaster shrinkage and stucco thermal movements are real and known causes of stucco cracking. If exterior stucco wall cladding systems did not experience shrinkage or thermal movements, then there would be only limited purpose for the SMJS subassembly. This webpage attempts to unpack and evaluate the plethora of accurate and inaccurate information in circulation and provide a rational, factual basis for providing correctly assembled, functioning SMJS subassemblies, including information on SMJS subassembly history, design, lath accessory and subassembly configuration requirements, performance testing and the capabilities and limitations of the SMJS subassembly that are useful towards minimizing stucco cracking and water intrusion.
Where the SMJS subassembly is the primary means of accommodating cement-based plaster shrinkage and stucco thermal movements, it is only when the SMJS subassembly is correctly located within a stucco wall cladding system relative to wall openings and larger wall expanses, and configured into panel areas of certain dimensions and geometries that it creates a functional cement-based plaster shrinkage and stucco thermal movement control Assembly. While this webpage focuses on the SMJS subassembly it also covers the essential parameters for a portland cement-based plaster shrinkage and stucco thermal movement control Assembly. If a stucco wall cladding system did not experience shrinkage and thermal movements, then there would be no essential purpose for the SMJS subassembly. This webpage explores the conditions that make the SMJS subassembly beneficial to the success of an exterior stucco wall cladding system on a building as a substrate support.
Visit the StuccoMetrics Reference Archives webpage for cited references and further information.
From Technical Manual,
Keene Corporation, Penn Metal Products, c.1980
From the earliest times when portland cement-based plaster first began to be applied onto framed building structures, the biggest concern was wall opening corner cracking at reentrant window/door corners. In the late 1910’s through early 1920’s, the US Bureau of Standards, as part of the Department of Commerce, performed a battery of testing on portland cement-based plaster and stucco wall cladding and its Components because it recognized the potential and importance of stucco to the economy and wanted to assist in its success as a building material. The Bureau of Standards constructed several full-sized test buildings with multiple test panels of stucco to test and observe the performance of various combinations of stucco components, lath types and materials, stucco mortar mix designs, framing/sheathing type alternatives, and application and curing methods for optimum results and minimized cracking. A significant part of the stucco testing at this time was devoted to evaluating the predominant stucco cracking conditions occurring at window corners. During this era, stucco cracking was most commonly attributed to building structure movements. Soil settlement and wind loads were also suspected causes of cracking. Further government testing determined that wood lath was a major cause of stucco cracking and was ultimately replaced in the marketplace by expanded sheet metal lath and wire lath. Various board sheathing types and installation methods (to minimize building movements) were evaluated, ultimately determining that horizontally-installed board sheathing was the best method for sheathing a building for stucco cladding to minimize reentrant window/door corner cracking. Yet despite these improvements, reentrant corner stucco cracking persisted.
1916 US Bureau of Standards Stucco Testing
to evaluate window corner and stucco panel cracking
Patent research: Not all SMJS lath accessory components are patented and not all patented SMJS lath accessory components were produced or are currently available. Select SMJS lath accessory components and subassemblies are discussed.
In 1920, inventor John Earley (who had participated in the Bureau of Standards testing program) devised the first stucco movement joint lath accessory and SMJS subassembly, Patent No. 1355756, Flexible Joint for Stuccoed Buildings, for a flexible metal joint screed lath accessory for use around window and door frames. Earley opined that a building and stucco cladding were stable using board sheathing, therefore stucco only cracked because the wood window/door frames and underlying framing at these wall openings expanded due to water absorption and swelling during seasonal wet/dry cycles, which put force on the stucco at window and door corners, causing reentrant corner cracking. Earley’s screed was intended to provide a safety compression zone and counteract the window/door frame expansion forces, and prevent stucco cracking that was prevalent at reentrant window and door corners. Yet reentrant corner stucco cracking persisted.
First stucco lath accessory patent to address stucco cracking
J.J. Earley, 1920
In the late 1940’s a serious stucco cracking problem was reported to have developed at the massive Grand Cooley Dam project(1), and the stucco community benefitted once again from a rigorous stucco system testing program, underwritten by taxpayers. Interior room suspended stucco ceilings cracked objectionably when their lath was continuous through the ceiling/wall juncture and secured to the adjacent concrete perimeter walls (the common practice of the day). It was discovered through extensive testing of many variables, that the ceilings did not crack when the lath was discontinuous at the ceiling/wall juncture, which allowed the ceilings to shrink away from the walls. The perimeter ceiling gaps that developed when the ceilings were isolated and shrank away from the walls were large and measurable, but no ceiling cracking occurred attributable to the isolated perimeter lath condition which was deemed to be of great benefit in minimizing cracking at the ceilings.
Discontinuous lath, which eliminated restraint conditions at stucco ceiling panel perimeters of interior rooms, was proven to minimize stucco cracking at Grand Coulee Dam. At Grand Coulee Dam lath and stucco was installed as an unrestrained composite panel, which accommodated shrinkage and thermal movements and minimized stucco cracking at interfaces of different substrate supports. The realized benefit of this newly developed discontinuous method of lath installation became the new paradigm for stucco on framed substrate supports to minimize cracking: Stucco on framed substrate supports performed better with less cracking when constructed as adjacent, discrete, segmented panels that accommodate shrinkage and thermal movements within each panel, as compared to a single, non-segmented, solid, continuous lath and stucco composite mass cladding the entire building. The stucco movement joint subassemblies pertaining to transitions between different substrate support materials or loadbearing conditions, or building substrate movement conditions are known on this website and BMJS and PMJS subassemblies. Stucco movement joints pertaining only stucco cladding shrinkage and thermal movement where the substrate support is continuous and produces no substrate movement, is known on this website as a Shrinkage Movement Joint Subassembly (SMJS). The SMJS subassembly was formed by adjacent casing beads with discontinuous lath, which is a SMJS subassembly still recognized by Minimum Stucco Industry Standards today.
By the mid-1950’s portland cement-based plaster shrinkage and stucco thermal movement were recognized causes of cracking. The one-piece stucco SMJS lath accessory component was developed and brought to market as a more convenient form than the two-casing bead configuration. The one-piece SMJS lath accessory component invented by Raymond Clark(2) was filed for Patent No. 3,015,194 in 1955, and the patent was granted in 1962. This product is known today as the Double-V lath accessory, and generically as a SMJS lath accessory component on this website. The Double-V SMJS lath accessory component features an expansible, resilient pleat at its center with expanded sheet metal flanges to each side for keying the SMJS lath accessory component to the adjacent lath edges as the result of discontinuous lath. The anticipated behavior of the continuous lath was described in numerous locations throughout the patent as “expansible” and was considered to be the primary zone of movement along with the pleat of the lath accessory component. The SMJS lath accessory component is of a vertical height dimension sufficient to function as a plaster thickness ground screed and its configuration provides a convenient location to stop plastering work where needed, to avoid the need for “joinings” (cold joints) in the plaster. The Penn Metal Company, and later the Keene Corporation and Metalex which later acquired Penn Metal Company, were the original manufacturers of this original one-piece SMJS lath accessory component.
Illustrations from Clark Patent 3,015,194
(Note the patent illustrates both a SMJS lath accessory component
and SMJS subassembly. The SMJS lath accessory is wire-tied to continuous lath. Arrows 40 and 41 depict anticipated SMJS subassembly movement behavior.)
From the Clark patent the SMJS lath accessory and SMJS subassembly functions as follows: “…due to the stresses and strains of expansion and contraction caused by initial drying, by subsequent variations in temperature and humidity of the surrounding atmosphere...”. “It provides narrow gaps at desired intervals across the face of the finished coating of [stucco] which extend through the entire thickness of the [stucco] on the lathing so that the [stucco] is installed, not in a continuous expanse or sheet, but in interrupted sections, which may expand and contract independently to an extent at least adequately to prevent cracking of the [stucco]”.
In the Clark patent, the pleat is described as a “resilient fold”, it was intentionally designed and described to function similar to an accordion – opening and closing, in response to the movements of initial stucco shrinkage and thermal expansion and contraction while in service. The SMJS lath accessory has expanded sheet metal flanges which key the stucco to the lath and SMJS lath accessory just as the lath does; it creates a flexible extension of the lath, in the plane of the lath. Clark termed this invention an “expansion joint” in his patent and this term is used in many of the Penn Metal, Keene, Metalex catalogs.
SMJS lath accessory, Double-V
A variant of the original SMJS lath accessory is the Double-V, Internal Corner SMJS lath accessory component, developed in the early 1960’s. It is nearly identical to the Double-V SMJS lath accessory other than its flanges are configured into a 90 degree to nest into an internal wall corner.
SMJS lath accessory, Double-V Internal Corner
Soon after the Double-V SMJS lath accessory component was introduced, it was observed that as its center pleat opens and closes from initial portland cement-based plaster shrinkage and stucco thermal movements, linear, parallel gaps can form at one or both edges of the pleat. These parallel edge gaps are an aesthetic distraction and may allow bulk water penetration into the stucco wall cladding system. By about 1969 Penn Metal Co. had been acquired by the Keene Corporation and in 1978 Keene brought an improved version of the Double-V SMJS lath accessory to market, with the proprietary designation as the Keene XJ15-3 and generically known as the Double-J SMJS lath accessory. The Double-J SMJS lath accessory is identical to the Double-V SMJS lath accessory, except it has additional locking edge flanges that grip the stucco edges of each adjacent stucco panel on both sides of the SMJS subassembly. This locking edge capability minimizes the parallel gaps that can form at the stucco panel edges that the Double-V SMJS lath accessory can experience when portland cement-based plaster shrinks away from it or stucco thermal movement occurs, and also conceals the parallel gaps to minimize the potential for water intrusion through the parallel edge gaps. No separate patent for this improved lath accessory is known to exist.
SMJS lath accessory, Double-J
SMJS lath accessory, Double-J, PVC
The SMJS lath accessories depicted above are available from several manufacturers today, in a variety of corrosion-resistant materials including galvanized steel, solid zinc alloy, stainless steel, extruded PVC and are available in a range of ground dimensions for various stucco panel thickness requirements.
Other SMJS lath accessory inventions after Clark, were either not commercially successful or may have been mistakenly inferred to be SMJS lath accessories, but were intended for larger movement conditions related to substrate support movements and therefore are considered Building Movement Joint (BMJS) lath accessories and subassemblies as described on this website.
In 2008 the SMJS lath accessory and subassembly evolved further when Mr. Don Pilz of Cemco brought a number of lath accessories and water management flashings to market, including the innovative Cemco Solid Leg #15 lath accessory(3). Generically referred to as the Double-V Horizontal Drainage SMJS lath accessory, it is most similar to a Double-V SMJS lath accessory, but it also includes a solid flange to integrate with the WRB which allows it to function as a horizontally-oriented drainage screed flashing for stucco wall cladding systems. This innovation is designed for use typically as a horizontal SMJS subassembly only, for use at design locations on walls occurring between the foundation drainage screed and the top of wall. It has dual function as both a horizontal drainage screed lath accessory because it is integrated with the WRB, and as an SMJS subassembly to minimize cracking related to shrinkage and thermal movements.
SMJS lath accessory, Double-V Horizontal Drainage (Cemco Solid Leg #15)
From the Stucco Material Properties webpage, we learn that the combined portland cement-based plaster shrinkage movement and stucco thermal movement accounts for approximately 130-240 mils (0.13-0.24 in.) per 10 lineal feet, or 13-24 mils per lineal foot of stucco wall cladding.
The Clark patent documentation clearly describes in written and graphic form the SMJS lath accessory intended configuration and function as a SMJS subassembly. The lath of the SMJS subassembly expands and contracts along with the portland cement-based plaster shrinkage and stucco thermal movements because the two are one composite material. The new invention was described as being applicable for both walls and ceilings and is wire-tied to the face of the lath. The lath was described as being “expansible” and the SMJS lath accessory was described as being “resilient” to accommodate movement which was anticipated by Clark to occur at SMJS subassemblies and SMJS lath accessories. It is essential to recognize and understand that Clark devised not only the SMJS lath accessory, but also the SMJS subassembly which includes the SMJS lath accessory in a specific installed configuration with other components of the subassembly such as the substrate condition, lath, fasteners and of course the cement-based plaster, a fact often overlooked.
Lath accessory product catalogs from the Penn Metal Company, Keene Corporation and lastly Metalex (all were the original producers of the SMJS subassembly) provide the first glimpse of SMJS subassembly installation requirements as provided by the manufacturer. A manufacturer’s installation requirements for their products are generally regarded as an authoritative resource in the construction industry. In the timeline that follows, the requirements described remain in effect until changes and additional requirements are noted in subsequent catalog issues. These original SMJS lath accessory manufacturers issued installation requirements for the SMJS lath accessory product from at least 1959 through about 2005. SMJS lath accessory manufacturers no longer publish SMJS subassembly installation requirements. Instead, since the 2006 IBC building code has adopted ASTM C1063, this Minimum Stucco Industry Standard is the current authority and reference resource on SMJS subassembly installation requirements wherever the IBC building code is in effect.
1959-2005 Penn Metal Company, Keene Corporation, and Metalex catalogs
It is interesting to review and understand the progression of “control joint” installation requirements, because they subtly changed as they were introduced over many years from the Penn Metal Company, Keene Corporation, and Metalex product catalogs, and to compare them to the requirements in the ANSI and ASTM specifications of the day up through today:
· 1955: “Expansion joint” lath accessory introduced to market, patent issued in 1962.
· Through early 1960’s: No installation requirements provided in product catalogs, the only guidance was described in Clark’s patent documentation.
· 1964: Maximum spacing for “control joints” is “generally 8 feet.” Solid zinc lath accessory was made for salt laden environments.
· 1971: “Install “joints” at all locations where panel sizes or dimensions change. Joints shall extend the full width or height of the plaster membrane.”
· 1973: Locate “control joints” where dissimilar materials join, where substrates support conditions change such as suspended ceilings to perimeter walls, 100-125 SF areas maximum, above door bucks and into the ceiling juncture on both sides of heavy frames or where heavy doors and hard use will be encountered. Maximum panel geometry 2-1/2 to 1. “All (Plaster-Stucco) expansion joints shall be installed…to provide a movement capacity of 1/4 in.”
· 1974: Only zinc shall be used at exterior locations, and the following requirements:
1978: Note discontinuous lath, separate (doubled) studs,
no sheathing, “control joint” lath accessory wire-tied or
nailed to/through lath edges and into studs
· 1983: Expansion joints must be placed at least every 100 SF and above door bucks as noted previously. Intersections of joints may be butted-and-calked and the following:
From 1983 Keene Corporation catalog
1983 Keene Corporation product data
(Note discontinuous lath, separate (doubled) studs, discontinuous sheathing,
fasteners for “control joint” lath accessory not indicated)
· 1985: “Joint materials must be held firmly to metal lath or separate (double) studs with wire ties, nails or staples.”
· 1990: Maximum panel area recommendation, 100 SF (not 144SF), square, and aligned with wall opening corner:
From 1990’s Keene Corporation Catalog
(Note SMJS aligning at door opening corner,
panel area size and geometry)
· 1994-2005: Where vertical and horizontal “control joints” intersect, the vertical joint should remain continuous and unbroken. Screws are not mentioned as an acceptable fastener either for lath edge fastening or fastening the “control joint” lath accessory.
· Sometime after 2005 Metalex ceased providing “control joint” products direct to the market, and hence product literature, but they continue to manufacture “control joints” and other lath accessories and provide them to other lath accessory manufacturers that distribute them.
Keene Corporation published a Technical Manual(4), c. 1980, as a general guide to “control joints”. Unlike other Keene documents of the era, this document is primarily a narrative and written in a language style more as a guide than specifying technical requirements. Interestingly, the term “control joints” is identified and intentionally used exclusively by Keene in this document after discussing other terms used in the era, for stated consistency reasons. None the less the manual does present the following technical installation requirements, “…a control joint…must be used...”:
· Where dissimilar materials join
· Where a large suspended ceiling is pierced by supporting beams or columns
· “Where portland cement plaster is involved, for allowance must be made for the inherent shrinkage in portland cement. In this instance areas of not more than 100-125 square feet are recommended on the premise that small areas disassociated from one another will allow for this shrinkage with no unsightly fracturing and that if the areas are isolated from one another and small enough to come and go without restraint, there will be no problems.”
· Above door bucks and into the ceiling juncture on both sides of heavy frames
· Around large penetrations such as light troffers, heavy access panels, etc.
· Discontinuous lath is required at “control joints”. “Walls and ceilings that use metal reinforcement for the base should be divided with a “control joint”… The metal reinforcement in the stucco or plaster must be separated and not extend across these “control joints”.” And the following:
· “Walls and ceilings that use metal reinforcement for the base should be divided into rectangular panels with a control joint at least every 20 ft.” [No basis for that 20 ft. dimension is mentioned].
ASTM C1007 Standard Specification for Installation of Load Bearing (Transverse and Axial) Steel Studs and Related Accessories(5):
· (7.1) Vertical stud alignment (plumbness): 1/960 of span (1/8-in. in 10-ft 0-in.)
· (A2.4) Allow additional studs at panel intersections, corners, doors, window, SMJS, etc.
ASTM C1280 Standard Specification for Application of Exterior Gypsum Panel Products for Use as Sheathing(5):
· (188.8.131.52) Offset gypsum panel edge joints at wall opening corners 4-in. (100-mm) minimum.
ASTM C1063 Standard Specification Installation for Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster(5):
· (3.2.3) SMJS, noun: A movement joint subassembly accommodating minor movement associated with plaster shrinkage and curing along designated lines.
· (7.5.4) Main runner splices: Nest channel flanges and overlap channel ends 12-in. (305-mm) minimum. Securely install ties near splice ends with double loops of either 0.0625-in. (1.59-mm) or twin strands 0.0475-in. (1.21-mm) galvanized wire. For splices located at BMJS and SMJS, loosely install ties holding splice together to allow for movement.
· (7.6.5) Splice main runners and cross furring at BMJS, SMJS. Reference 7.5.4
· (184.108.40.206) Discontinue lath through SMJS, and attach the lath edges to the substrate support at each side of the SMJS.
· (7.11.4) SMJS subassembly: A plaster separation gap 1/8-in. (3.2-mm) minimum or as required by anticipated shrinkage and thermal movement, conforming with 220.127.116.11 using a one-piece manufactured SMJS lath accessory or back to back casing beads configured over a flexible barrier membrane.
· (18.104.22.168) SMJS assembly: Install and configure SMJS subassemblies into plaster panels with surface areas not exceeding 144-ft2 for walls and 100-ft2 for horizontal applications such as ceilings and soffits.
· (22.214.171.124) Maximum plaster panel dimension and proportion: 18-ft. (5.5-m) panel dimension, 1:2.5 panel length to width proportion. Provide SMJS at ceiling framing or furring directional changes.
· (126.96.36.199) Plaster panel edges at floor to ceiling height door frames are SMJS.
· (A1.2) Provide BMJS, PMJS to accommodate building substrate movement and to minimize movement related stucco and WRB damage.
· (A1.3) Provide SMJS to minimize plaster shrinkage, curing stress and minor movement, along designated, typically straight lines and as a plaster thickness control screed.
ASTM C926 Standard Specification for Application of Portland Cement-Based Plaster(5):
· (7.1.5) Apply plaster continuously at walls and ceilings to avoid cold joints and abrupt appearance changes in each plaster coat. Abut wet plaster to set plaster at planar interruptions such as corners, rustications, openings, BMJS, PMJS and SMJS where possible. Cut joinings, square and straight, 6-in. (152-mm) minimum away from joining in previous coat, where they are necessary.
· (A2.1.3) Seal separation gaps between weather exposed plastered panel edges and dissimilar materials to prevent water penetration.
· (A2.3.1) Reference the Installation Section of Specification C1063 for PMJS and SMJS installation requirements used with metal plaster base. PMJS and SMJS are not required at solid plaster bases, except as stated in Specification C1063 188.8.131.52.
· (A184.108.40.206) Remove plaster from pleat area of SMJS before applied plaster hardens.
· (A220.127.116.11) Evaluate the characteristics of the substrate and indicate the requirements for BMJS, PMJS and SMJS on construction documents, including type, location, depth, installation requirements. Install BMJS, PMJS and SMJS before plastering.
· (A18.104.22.168) A groove in plaster is not a BMJS, PMJS or SMJS.
· (A2.3.3) Provide a BMJS, PMJS or SMJS at transitions between dissimilar substrate support materials that receive continuous plaster.
In addition, various non-codified stucco industry reference resources establish a strong precedent for stucco wall cladding systems to include “control joints”:
PCA Portland Cement Plaster / Stucco Manual, EB049:
· A narrative description of requirements for “control joint” usage and installation requirements identical to those stated in ASTM C1063 and C926
ACI Guide to Portland Cement-Based Plaster, ACI 524R:
· A narrative description of requirements for “control joint” usage and installation requirements identical to those stated in ASTM C1063 and C926
· “Specify and detail measures to minimize plaster cracking, including…Proper location of “stress-relief joints” in accordance with ASTM C1063; additionally, “control joints” at re-entrant corners may be recommended, though not required by code”.
· “Stress-relief joints” should be located…where cracking is more likely to occur:
1. Headers and sill corners of windows, doors and other architectural projections or penetrations into plaster;
2. Edges and corners of ventilation or heating vents;
3. Structural plate lines or concentrations of large timber members in wood construction;
4. Midpoints between frame supports or midpoints between maximum control-joint spacings
5. Junctures where main columns or structural beams meet walls or ceilings;
6. Plastering over “expansion joints” or “control joints” of a solid plaster base; or
7. Plastering over junctures of dissimilar plaster bases.”
· “Terminations or splices in “stress-relief joints” should be embedded within a weather-resistant elastic sealant to prevent moisture penetration.”
· “Specify and detail the installation of flashing, weep screeds, and sealant at doors and around windows, vents, and all other wall penetrations to ensure that water will be diverted or channeled to the outside of the wall assembly in accordance with ASTM C1063 and E2112.”
· “Specify and detail the use of sealant at “stress-relief joint” terminations and splices”
· “Specify and detail the use of sealant at wall penetrations to prevent leakage at these points.”
EMLA 920-09 Guide Specifications for Metal Lathing and Furring:
· A narrative description of requirements for “control joint” usage and installation requirements identical to those stated in ASTM C1063 and C926
· “Joinery of abutting ends of trim accessories should be spliced or lapped and sealed with appropriate sealant.”
· “Joinery of “control joint” intersections should be spliced or lapped and sealed with appropriate sealant.”
· ““Control joints” should be sealed at inside/outside corners and termination points.”
· “Less than ceiling height door frames shall have “control joints” extending to the ceiling from both top corners, or ceiling height door frames may be used as “control joints.””
· “All intersections and terminations of “control joints” must be embedded and weather sealed in a bed of caulking material.”
A stucco wall cladding system can be constructed without SMJS subassemblies, but this practice is only applicable to direct-applied, continuously-bonded portland cement-based plaster onto mass masonry or solid concrete substrate supports. No WRB is used in this stucco wall cladding system; the mass of the wall is the weather protection as allowed by the building code. Stucco applied in this manner is intended to be continuously bonded to its substrate, is an acceptable method for applying stucco onto a mass masonry or solid concrete substrate and has performed well as an assembly for centuries. It must be mentioned that solid concrete or masonry buildings can be vulnerable to cracking and some have cold joints expressed at the surface, either condition of which can be vulnerable to water intrusion. Relatively few buildings are constructed with mass masonry or solid concrete walls in many parts of the USA, the southeastern and tropical USA regions being exceptions. Higher performance finish coatings can be applied if water intrusion through these solid wall assemblies is a concern. Generally, this wall assembly, without high performance exterior coatings is most appropriate for where some tolerance for water intrusion may be acceptable, such as for unconditioned parking garages, industrial or utility buildings.
Stucco on framed or framed/sheathed building substrate supports however behaves in an entirely different manner than when directly-applied to solid concrete or mass masonry substrates. Stucco cracking at wall opening reentrant corners such as window and door corners has been problematic for stucco on framed or framed/sheathed substrate supports since the earliest use of stucco on framed or framed/sheathed buildings. At the time, stucco on wood lath and later metal lath, was applied continuously over all framed or framed/sheathed substrate supports without interruptions, as if it were emulating stucco directly-applied to mass masonry or solid concrete buildings. The earliest causes of stucco cracking on framed or framed/sheathed buildings were commonly regarded to be building movement and expansion of wood windows due to water absorption.
Stucco cracking remains a concern by a variety of interests in the building industry today. Stucco crack typologies suggest patterns of recurring cracking conditions that can be addressed during stucco wall cladding system design and construction. It can be a challenge to correctly identify stucco shrinkage and thermal movement stress concentration conditions with 100% accuracy for strategically locating SMJS, but obvious and typical patterns and conditions exist that can be addressed. Primary design and installation considerations include conditions such as non-functional SMJS, reentrant wall opening corners at windows, doors, louvers etc., maximum stucco panel areas, dimensions and geometries, SMJS splices, intersections and terminations, three-plane intersections, and stucco cladding penetrations. Reference the Stucco Cracks webpage for further information.
Minimum Stucco Industry Standards ASTM C1063 and C926 as referenced in the building code or as specified in contract documents, provides the minimum prescriptive-based design and installation requirements for SMJS. SMJS lath accessory manufacturers do not publish engineering information, performance testing information or installation instructions for SMJS lath accessory products.
Other stucco industry reference resources suggesting design and installation conditions for SMJS, where they conflict with Minimum Stucco Industry Standards, codified as ASTM C1063 and C926 requirements, ignore basic behavioral characteristics of portland cement-based plaster shrinkage and stucco thermal movements.
In the late 1940’s, the role of portland cement-based plaster shrinkage and stucco thermal movements in stucco cracking became better understood. Lessons learned included that separating stucco and lath composite panels into segregated, discrete panels of smaller areas, eliminated the perimeter edge restraint of the once continuous lath to accommodate shrinkage and thermal movements, which minimized cracking. It was proven by extensive field testing which evaluated a number of factors, that continuous, restrained lath at perimeters of stucco panel areas exceeding a certain area or geometry, caused stucco panel area cracking. Stucco panel area perimeters were defined by a new isolation joint subassembly configuration consisting of paired casing bead lath accessories, and the lath was terminated or discontinuous at the isolation joint subassembly to accommodate portland cement-based plaster shrinkage movement and stucco thermal movement to minimize cracking. Keep in mind that these initial example installation conditions, evaluation and follow-up testing concerned a stucco and lath composite, wire-tied to a suspended ceiling grillage, conditions that are about as ideal as possible for isolating shrinkage and thermal movement from the substrate support, and quite different from a lath fastening and load transfer perspective than a stucco clad wall system where the lath is nailed, stapled or screwed, where the gravity load of the stucco is transferred in shear through lath fasteners to framing in the substrate support.
The Clark patent describes the functional purpose and a general description of the “control joint” lath accessory component and subassembly, but provides little other substantive installation requirement specifics. The SMJS lath accessory component and subassembly were brought to the marketplace by a succession of companies over many years, Penn Metal Company, Keene Corporation, and Metalex.
The Penn Metal Company, Keene Corporation, and Metalex manufacturers’ product catalogs from at least 1959 through 2005 included “control joint” installation instructions which indicate a number of consistent requirements that did not change much in over 45 years of publication. But also indicated are subtle, progressive variations in other important “control joint” installation requirements over that same time span, which have certainly contributed to the struggles the stucco industry has historically had concerning “control joints”. For example the following is a compiled chronology of published installation requirements from Penn Metal, Keene and Metalex over the years they produced and marketed the SMJS lath accessory and subassembly:
· Consistent: From 1971 forward, the stated requirement to provide joints at “locations where panel sizes or dimensions change” is a reference to panel shape or geometry. This requirement describes irregular panel area geometries such as C-shaped, L-shaped and doughnut, all of which contain reentrant corners. The requirement is to avoid panel area geometries with reentrant corners.
· Consistent: The stated requirement for discontinuous lath at “control joints” did not change since it was first mentioned in 1973.
· Consistent: The stated requirement for “control joint” locations above door bucks and into the ceiling juncture on both sides of heavy frames (reentrant corners), later described as above wall openings and window openings did not change since it was first mentioned in 1973.
· Consistent: The stated requirement for maximum stucco panel area geometry of 2-1/2 to 1 and 18 LF maximum panel dimension did not change since it was first mentioned in 1973.
· Consistent: Throughout the over 45 year history of the Penn Metal-Keene Metalex SMJS lath accessory installation publications the accessory is indicated to be installed over the lath, not mounted to the wall first with the lath over it and not shimmed.
· Variable: The inconsistent and interchangeable use of different terminology on what exactly to call this lath accessory and/or subassembly – “control joint” or “expansion joint” and other terms.
· Variable: Written descriptions and graphic illustrations that depict subassemblies of related components including the “control joint” lath accessory in several different subassembly configurations. The lath, fasteners, framing and sometimes sheathing and a WRB are indicated.
· Variable: Variations in fastening requirements for the “control joint” lath accessory from the patent application in 1955 where the lath accessory was required to be wire-tied to the face of lath, to 1974 when it was depicted as nailed to framing, to 1978 when each flange was depicted to be nailed or wire-tied to separate framing (double studs) where sheathing is not installed. In 1983 staples were identified and allowed, and also the clarification, “When used with metal lath wire-ties must be used. Install the joints with attachment only to the edges of the abutting sheets of lath, so that the lath is not continuous or tied to across the joint.”
· Variable: In 1983 it was first mentioned to require that lath, sheathing and the “entire wall assembly” be discontinuous including the requirement that the adjacent lath edges not be fastened to the same framing member, effectively requiring double studs. This new requirement for discontinuous sheathing and separate framing at “control joints” is a giant conceptual leap from Keene’s original vision of “control joints” that only accommodated portland cement-based plaster shrinkage and thermal movements, to “control joints” that accommodated building substrate support movement – a joint we now term as an “expansion joint” in ASTM C1063 today, or preferably a BMJS or PMJS as used on this website.
· Variable: “Control joint” spacing requirements, which determine maximum stucco panel areas:
è Beginning with the 1959 Penn Metal catalog: No spacing or panel area requirements were initially indicated.
è 1964-71 Penn Metal catalogs: “Maximum spacing for exterior stucco is generally 8 feet.” 64 SF panel areas(?), no conditions or other justification for the spacing is indicated.
è 1973-74 Keene catalogs: “Where portland cement plaster is involved, for [sic] allowance must be made for the inherent shrinkage in portland cement. In this instance areas of not more than 100-125 square feet are recommended on the premise that small areas disassociated from one another will allow for this shrinkage with no unsightly fracturing and that if the areas are isolated from one another and small enough to come and go without restraint, there will be no problems.” Later, these two catalogs indicate the conflicting requirement: “…install joints to create panel no larger than 144 SF…”.
è 1977 Keene catalog: Identical text to the 1973-74 catalogs for 100-125 SF maximum panel areas, but the later indication of 144 SF is not mentioned in this or later Keene catalogs.
è 1978-80 Keene catalogs: “Where portland cement plaster is involved, there is an inherent shrinkage expected. “Expansion joint” should be spaced to allow areas of not more than 100-125 square feet. Unsightly fracturing is less likely to happen as long as wall/ceiling surface areas are small and disassociated to “come and go” without restraint.”
è 1983 Keene catalog (Sweets): “For exterior portland cement plaster, install joints to create panels no larger than 125 sq. ft….”
è 1983-90 Keene catalogs including product data for Keene #15 and Unijoint II, a separate loose leaf catalog, and the later years Sweets catalogs: “Expansion joints must be placed at least every 100 square feet…”
è 1994 Keene-2005 Metalex catalogs: No spacing or maximum panel areas indicated.
è Note that only the 1973-74 Keene catalogs indicate two different conflicting panel area maximums and are the only references to 144 SF maximum panel areas in the Keene product literature. Even though Keene product literature reverted back to smaller panel areas in 1983-85, the first official 1986 ASTM C1063 indicates 144SF which remains unchanged to this day. The lack of change back to 100 SF after 1985 is not explained in stucco industry literature.
è Note that Keene produced their “control joint” lath accessories in both galvanized steel and solid zinc alloy, and no mention of different “control joint” spacing requirements based on different material performance is ever mentioned.
Minimum Stucco Industry Standards ASTM C926, C1007, C1063 and C1280 are referenced standards in the building code which state minimum installation requirements for SMJS lath accessories and subassemblies.
From ASTM C926 we note:
· 7.1.5: “Control joints” are a naturally occurring interruption in the plane of the plaster that plaster application can work to, to eliminate cold joints
· A2.1.3 Weather-exposed SMJS lath accessories and casing bead lath accessories at stucco panel edges must be sealed to prevent bulk water intrusion
· A2.3.1 Requirements for “control joints” and perimeter relief, are referenced to ASTM C1063. Solid plaster bases do not require stucco movement joints, except at expansion joints occur.
· A22.214.171.124 Remove plaster from “control joints” from the pleat before plaster hardens.
· A126.96.36.199 Install “control joints” before applying of plaster. Determine their type, location, depth, and method of installation by the characteristics of the substrate and indicate requirements in the contract documents.
· A188.8.131.52 A groove or cut in plaster is not a “control joint”.
· A2.3.3 Where plaster is applied continuously over dissimilar base materials, provide…casing beads back-to-back, or a “control joint”
From ASTM C1007 we note:
· 7.1 Steel stud framing must be plumb within 1⁄8 in. in 10 ft.
· A2.4 Steel studs must be allowed for at …“control joints”, etc.
From ASTM C1063 we note:
· 3.2.3 A “control joint” is a joint that accommodates shrinkage and curing movement, usually as straight lines.
· 7.5.4 At suspended ceiling grillage, main runners must allow “control joint” movement.
· 7.6.5 At suspended ceiling grillage, main runners and cross furring must allow “control joint” movement
· 184.108.40.206 Lath must be discontinuous through “control joints” and fastened to substrate support at each side.
· 7.11.4 “Control Joints” are a one-piece prefabricated member or paired casing beads back to back with SAF behind the casing beads. Provide a 1⁄8 in. minimum gap.
· 220.127.116.11 Provide “control joints” at defining panel areas at walls not exceeding 144 SF, and at ceiling areas not exceeding 100 SF.
· 18.104.22.168 The maximum panel dimension between “control joints” is 18 ft or a panel area length-to-width ratio of 21⁄2 to 1. Provide a “control joint” where ceiling framing or furring changes direction.
· 22.214.171.124 Wall or partition height door frames are “control joints”.
· A1.2 “Control Joints” accommodate stucco curing and drying shrinkage movement, along usually straight lines and function as a screed to aid in stucco thickness control.
From ASTM C1280 we note:
· 126.96.36.199 Gypsum sheathing panel corners must be notched a minimum of 4-in. at window, door and similar wall opening corners.
Today’s Minimum Stucco Industry Standards ASTM C926 and ASTM C1063, describe various general prescriptive requirements on what a SMJS is, how it is configured, and on where to locate the SMJS on a building where it is intended to minimize stucco cracking by accommodating portland cement-based plaster shrinkage and stucco thermal movements.
Since the mid-1950s, the stucco panel isolation joint has been termed the “control joint,” however this website preferentially uses the term Shrinkage Movement Joint Subassembly (SMJS) and SMJS lath accessory. The one-piece SMJS lath accessory is used in a SMJS subassembly which includes the substrate support (continuous), the configuration of the lath (discontinuous), framing/blocking to receive lath edge fasteners, the SMJS lath accessory and its fasteners, and of course the portland cement-based plaster.
Clark’s SMJS requirements: Clark designed and patented the SMJS lath accessory to be wire-tied to the outer face of continuous “expansible” lath and not fastened with nails, screws or staples to the substrate support framing or blocking. Throughout the patent documentation, the SMJS lath accessory is clearly described to be a flexible extension to allow concentrated movement within the lath, and is isolated from and not connected to the substrate support framing or blocking. For movement to occur Clark clearly describes and states throughout the patent that the lathing was “expansible” which he appears to have assumed was capable of expanding and contracting according to the movement needs imposed upon the SMJS by portland cement-based plaster shrinkage movement and stucco thermal movement. A SMJS is part of the lath and stucco composite wall cladding assembly only, and is isolated and not part of the continuous substrate support. Mechanically fastening the SMJS lath accessory to the continuous substrate support framing or blocking with nails, screws or staples makes it physically attached to and part of the substrate support which significantly reduces its ability to accommodate shrinkage and thermal movements, and minimize cracking.
As it turns out, and contrary to the depictions and assumptions in Clarks patent, as proven by performance testing, where the lath is continuous through the SMJS, metallic lath in any of its forms, expanded sheet metal, woven wire or welded wire, is not expansible enough to accommodate the amount of dimensional movement SMJS are subjected to, capable of, or required to accommodate to minimize cracking. ASTM C1063 recognizes this materials property characteristic of lath products and requires the lath to be discontinuous at SMJS to minimize the restriction of lath continuity to movement at the SMJS and mitigate its ‘non-expansibility’ characteristic, to allow movement and minimize cracking. The requirement for discontinuous lath at SMJS in Minimum Stucco Industry Standards has not changed since it was first introduced in the 1971 ANSI A42.3(6).
The subject of stucco “control joints” was something I had to wade through myself and this webpage is a summary of the highlights from that journey. First and foremost, I decided from the onset to have an open mind and was committed to locate and consider all information and evidence available and not approach this subject with any predispositions or preconceptions, in a comprehensive, considered and rational manner. I have been interested from the earliest days of my professional career (mid-1980’s) about all things related to stucco, and “control joints” is central to that interest. I set out to locate and evaluate every bit of information available about stucco “control joints” that I could find, and a number of significant, meaningful, informational and related resources that had been forgotten about or overlooked through time, have surfaced and been brought together here, for the first time together in one place – for careful re-evaluation and to illuminate a progressive context to understand the broader perspective. Don’t shoot the messenger – it is what it is! While we can develop conclusions and suggestions going forward, they should be based on the broad spectrum of complete information, evaluated collectively, which considers a thorough review of all information resources available on the subject. It is too easy to cherry-pick information that may support one position or another and what a tremendous disservice that is to gaining a complete understanding of “control joints”. The conclusions and suggestions presented are based on the considerations of rational inquiry, on laboratory performance testing, on material properties and characteristics, on the realities of construction document and jobsite-related challenges, on observations of stucco cladding systems and their components, behavior and performance of different SMJS configurations since the mid-1980’s, from the perspective of a technically-oriented architect. When evaluating the various available resources, conflicts, misinformation, incomplete information and other conditions may at first be problematic. Use professional discernment, and disregard unsubstantiated information. The hope is that the reader will consider approaching this topic with the same rigor as together we seek a unified industry voice on this very important subject.
The Minimum Stucco Industry Standards regarding SMJS provide limited but important and useful basic, prescriptive-based criteria. The specific Minimum Stucco Standards of Care for SMJS are defined in the building code which includes adopted reference standards ASTM C926 and C1063. Because these are the only applicable codified references, other stucco industry references, where they conflict with ASTM C926 and C1063 may result in SMJS that do not comply with the building code and result in less than minimally acceptable stucco performance. Following manufacturers written requirements for their prefabricated SMJS lath accessory products is also a Minimum Stucco Standard of Care.
That term “control joint” is a significant part of the struggle the industry has experienced through the decades because it is impossible to know or understand intuitively exactly what the “control joint” controls, or to understand if it is a dynamic or static joint, from its name alone. The term is also vague as to what it is in reference to, and has been commonly and incorrectly used to describe merely the lath accessory, and not a functioning stucco movement joint as a subassembly of various components working together. Complicating things, for decades this term was also used interchangeably throughout the industry, by the lath accessory manufacturers, and in ASTM C1063 with the term “expansion joint”, “stress-relief joint”, “expansion/contraction joint” and other similar and vague terms, without a clear understanding of what exactly was expanding (the joint, the stucco, or the substrate?), and even sometimes used to describe decorative joints such as formed grooves, cuts and reveals. What is abundantly clear is that the “control joint” lath accessory has been promoted and used since its introduction without a definitive understanding of its purpose, intended and actual function, intended and actual performance, or intended and actual installation requirements to achieve its functions, all of which have seemingly mutated into multiple variations over time.
The term “control joint” is unfortunately too generic, broad, nebulous, ambiguous, and obscure to have any real, intuitive or commonly accepted meaning – which in part has merely just fueled the fire of dissension in the stucco industry. The term “control joint” has not served us well, is obsolete and does not clearly describe that this is a subassembly of the larger stucco wall cladding system, which includes adjacent components within the subassembly, arranged in a specific configuration for performance reasons, and is not just a lath accessory component, and it does not describe the purpose or function of this stucco movement joint subassembly. We as the stucco industry would be well-served to abandon the term “control joint” from our vocabulary because it is vague and replace it with a more accurate, clearly understood, descriptive term. Where the term “control joint” is used on this website, it will be surrounded by quotes in respect of its common or historic contextual usage, and will only be used in homage to its original context.
ASTM C1063 added clarity, definition and distinction to the two terms “control joint” and “expansion joint” in 2007. These two different stucco movement joint subassembly categories are not the same; they are different in many physical, installation and functional characteristics. In ASTM parlance, a “control joint” is intended to accommodate portland cement-based plaster shrinkage and stucco thermal movements, and an “expansion joint” is to accommodate building substrate support movements.
In search of clarity I derived what I suggest is a more accurate descriptive term for “control joint”. The term Shrinkage Movement Joint Subassembly (SMJS) as used on this website is used for easier, more intuitive recognition, to promote understanding, to facilitate its effective use and because it more clearly describes the purpose and function of this stucco cladding movement joint subassembly. The term Shrinkage Movement Joint lath accessory is used to describe just the lath accessory component where that is appropriate for the context. Because the subassembly and lath accessory are directed at accommodated shrinkage and thermal movements, the term could have been “shrinkage and thermal movement joint”, but since thermal movements impart similar behaviors as shrinkage movements but at much smaller magnitudes, it is reasonable to shorten the name to just Shrinkage Movement Joint Subassembly (SMJS), for simplicity.
The term SMJS is the complete term for the subassembly and the term SMJS lath accessory is the complete term for the SMJS lath accessory component. These terms are not interchangeable because they mean different things, and neither of these terms should be shortened to simply “SMJS”, because the term SMJS is incomplete and could cause misunderstanding and miscommunication. The intention of this key point about clarity of terminology is to avoid the potential misperception, confusion and miscommunication that may occur, which has occurred with use of the term “control joint”, in that the duality and unclarity of meaning that the term SMJS could be in reference to either a lath accessory or a subassembly. The term SMJS is not used in the building code or Minimum Stucco Industry Standards, where it is generically described as a stucco “control joint” and the term SMJS lath accessory is not used by any known lath accessory manufacturers.
Portland cement-based plaster shrinkage movement during curing and stucco thermal movements while in service are significant factors that are a primary cause of stucco cracking, most will agree. While shrinkage and thermal movements are important, it is recognized that other factors can contribute to stucco cracking also so addressing merely shrinkage and thermal movements alone, are not the only factors contributing to stucco cracking, but they are primary considerations substantiating the use of SMJS. This webpage focuses primarily on the issue of portland cement-based plaster shrinkage movement and stucco thermal movement and a primary method of utilizing the SMJS to minimize their contributory effects regarding stucco cracking.
The SMJS developed from the Perimeter Movement Joint Subassembly (PMJS), so a complete understanding of the principles and context presented on the PMJS subassembly webpage and Stucco Material Properties webpage is fundamental to an understanding of SMJS along with the additional information specific to SMJS presented on this webpage.
The stucco SMJS by definition in ASTM C1063 is: “3.2.3 control joint, n—a joint that accommodates movement of plaster shrinkage and curing along predetermined, usually straight, lines. Note that this definition does not specifically identify stucco thermal movement while in service. Thermal movements are mentioned in ASTM C1063 at 7.11.4 where it says: “Control joints shall be formed by using a single prefabricated member or fabricated by installing casing beads back to back … The separation spacing shall be not less than 1⁄8 in. (3.2 mm) or as required by the anticipated thermal exposure range.” Thermal-related materials expansion or contraction movements in stucco are identical in behavior to shrinkage of portland cement-based plaster, just of a much smaller magnitude than shrinkage movements. Because of their similar behaviors, the two movements can be considered as additive together for purposes of evaluating SMJS movement behavior as long as their individual magnitudes are understood and accommodated.
Stucco behaves differently when installed on framed or framed/sheathed substrate supports than when directly-applied and continuously-bonded to a solid substrate support. Where directly-applied to mass masonry and solid concrete mass wall substrates without a WRB, stucco cladding is continuously-bonded and therefore continuously-restrained from movement caused by shrinkage and thermal movement as a result of its continuous bond to the masonry or concrete substrate support. Cracking occurring where directly-applied is typically translational cracking from localized movement in the substrate support, or is the result of movements at discrete locations where the stucco is not continuously-bonded to (delaminated from) the substrate support. As a result, SMJS serve no purpose for directly-applied stucco on masonry or concrete walls, in addressing portland cement-based plaster shrinkage or stucco thermal movements.
Portland cement-based plaster installed over lath creates a new homogenous lath and stucco composite material possessing the newly combined properties of both materials. Portland cement-based plaster installed on framed substrate supports over lath and a water-resistive barrier is not continuously-bonded to the substrate support and dynamically shrinks, expands and contracts relatively independently from its substrate support – it is conceptually isolated perhaps, but is not a complete isolation. The lath is mechanically fastened to the substrate support at periodic intervals which inherently restricts movement of the plaster or stucco membrane at each lath fastener attachment location, yet allows shrinkage and thermal movement independent of the substrate between the mechanical fasteners. Lath fasteners such as nails, screws and staples used to mechanically fasten lath to the substrate support can be a restriction to shrinkage and thermal movement in the field of a stucco panel, but a restrictive condition at lath fasteners is not absolute and depends on the conditions at a specific fastener location. For example, where the lath fastener head is not drawn tight to the substrate support surface, limited slippage can occur between the lath fastener head and the lath to allow limited movement. Or when the lath fastener head is drawn tight to the lath and substrate support, fasteners at self-furred points or even in the field of the lath, may draw the lath to the rearmost plane of the stucco, and the stucco immediately proximate to the lath fastener may not embed the lath. The localized unembedded lath has been observed to distort around lath fasteners in this configuration to allow limited movement, similar to what has been observed to occur proximate to lath edge fasteners at SMJS. A lath fastener that could be devised to allow a greater degree of predictable shrinkage and thermal movement for the field of the lath would be beneficial in minimizing stucco cracking.
Lath not embedded proximate to lath fastener can allow limited movement
by slippage or lath strand distortion around lath fastener.
(Lath fastener removed, but imprint visible in stucco behind lath)
The process and duration of stucco mortar curing involves water loss and compositional changes in the mortar that result in both strength increase and volumetric reduction or shrinkage of the stucco mortar. The curing rate, and related shrinkage movement amount is discussed in more detail elsewhere on this website, but summarized here. The combined shrinkage and thermal movements from ambient temperature variations for design purposes is in the approximate range of 130-240 mils for each 10 feet (13-24 mils per lineal foot) of stucco over a 100 degree F temperature range. While this may seem small dimensionally, it is a significant amount of movement for portland cement-based plaster while in its plastic state, and for stucco as a dense, brittle material where movement can cause cracking if not accommodated.
To determine which stucco movement joint is appropriate for a given condition, one must understand the differences in anticipated movement at a given condition. Shrinkage and thermal movements occur in the composite lath and stucco membrane. BMJS, PMJS and SMJS each accommodate shrinkage and thermal movements because the composite lath and stucco membrane is discontinuous through and terminates at each edge of these subassemblies. A SMJS is not intended to accommodate substrate support movement both by definition and because the substrate support is continuous at SMJS. BMJS and PMJS accommodate substrate support movement by their definitions and because the substrate support is discontinuous and each side of the BMJS and PMJS and their substrate supports can move independently.
Shrinkage Movement Joint Subassemblies (SMJS) serve these purposes:
· 1st Purpose: Accommodates portland cement-based plaster shrinkage movement occurring soon after plaster application, and stucco thermal movement after the plaster has cured, resulting from daily and seasonal thermal expansion and contraction, by using a flexible portion and extension of the lath created by discontinuous lath and a flexible SMJS lath accessory in the plane of the lath, mounted to the lath only and not the substrate support.
· 2nd Purpose: As a ground screed to gauge the application thickness of portland cement-based plaster to assist in the achievement of its intended nominal thickness and finish planarity.
· 3rd Purpose: Segments and panelizes continuous stucco wall cladding assemblies into functionally isolated, discrete, smaller, adjacent wall cladding panel areas with panel edges that define portland cement-based plaster work stoppage locations and prevent cold joints.
1st Purpose: Shrinkage Movement Joint Subassemblies (SMJS) are generally perceived as a complex, misunderstood and hotly-debated subassembly in the stucco industry, but it does not need to be that way. Throughout the industry we have made the topic of SMJS much more difficult than it needs to be, if collectively we can recognize, understand, and agree on a few inherent stucco movement characteristics. It is not surprising that the complexity surrounding the characteristics, requirements and expectations for SMJS has caused misunderstandings, misinformation, insufficient design, insufficient installation, insufficient inspection and ultimately less than reasonable quality stucco performance as manifested by excessive cracking, water intrusion and other stucco and SMJS performance-related issues.
Fundamentally, most people will agree that it is an intrinsic characteristic of portland cement-based plaster to shrink as it cures and hardens, because it contains water that evaporates or is consumed in the process of curing causing volume loss, and is applied in a wet, plastic state, unlike most other contemporary exterior wall cladding materials. Portland cement-based plaster shrinkage is a one-time event that only occurs subsequent to application of the wet plaster. If portland cement-based plaster shrinkage did not occur and cause cracking, then the stucco SMJS would not be necessary to accommodate shrinkage, or at least the accommodation of shrinkage movement would not be of concern. The other type of movement the SMJS accommodates is stucco thermal expansion and contraction after the stucco has cured and hardened; a behavior that occurs cyclically on daily and seasonal intervals, throughout the service life of stucco. The dimensional movement related to stucco thermal expansion and contraction behavior is of comparatively minor magnitude relative to stucco shrinkage movement, but similar in dynamics, and because thermal movements are minor and similar, its effects are considered in parallel along with shrinkage because the movement dynamics are similar, if unequal but complementary in magnitude. The essential point to comprehend about SMJS is that without an appreciation for and understanding of the basic behavior and characteristics of portland cement-based plaster shrinkage and stucco thermal movement, one cannot expect to appreciate or understand the design, function and installation requirements of the SMJS and the role it performs towards minimizing stucco cracking. We need to agree on the basic functional requirements of SMJS before proceeding, because they are based upon the shrinkage and thermal movement-related behavior of portland cement-based plaster and stucco.
Stucco cracking in general terms is an inherent characteristic of stucco, not a defect in and of themselves but they may be the manifestation of other defects, latent or patent. Cracking are generally the manifestation of unaccommodated stresses within the portland cement-based plaster or stucco membrane and are not only a visual distraction but they may also allow water intrusion. Generally, cracking are not considered to be defects, unless they are part of a larger context of causes or contribute to resultant damage such as water intrusion and/or building substrate support deterioration. SMJS correctly designed, located and installed may assist in minimizing the most common types and occurrences of stucco cracking which is related to initial portland cement-based plaster shrinkage and stucco thermal movements.
In the broadest sense and by definition, an SMJS is a “joint that accommodates movement of stucco shrinkage and curing along predetermined, usually straight, lines”. These comparatively small magnitude movements occur primarily in the portland cement-based plaster membrane and if not accommodated, can result in stucco cracking.
SMJS may also assist in minimizing stucco cracking from the contributory effects of other ancillary, induced, cement plaster and stucco stresses, which are not substrate support-related, and which are in addition to and may exacerbate cement plaster shrinkage and stucco thermal movements. These concentrated stresses may include stucco membrane thickness variations resulting from wrinkly building paper, stresses proximate to the expansion and contraction of wood-based sheathings while in service and stresses related to lathing anomalies.
SMJS, BMJS and PMJS individually exist for the sole purpose of managing portland cement-based plaster and stucco movement in different conditions, to minimize stucco cracking. However each of these stucco movement joint subassemblies functions similarly to the SMJS to manage portland cement-based plaster shrinkage and stucco thermal movements, because the lath is discontinuous and terminates at these stucco movement joint subassemblies.
From ASTM C926 we read “…control joints and expansion joint[s]…shall be installed prior to the application of plaster. Their type, location, depth, and method of installation shall be determined by the characteristics of the substrate and included in the project contract documents.” Underlining is by this author for emphasis. This text is broadly worded determining criteria for deciding whether a SMJS, BMJS or PMJS is required and is often overlooked. It can mean that if there is no movement within the substrate support to accommodate (where the substrate support is continuous), the primary movement that does occur is portland cement-based plaster shrinkage and stucco thermal movement only, and the appropriate stucco movement joint subassembly is an SMJS. If the building substrate support is discontinuous at a stucco movement joint location, then the more significant movement is substrate support movement, and the appropriate stucco movement joint is a BMJS if along a wall or PMJS if at an internal corner.
Effective utilization of a SMJS is no guarantee against stucco cracking. SMJS are only one of several elements in a complete stucco wall cladding system that may be useful towards minimizing cracking – mix design, aggregate selection and gradation, fiber admixtures, lath type, finish type and texture, workmanship, curing, and many other factors – all have a contributing role towards either contributing to or minimizing cracking.
SMJS are not complicated if one understands why they are needed, how they function and where best to locate them. It is a fundamental characteristic of portland cement plaster to shrink as it cures and hardens into stucco. Portland cement-based plaster shrinkage movement occurs as a manifestation of the plaster curing and hardening process. If the inherent process of portland cement plaster shrinkage movement is restrained, then shrinkage cracking can form in the plaster as it cures. A SMJS provides a comparatively unrestrained location for shrinkage movement to occur that helps minimize cracking within a stucco panel. It’s no more complicated than that.
We observed from a detailed review of the historical development of the “control joint” subassembly and lath accessory that Clark, Penn Metal and Keene initially focused on mitigating portland cement-based plaster shrinkage and stucco thermal movements. Early “control joint” illustrations depicted ceiling locations which did not include the support substrate of framing and sheathing. Discontinuous lath at “control joints” has been consistently required in stucco industry standards since at least the 1971 ANSI A42.3. In the late 1970’s, over twenty years after introduction of the “control joint”, manufacturers’ wall assembly illustrations depicted separate framing at each side of the “control joint assembly” followed a few years later with recommendations that the entire wall assembly be separated – the framing and sheathing. It is clear from the historical record that the purpose and installation requirements for “control joints” evolved and mutated from simply shrinkage and thermal movement purposes, to what we know today as “expansion joints” to accommodate substrate support or building movement purposes, movements this website refers to BMJS and PMJS. “Control joints” or SMJS are not intended to address movements related to the building substrate support – wall sheathing expansion/contraction, building structural movement, deflections, deformations or settlement, wind loads, seismic activity, story drift, beam or floor deflections or similar movements which are the domain of the BMJS and PMJS. While a SMJS may accommodate incidental substrate movements in small magnitudes, they are not the primary function of a SMJS.
It should be obvious that a SMJS lath accessory independent of its subassembly installation configuration is not a SMJS subassembly. At its essence, we must realize that an SMJS subassembly is not a manufactured product one buys off the shelf, it is a jobsite-fabricated subassembly installed on a building as a subassembly of an exterior stucco wall cladding system, that is created by the interrelationship of several components: The continuous substrate support with framing and blocking, continuous WRB, discontinuous lath and lath edge fastening using nails, screws or staples into framing/blocking in the substrate support, a SMJS lath accessory wire-tied to the face of adjacent lath edges created by discontinuous lath at the SMJS subassembly, and the portland cement-based plaster which hardens to become stucco. A SMJS subassembly must be assembled by installing the different related Components in a specific configuration, so that portland cement-based plaster shrinkage and stucco thermal movements can be most effectively accommodated and cracking minimized. Assembled incorrectly, the SMJS will not accommodate movements and excessive stucco cracking will occur. Variations in any of these components or conditions affect the SMJS performance and its ability to minimize stucco cracking.
2nd Purpose: Shrinkage Movement Joint Subassemblies (SMJS) and the SMJS lath accessory, function as a ground screed as an aid towards achieving the intended portland cement-based plaster membrane thickness and finish planarity within accepted tolerance. The SMJS lath accessory is either two casing beads back to back or a premanufactured one-piece Component. The SMJS lath accessory in either form includes two foraminous flanges as an aid to attaching the accessory to the lath and keying with the portland cement-based plaster mortar and lath, a pleat which is configured to accommodate movement parallel in plane with the portland cement-based plaster (and stucco), and a pair of fixed dimension ground screeds at both edges of the pleat. The fixed dimension ground screeds are oriented perpendicularly to the substrate support and the outer screed edges are installed to provide planar alignment of the base coat which is the substrate for the finish coat, which has a specified or required tolerance. The dimension of the fixed dimension ground screeds is selected or specified to provide the required thickness of the portland cement-based plaster membrane. Typically available SMJS lath accessory ground screed dimensions range from 3/8” to 7/8”.
The nature of the fixed ground screed dimension of the SMJS lath accessory is that it is not adjustable, which may not be beneficial. Where the substrate support surface is within the specified or required planarity tolerance of the finish coat, and where fixed ground dimension SMJS lath accessories are used, then portland cement-based plaster can be installed to a uniform thickness which is desirable. But in the real world of construction, substrate support conditions may not be within finish coat planarity tolerances. Wall framing may not be sufficiently plumb, wall framing Components may be cupped or bowed, steel framing components may nest and result in out-of-plane localized anomalous conditions such as substrate support misalignments, SAF thickness build-ups, bumps at flanged window fins and their fasteners heads bolt/lag screw heads, framing straps and hardware etc., which can cause localized portland cement-based plaster thickness variations, which may contribute to stucco performance issues. The fixed ground screed dimension of the SMJS lath accessory has limited capability to address these conditions and other methods may need to be necessary to achieve a uniform thickness portland cement-based plaster membrane thickness and a finish coat surface within an acceptable planarity tolerance.
3rd Purpose: In the era before the SMJS lath accessory and subassembly were available, framed buildings were clad in continuous stucco over continuous lath. Buildings were smaller than those today and some could receive a complete continuous portland cement-based plaster coat in one work day. As buildings became larger the plastering crew simply stopped the plaster at a convenient location at the end of each work day and resumed the next. The condition where plastering is started and stopped are called joinings or cold joints, which were susceptible to cracking where fresh new plaster met set stucco.
The advent of the SMJS lath accessory provides a device that when installed, divides and segments the otherwise continuous portland cement-based plaster wall cladding assembly into discrete adjacent panels, perimetered by SMJS that define each segmented panel. The SMJS located at panel perimeters function in part as a plaster work stoppage location which effectively eliminates joinings and the resulting plaster cold joint, from contemporary portland cement-based plastering practice. The SMJS provides a solution to the problems related to cracking at joinings.
Isolated stucco wall cladding panels of certain areas, dimensions and geometry, using horizontally-oriented and vertically-oriented SMJS located in certain locations such as reentrant corners, configured in a certain way to accommodate shrinkage and thermal movements, sealed at critical junctures to effectively manage the effects of water, create a Shrinkage Movement Joint Assembly that minimizes cracking. The Assembly is the sum of its parts, arranged and functioning together to serve the purpose of minimizing cracking. No singular SMJS lath accessory component or SMJS subassembly in isolation can accomplish the effect of the Assembly as a whole on the stucco wall cladding system.
Traditional 3-coat, continuous, exterior stucco wall cladding in the field of the stucco cladding away from edges and cracking and when of a uniform 7/8 inch nominal thickness, performs sufficiently at deflecting bulk water from entering the wall assembly. Bulk water can potentially intrude into and behind the exterior stucco wall cladding at full thickness cracking, at unsealed gaps at stucco panel perimeters and penetrations, at exposed lath accessories such as splices, terminations and intersections of casing beads, stucco movement joints, at unsealed drainage flashing laps and at anomalous construction conditions. Exterior stucco wall claddings installed over a continuous WRB and drainage flashing system integrated watertight with adjacent wall components is a drainage wall assembly, and dependent on the performance characteristics and workmanship of the WRB and drainage flashing system, the exterior stucco wall cladding system can accept and manage bulk water.
During the process of applying wet portland cement-based plaster to lath over a WRB, moisture from the wet plaster is released during curing, expelled in all directions from the plaster into the air and into the WRB and wall assembly. When the WRB material is capable of absorbing moisture such as with asphalt saturated building paper, the building paper absorbs moisture, distorts to create wrinkles in the building paper. The wrinkled building paper surface functions as an irregular formwork for the interior surface of the portland cement-based plaster which results in a non-uniformity in thickness of the portland cement-based plaster which remains of variable thickness when cured into stucco. Incidentally this condition is one reason why cement plaster and stucco thickness measurements are stated as nominal dimensions in the Minimum Stucco Industry Standards. As the plaster and building paper dry out with moisture evaporation, the WRB shrinks away from the inner surface of the plaster, leaving an irregular, unpredictable, compact network of drainage gaps between the stucco and WRB to facilitate gravity drainage of bulk water towards drainage flashings. Polymeric WRB materials do not exhibit this behavior to wrinkle with moisture absorption and form a network of drainage gaps.
Wrinkled building paper under removed stucco after being in service several years
Localized wrinkled building paper during adjacent window water testing
(Note non-wrinkled building paper where not wetted)
While the compact drainage capabilities of this assembly are noted, the WRB wrinkles invariably cause thickness variations in the portland cement-based plaster. Thickness variations in any homogenous material by definition reduce the cross-sectional area of the material and for a material that is subject to shrinkage and thermal forces that cause movements, those forces are magnified and are stress concentrations at the thickness variation. Consider that a 1/4 inch thickness variation, in a 3/4 inch thick scratch/brown portland cement-based plaster membrane is a 33% thickness and stress variation from the thickest to thinnest conditions, concentrated at the thinnest location of the cladding. Where only minor thickness dimensional variations occur sufficient portland cement-based plaster thickness often provides enough strength in cross sectional area to accommodate the increased stresses without performance issues. Where the magnitude of the stress concentration exceeds the structural capacity of the curing plaster or cured stucco at a thickness variation, cracking can occur. SMJS assist in minimizing stresses caused by thickness variations that may contribute to cracking.
Extreme stucco thickness variation caused by wrinkled building paper
Stucco thickness variation, no cracking from wrinkled building paper
Stucco thickness variation and cracking from wrinkled building paper
Stucco thickness variation and cracking from wrinkled building paper
A concern with SMJS amongst waterproofing professionals is bulk water intrusion at unsealed gaps at exposed lath accessories primarily at the finish stucco surface of corners, intersections, end terminations and butted termination splices. Splice plates and end caps that can be sealed watertight are not available for any SMJS lath accessory. At weather-exposed surfaces these conditions should be sealed watertight at the stucco surface to minimize bulk water intrusion behind the stucco cladding system, by embedding the condition in a sealant bed from behind as the lath accessory is installed. SMJS are hollow profile sections that function as water collection channels and that divert water wherever it will go, concealed behind the lath accessory.
Butted termination splices of adjacent SMJS terminations, unsealed and occurring along stucco panel edges are problematic because they can cause perpendicular splice cracking which appear unsightly and can allow water intrusion. Butted termination splice cracking can be avoided by limiting a stucco panel edge to 10 ft. (length of a SMJS lath accessory without splices) or less and not allowing a butted termination splice to occur along a stucco panel edge, by locating termination splices only at SMJS lath accessory intersections where a crack is not possible. The potential for water intrusion at butted splice terminations can be minimized by embedding the lath accessory terminations at the splice location in a sealant bed.
A recent variant to the one-piece SMJS lath accessory has been developed with one solid flange and one expanded sheet metal flange (Pilz, 2008) to provide drainage capability for horizontally-oriented SMJS installations on walls by providing a solid sheet metal flange as an upward-oriented flange which integrates with the WRB and flashings. Coordinate the locations of butted termination splices of the Double-V Horizontal Drainage SMJS lath accessory at intersections with vertical SMJS, to avoid perpendicular butted termination splice cracking.
Open stud framing is the original form of framed substrate support and can be constructed using either wood or steel studs. Without exterior sheathing, the building structure must be braced with diagonal bracing or interior panel sheathing, or by other means and is categorized as “conventional framing” in the building code. Without exterior sheathing a support backing for the wet plaster application is needed, which traditionally takes the form of either individually installed line wires at 6 in. on center, attached to studs, over which a WRB and lath are installed, or more frequently an integral paper-backed, wire lath with integral line wires is installed in one operation. The wire lath can be either welded wire or woven wire. Another option is expanded sheet metal lath over the WRB, each layer individually installed. Open stud framing is more common with smaller-scale buildings such as residences but there are no limitation conditions regarding use of this substrate support assembly.
Line wire backing as a substrate is pulled taught when installed, but can still deflect between the studs during the process of plastering which requires pressure to key the plaster with the lath. This configuration can result in a plaster application that is thinner at studs than in between studs, a differential thickness condition which can cause cracking. Stud line cracking is typically expressed in the finished stucco as a series of vertical cracking whose location correlates with the underlying studs.
With open stud framing, additional vertical wood framing or blocking members may be needed at lath accessories and SMJS subassemblies, the same as for panelized sheathing over framing support substrates.
Structural engineers may engineer a building and require sheathing only at specific exterior wall locations where needed for a buildings’ structural purposes as shear walls, the remaining walls may remain as open stud framing. Nothing is technically incorrect with this change in substrate support condition because both substrate support conditions meet Minimum Stucco Industry Standards requirements, although it presents installation and performance challenges for the exterior stucco wall cladding assembly. Where the change in substrate support from open stud to sheathing over framing occurs along a contiguous wall, the stucco finish surface must either transition at the change in substrate to maintain a uniform thickness, or the thickness must vary which is the usual case. Either condition is likely to result in a stucco crack at the transition in substrate support conditions.
To avoid cracking resulting from open stud framing conditions and to provide a uniform substrate support condition for the stucco, provide continuous panelized sheathing correctly installed over stud framing at all framed substrate support locations under exterior stucco wall cladding.
A vertical framing or blocking substrate support member may be required at vertically-oriented SMJS for fastening the adjacent vertical discontinuous lath edges if and where framing or blocking is not already part of the typical framing layout at the SMJS location. Often the typical framing already present including the king/jack/cripple stud framing members at wall opening corners above and below window and door corners is sufficient. Additional framing or blocking needed solely for vertically-oriented SMJS may only be needed where the SMJS is located away from wall openings that do not correspond with the normal framing layout. For steel stud frame substrate support systems, ASTM C1007 at A2.4 specifically requires that “Care be taken to allow for additional studs at…control joints...” So no ‘additional’ framing or blocking is needed for vertically-oriented SMJS at steel stud frame substrate support systems, which should be routinely provided.
Additional framing or blocking substrate supports at horizontally-oriented SMJS locations on vertical walls are not needed where the SMJS is perpendicular to the framing member span. No horizontal framing or blocking is needed at the adjacent lath edges of horizontally-oriented SMJS because the required lath edge fasteners are only attached into vertical framing substrate supports and the SMJS lath accessory is wire-tied over the lath. At vertical walls, ASTM C1063 requires lath to be attached to vertical studs only, no attachment to horizontal members at floor lines, sill plates or any other similar condition is required or necessary – to do so is overfastening the lath which contributes to cracking. ASTM C1063 also requires accessories to be attached at 7 in. O.C., where some interpret this to require horizontal blocking between the studs. For horizontally-oriented SMJS on a wall, the lath is discontinuous and attached to studs on both sides of the SMJS, and the SMJS lath accessory flanges are wire-tied to the lath edges at 7 in O.C.. No additional horizontal blocking is required or necessary between the vertical studs at horizontally-oriented SMJS for lath attachment purposes.
The continuity of the substrate support (usually gypsum-based sheathing or wood-based sheathing) including framing/blocking, is not a factor for a SMJS because a SMJS is only intended to accommodate lath and stucco composite shrinkage and thermal movement, not substrate support movement. Therefore, a gap or discontinuity in the substrate support to accommodate movement is not required or essential for a SMJS. SMJS are primarily intended to counteract the effects of shrinkage and thermal movements of the lath and stucco composite only. A gap or discontinuity in the substrate support for substrate support movement purposes requires a BMJS or PMJS.
Sheathing panels such as wood-based plywood and OSB and gypsum-based sheathing panels are typically installed over framed wall substrates under stucco for multiple purposes, both as a component of the building structural system and as an important component in for building enclosure and exterior stucco wall cladding systems. They provide a smooth continuous support surface for the WRB and drainage components, and also facilitate the cement-based plaster application in achieving uniform plaster thicknesses that minimizes cracking. Gypsum-based sheathings can provide fire-resistivity to the wall assembly.
Wood-based and gypsum-based panelized sheathings may expand and contract with variations in the transient moisture content that may permeate through permeable WRB’s into the sheathing. After the plaster is applied and is curing, the plaster is shrinking and moisture from the plaster curing process permeates through the WRB into the sheathing which may be expanding from moisture absorption and potentially confined at its panel edges – sheathing movement conditions contradictory to plaster shrinkage movement that is a known cause of stucco cracking. For this reason the building code and Minimum Stucco Industry Standards require wood-based sheathing panels located under portland cement-based plaster to be gapped 1/8 in. at panel edges to accommodate wood based sheathing panel movement, to mitigate potential stucco cracking. No similar panel edge gap requirement exists for gypsum sheathing which requires ‘moderate contact’ between adjacent panels. Sheathing panel movement under these conditions does not require BMJS or PMJS, but the stress concentrations panelized sheathing substrate movement creates on the portland cement-based plaster/stucco membrane can cause cracking that effective SMJS can help minimize.
No gaps at wood-based sheathing panel edges at this condition
provide no movement capability for the panelized sheathing substrate support, movement which may impart expansion/contraction stress to the portland cement-based plaster and stucco membrane to cause cracking
(Photo used with permission of Chris Nelson PE, Technical Roofing Services, Inc.)
Stucco crack at gapped horizontal wood-based sheathing panel edge
(SMJS at this stucco wall cladding system had continuous lath
and other restrictions to stucco movement)
Gypsum-based sheathing panels must be installed per the requirements of ASTM C1280, which requires among other things, that the panel be notched by 4 inches minimum at wall opening reentrant corners such as windows and doors. Aligning the sheathing panel joints at wall opening reentrant corners creates an obvious localized weakness condition in the substrate support for stucco, which can move incrementally with shrinkage and thermal movements which can cause cracking. Offsetting the sheathing panel joint away from wall opening reentrant corners by notching the panel sheathing, is known to minimize stucco cracking at wall opening reentrant corners. While no similar standard for wood-based sheathings exist, offsetting wood-based sheathing panel joints away from wall opening reentrant corners in wood-based sheathings in a similar manner is also beneficial for the same reasons, in reducing stucco cracking.
Gypsum-based sheathing panels with 4-in. minimum offset
at wall opening corners per ASTM C1280
It is an all-too-common occurrence that wall assembly structures as substrate support for stucco, are sometimes built beyond acceptable planar tolerance resulting from a variety of circumstances, but at the same time, finish planarity tolerances for stucco wall cladding systems are still required. So one question is, can and should SMJS, in their function as screeds for thickness control, be shimmed to achieve acceptable finish surface alignment? Succinctly no, for multiple reasons. SMJS lath accessories should not be shimmed and the need to shim indicates problematic substrate issues that require resolution independent from the stucco wall cladding installation to avoid stucco cladding system performance problems:
· Shimming SMJS lath accessories is not a recognized lath or lath accessory installation method in ASTM C1063 which requires a substrate within planar tolerance by reasonable inference.
· Shimming SMJS lath accessories is not recognized or discussed in SMJS lath accessory manufacturer’s product literature.
· Shimming SMJS lath accessories indicates that the SMJS lath accessory is installed onto the substrate support first, mounted with fasteners through its flanges and shims to the substrate support, a condition which significantly restricts SMJS movement and causes cracking.
· Shimming SMJS lath accessories may create excessive differential stucco thickness variations and may allow wet portland cement-based plaster mortar migration behind the SMJS lath accessory, which can cause cracking.
· Shimming SMJS lath accessories causes fastener penetrations through the WRB that are often not in compression to the WRB, potentially allowing water intrusion.
Where lath is discontinuous at SMJS does the stucco mortar flow behind the SMJS lath accessory and join the lath ends together preventing the SMJS from functioning? No, not typically, not when it is not shimmed. When the SMJS lath accessory is correctly installed over the face of the lath and not shimmed, its section profile includes two parallel linear shield flanges that block wet plaster from migrating behind the SMJS lath accessory which prevents fusing the two discontinuous adjacent lath edges together and avoids creating a continuous stucco membrane. Where shims are used under SMJS lath accessories, it is possible that wet stucco mortar may migrate behind the SMJS lath accessory and functionally connect the lath edges, potentially restricting movement and causing cracking.
Backside surface of installed SMJS
Note discontinuous lath, and shield flanges (arrow).
Note also the hollow cavity that can function as a water channel.
Stacked, plastic horseshoe shim condition,
with screw fasteners through SMJS lath accessory flanges
Folded WRB wad as shim - Really?
The Minimum Stucco Industry Standard substrate support planar tolerances for stucco wall cladding are as follows:
ASTM C926 specifies the nominal thickness of portland cement-based plaster over various substrate supports. Substrate planarity is specified for solid bases and steel stud framing in their respective ASTM standards, but for wood stud framing planarity criteria, no literal Minimum Stucco Industry Standard exists. For plaster applied over metallic lath, ASTM C1063 specifies that the finished plaster surface must be within ¼ in. in 10 feet. By reasonable inference the planar tolerance for wood stud framing, including panelized sheathing as a substrate support, must also be within ¼ in. in 10 feet per ASTM C1063, so that stucco can be installed to the specified, nominal uniform thickness. If and where the substrate support surface exceeds acceptable planarity tolerance, corrections are necessary to bring the substrate support surface within tolerance using means other than the stucco wall cladding assembly, before lath and stucco are installed. Shims at lath accessories including SMJS lath accessories should not be required, necessary, or allowed to correct out-of-tolerance substrate conditions.
Stucco is a wall cladding system not a shimming device, although it is capable of correcting minor planar irregularities within the allowable nominal tolerance range of 1/4 in. over 10 feet. Metallic SMJS lath accessory flanges can be distorted slightly before they are wire-tied to the face of lath, to accommodate minor gradual thickness alignment irregularities of a total dimension not exceeding 1/4 in. in 10 ft. Avoid shimming SMJS lath accessories to avoid stucco performance problems.
A specified or required substrate support planarity tolerance of 1/8 or ¼ inch in 10 feet is helpful in achieving generally uniform portland cement-based plaster and stucco thickness which minimizes cracking, but a 10 foot datum can be too large to address localized anomalies even of small dimensional magnitude, that may also cause cracking. Localized offsets in planarity should be eliminated or minimized where possible because they cause localized stress concentrations in the portland cement-based plaster or stucco membrane that can cause stucco cracking. Stress concentrations in the portland cement-based plaster or stucco transition from open stud to sheathed substrate supports is an obvious example, but cracking can occur from the thickness variation caused by even the thickness of overlapping WRB membranes, which effective SMJS can help minimize.
Variation in portland cement-based plaster or stucco membrane thickness
at transition between open stud framing and sheathed substrate support
(Photo used with permission of Chris Nelson, Technical Roofing Services, Inc.)
Stucco crack at gapped horizontal wood-based sheathing panel edge
with localized 1/8 inch planarity offset (See next photo below)
(SMJS at this wall had continuous lath)
Stucco crack at gapped horizontal wood-based sheathing panel edge
with localized 1/8 inch planarity offset
(See photo above)
Stucco crack at WRB lap
(SMJS at this wall had continuous lath)
No evidence has been located to indicate that Clark performance tested the Double-V SMJS lath accessory invention. The lack of evidence suggests that Clark simply theorized that his invention would work, designed and patented it and took it to market. Manufacturers produced and sold the Double-V lath accessory, but what we would consider today as sufficiently detailed installation instructions were not fully-developed. It is obvious that Clark’s functional intentions and installation expectations described in the patent have been overlooked or forgotten over the decades since Double-V lath accessory and SMJS were invented. So the stucco industry has experimented with various installation techniques and derived how they thought this SMJS lath accessory should be installed and how it should work. One of the inherent problems with the Double-V SMJS lath accessory is that it is physically capable of being be installed in various different configurations as related to the lath, fasteners and substrate support. Different regions arrived at different conclusions and methods – none are based on actual performance testing.
From the Keene Technical Manual issued by the original manufacturer of the SMJS lath accessory approximately 25 years after the SMJS lath accessory was introduced to the market: “In the design of control…joints, architects must consider portland cement plaster in the same way they would a pane of glass. Glass is never installed without relief in all directions. The same is true of portland cement plaster.” Some twenty-five years later in 2005, respected stucco consultant Walter Pruter wrote about the SMJS configuration in an article entitled Crack Control in Portland Cement Plaster, published in The Construction Specifier regarding minimizing stucco cracking. He quoted the identical phrase from Keene as above and added “Until a testing program can be initiated to prove unequivocally the preferred method, it is this author’s recommendation unrestrained lath and plaster panels be built and, detailed to the best degree possible, which includes cutting the lath and wire-tying the accessories to the lath edges.” Mr. Pruter concurs that as of 2005 he had no knowledge of any SMJS performance testing and recommends discontinuous lath at SMJS and wire-tying the SMJS lath accessory to the lath edges onto the face of the lath.
In 2008 an extensive SMJS performance testing program was completed by this author with laboratory assistance from colleagues at Simpson Gumpertz and Heger, to evaluate the actual performance characteristics of the most common SMJS lath accessories and SMJS installation configurations. More than 70 test samples of SMJS assembled in various configurations and with various combinations of lath, lath accessory materials and fastening methods were tested in triplicate. Control samples were included. The testing program and results were presented and summarized in an April 2009 article published, in homage to Mr. Pruter, in The Construction Specifier, which described several significant observations and conclusions about SMJS performance. SMJS performance was determined to be primarily dependent on subassembly configuration and Component material properties, and independent of lath type, including:
· Continuous lath at SMJS is the most significant restriction to shrinkage and thermal movement of all SMJS configuration variables. With continuous lath, movement capability is dependent on lath material properties. Where the SMJS lath accessory is wire-tied to the face of continuous lath, woven wire lath allowed limited movement at SMJS, whereas expanded sheet metal lath and welded wire lath allowed virtually no movement.
· Nailing, screwing or stapling SMJS lath accessory flanges to the substrate support significantly restricts SMJS lath accessory and SMJS subassembly movement.
· The optimum SMJS configuration allowing the most shrinkage and thermal movement which also minimizes stucco panel edge curling, includes discontinuous lath where each separate lath edge is fastened to the substrate support with nails, screws or staples, and where the SMJS lath accessory is wire-tied to the face of the lath at the lath edges.
· The dimensional movement capability of the SMJS, all other factors identical including discontinuous lath at the SMJS, is dependent on the material property characteristics of the SMJS lath accessory.
SMJS performance testing in progress
(Double-V SMJS lath accessory)
SMJS performance testing in progress
(Failure mode of Double-J SMJS lath accessory,
note discontinuous lath behind lath accessory)
SMJS performance testing variation diagrams
and comparative performance results
Double-V SMJS lath accessory performance: Double-V SMJS lath accessories are the original design, originally patented SMJS lath accessory, but the industry has moved away from this accessory. Use of this lath accessory results in the narrowest shadow line which may be a preferable aesthetic design choice, but as the lath accessory opens up during shrinkage and thermal movement, especially with large stucco panel areas, two continuous gaps can occur parallel to and on each side of the lath accessory, creating three linear parallel shadow lines. Avoid the Double-V SMJS lath accessory to avoid these conditions.
Double-J SMJS lath accessory movement capability performance: Performance testing confirmed that zinc alloy and PVC materials provide more movement capability than galvanized steel and accommodate the most shrinkage and thermal movements. Stainless steel SMJS lath accessories are new to the market since the performance testing and have not been independently performance tested.
One conceptual variant for SMJS would be if it were possible to temporarily locate the SMJS lath accessory on the wall without mechanically fastening it to the framing, then bring the edge of lath into it and wire-tie the lath edge to the flange of the SMJS lath accessory. This could potentially be a functionally viable stucco SMJS, but obviously it is unrealistic to construct this subassembly from a practicality perspective.
A similar variant for SMJS is to nail, screw or staple the SMJS lath accessory directly to the substrate support. The lath is then brought into the SMJS lath accessory, terminated (discontinuous lath), and nailed, screwed or stapled to the substrate support over the flange of the SMJS lath accessory. The functional concerns with this of course is that the SMJS lath accessory now has multiple fasteners through its flanges and cannot move to accommodate shrinkage and thermal movements. The discontinuous lath feature of this subassembly is beneficial over continuous lath so the amount of restraint is less than it would be with continuous lath, but in terms of minimizing stucco cracking, it is not an optimally functional SMJS. This SMJS configuration only allows marginal movement when compared to an ASTM C1063 compliant SMJS as the performance testing confirmed.
Yet another variant for SMJS configuration provides continuous lath, and then wire-ties the SMJS lath accessory to the face of the lath, which is functionally a waste of time. The SMJS lath accessory might as well be screwed, nailed or stapled when it is over continuous lath because configured this way it only functions as a thickness screed. Continuous lath at SMJS restrains movement of the SMJS and does nothing to minimize cracking.
Do regional or environmental variations substantiate differences in SMJS performance or installation, location or configuration requirements? All SMJS lath accessory products are available in all regions throughout the USA although specific materials from specific manufacturers may require special order or minimum quantity orders. Stucco, lath and SMJS lath accessories are generic materials with physical properties and performance characteristics that are not regionally variable or dependent. Stucco shrinkage rates for stucco materials are the same in all regions. The single variable may be the service temperature range of a given installation, but stucco thermal movement is relatively minor dimensionally as comparative to stucco shrinkage. Regional variations for SMJS usage, location, configuration, are not valid justifications for regional differences in SMJS performance or installation, location or configuration requirements.
The local building official has the authority, at their sole discretion, to consider and approve alternates as applicable to exterior stucco wall cladding system installations under the following conditions:
· Approvals of alternates if granted, only apply to specific individual cases, such as a given building (i.e. a building permit)
· Claims made regarding the proposed alternate must be supported by submitted evidence to substantiate that the proposed alternate is equivalent in performance to the building code requirements for the building officials consideration
· Testing to substantiate compliance may be required by the building official, performed by an approved testing agency and paid for by the owner
Alternates to SMJS materials, designs and methods of construction may be considered and evaluated by the local building official based on the criteria stated in the code, as previously summarized. The process of consideration and approval of alternates requires the submittal of evidence and possibly testing if required, to support the performance equivalency of the proposed alternate, and the issuance of approval by the building official. Commonly encountered alternates in the stucco industry include the use of proprietary stucco systems, and EIFS, which are typically approved on the basis of their code evaluation reports. Use of approved alternates based on code evaluation reports or other submitted evidence, may include conditions to the approval such as the requirement of manufacturer-approved applicators, limitations to the installation, special inspections or other requirements or limitations.
The exterior stucco wall cladding System, Assemblies, Subassemblies and Components required and described in the building code are established as generic minimum acceptable construction. Use of SMJS requires compliance with multiple related requirements such as SMJS lath accessory material, locations, panel areas and geometry, methods of installation etc., all of which are minimum code requirements for stucco wall cladding systems. The building code is not intended to exclude other materials, designs or methods of construction that are, in the sole determination of the building official, equivalent in performance to the minimum building code requirements. The use of SMJS in an exterior stucco wall cladding system provides three separate functions to that stucco wall cladding system, all related to the purpose of minimizing cracking:
· Provides locations for plaster shrinkage and stucco thermal movement to occur
· Provides a plaster gauge for achieving uniform plaster thickness
· Provides a work stoppage locations to prevent cold joints
Proposed alternates to one or more of the generic requirements for SMJS as required by the building code require submitted evidence and testing upon request of the building official, to substantiate equivalent performance, for consideration, before approval of the alternate can be granted to be considered equivalent to building code requirements.
SMJS must conform with a number of requirements related to lath and fasteners, locations, panel sizes and geometry etc. to minimize stucco cracking. Fiber additives, polymer admixtures and other methods such as intensive moist curing have been used successfully to minimize stucco cracking to some extent, but none have proven to be of equivalent performance or accepted as substitutes for SMJS. Sometimes owner and design authorities seek out alternative solutions to SMJS for a variety of reasons. A primary example of a potential alternate to one or more or even all of the several SMJS requirements is the use of proprietary continuous fabric-reinforced basecoat and polymer-based finish coats assemblies that purport to minimize stucco cracking. These “base and mesh” and finish coat assemblies also promoted as “crack isolation” assemblies have become popular in the marketplace for minimizing stucco cracking as expressed at the finish coat surface.
ASTM C926 mentions polymer-based finish coats as a potential alternative for cement-based finish coats where specified, but does not mention either continuous fabric embedded into the brown coat or the continuous fabric in a separate polymer-modified basecoat lamina over the brown coat – a technology that is ported over from the EIFS industry. The building code and ASTM C926 do not currently recognize the use of continuous fabric in either configuration as an alternate to one or more of the SMJS requirements. Given that these continuous fabric reinforced lamina and finish coat assemblies are proprietary solutions, not generic, it is incumbent upon the manufacturers of these assemblies to substantiate and document the performance of their assemblies, identify conditions and limitations for their use and submit them to the building official for evaluation, who will likely require testing or testing reports by an approved testing agency upon which to base their evaluation on, as is required by the building code. Comparison testing can be performed to compare the crack minimizing performance of these “base and mesh” and finish coat materials and assemblies, with the performance of conventional SMJS with generic exterior stucco wall cladding systems, and to rationally quantify their effectiveness. Actual comparative performance, functional equivalency as an alternative to the use of SMJS and any limitations such as required materials and admixtures, panel size and configuration criteria, and finish coat materials should rightfully be evaluated by an approved testing agency and documented in a code evaluation report, which would benefit the effective use of these approaches and the stucco industry overall as well as the respective manufacturers of these systems. Without a bona fide testing and evaluation process with documentation, it can be nothing more than a guess as to how effective these assemblies are at minimizing stucco cracking because they are not a universal panacea. While their effectiveness at minimizing cracks is promising, their limitations have also been observed and the boundaries to their effective use have not been established.
In the following example from a major Northern California installation completed in 2012, the stucco wall cladding assembly was designed and intended to use one-piece EATS reveals in lieu of conventional SMJS. The reveals were located to define stucco cladding panels of typical size and geometry conforming to conventional ASTM C1063 SMJS requirements. The beneficial part of this solution was that the base and mesh and polymer finish coat allowed few if any cracks in the field of the panels defined by the EATS reveals. Lath was installed discontinuously at the EATS reveals and fastened to the framing/blocking at either side of the reveal component to allow stucco movement. And the stucco did move. The stucco cladding panels experienced shrinkage and thermal movement as could be anticipated and the results are obvious and less than acceptable. The stucco cladding along with the base and mesh and polymer-based finish coat fractured at its weakest location along the edges of the EATS reveal mounting flanges even though the lath lapped over these flanges. The point is that even base and mesh and a polymer finish coat were not sufficient to mitigate the stucco shrinkage and thermal movement at EATS reveals, as a replacement for SMJS, to minimize these cracks. This study illustrates and documents the shrinkage and thermal movement realities of stucco wall cladding assemblies, and a limiting condition of base and mesh and polymer finish coat assemblies.
Base and mesh and polymer-based finish coats are not sufficient
to eliminate shrinkage and thermal movement cracks at EATS,
where use is contemplated to replace SMJS
In another Northern California stucco wall cladding assembly example, 2-piece BMJS (in lieu of SMJS) were installed with discontinuous lath to define stucco wall cladding panels of conventional size and configuration. Base and mesh and a polymer-based finish coat were installed at the panels between the BMJS. Once again the base and mesh and polymer finish coat allowed few if any cracks within the field of the panels between the BMJS, but narrow separation gaps did occur at the corner edges of certain panels, not all, at the farthest corners of the panels away from the center of the panels.
Separation gaps at certain panel corners, not all, where the base and mesh and finish coat assembly pulled away from the BMJS
From a purely pragmatic perspective, a SMJS lath accessory is not required to create a functional SMJS. Blasphemy you say? That is because a SMJS lath accessory does not define a SMJS, discontinuous lath does. A functional SMJS simply provides a location within the stucco wall cladding system, where portland cement-based plaster shrinkage and stucco thermal movements that may cause cracking, can occur. Shrinkage and thermal movement and a stucco crack will occur at the point of discontinuous lath where adjacent stucco panels are large enough for significant shrinkage and thermal movements to occur and result in a crack.
Materials: SMJS lath accessories are available in galvanized steel, solid zinc alloy, stainless steel sheet metals and extruded PVC. Not all SMJS lath accessories are available in all materials from every manufacturer. SMJS subassembly movement performance is affected by the resiliency of the material of the SMJS lath accessory. SMJS lath accessories and adjacent lathing materials and fasteners must be carefully selected for material compatibility. Consider that materials such as stainless steel and galvanized steel/solid zinc alloy are at opposite ends of the galvanic scale and may corrode if used together. Galvanic action is possible with galvanized steel or solid zinc alloy materials in a shared environment with stainless steel in the presence of water.
· Galvanized steel sheet metal: The most common SMJS lath accessory material used in most regions throughout the USA, with the lowest SMJS movement capability and lowest installed cost resulting from its rigidity. Typically G60 galvanization.
· Solid zinc alloy sheet metal: Several manufacturers recommend solid zinc alloy SMJS lath accessories for all exterior building locations as a precaution against corrosion. Solid zinc alloy has 2x the movement capability of galvanized steel as a SMJS lath accessory. Zinc SMJS lath accessories cost more than galvanized or PVC to both purchase and install as a result of its inherent flexibility.
· Extruded Polyvinyl Chloride (PVC): Lowest cost and offering excellent corrosion protection, PVC SMJS lath accessories are primarily used at coastal and corrosive environments where corrosion protection is essential. PVC SMJS lath accessories offer the most movement capability of all available SMJS lath accessory materials. More expensive to install than galvanized steel resulting from its flexibility.
· Stainless steel sheet metal: Available in both 304 and 316 stainless steel, these materials have excellent corrosion resistance. Stainless steel lath accessories have not been tested for SMJS movement capability. Stainless steel SMJS lath accessories are only available in the Double-V profile.
SMJS lath accessory ground dimensions: Where most stucco over lath is required to be a minimum 7/8 in. nominal thickness including a 1/8 in. nominal cementitous finish coat, this results in a ¾ in. minimum nominal thickness for base coats. With ¼ in. self-furred lath and the SMJS lath accessory installed over the lath, the minimum SMJS lath accessory ground dimension is ½ in. to satisfy minimum stucco thickness requirements on framed and sheathed substrate supports.
Only five different generic SMJS lath accessory profiles are available in the marketplace today with variations of materials, ground dimensions and manufacturer. It is reasonable to suggest that none of the SMJS lath accessory products available on the market are ideal solutions, although if utilized correctly all can be effective within their individual limitations and are the best solutions currently available. All are made of reasonably corrosion-resistant materials, but if their installed location is in a marine or corrosive environment, avoiding galvanized steel is beneficial. Profiled splice plates and end caps for sealing splices, intersections and terminations are not available for any SMJS lath accessory profile from any manufacturer, setting the condition in a sealant bed is the primary solution to seal these conditions watertight. Hollow-profile SMJS lath accessories can function as water channels and drainage must be considered in their use and installation. Galvanized sheet metal materials are the most commonly used SMJS lath accessory but least resilient, with limited movement capability, whereas SMJS lath accessories of solid zinc alloy and PVC materials accommodate greater movement. Stainless steel SMJS lath accessories have not been tested for movement capability. Paired casing beads require more labor than most to align, install and seal. Double-V profiles open during shrinkage and thermal contraction, and parallel separation gaps can occur to one or both edges of the SMJS lath accessory, especially with larger stucco panel areas. The gaps are not only unsightly some waterproofing consultants express concern that these gaps may allow bulk water intrusion behind the stucco wall assembly. The Double-V Horizontal Drainage profile drainage surface does not slope and its lower V-point does not have a locking flange, both of these features would be welcome improvements to this SMJS lath accessory. Double-J profiles expose more metal (or PVC) material at the stucco finish surface resulting from their exposed locking flanges which some designers find concerning, but parallel separation gaps rarely occur with Double-J profiles.
No stucco industry product standard exists that specifies the engineering technical aspects of the SMJS lath accessory and no two manufacturers make them identically, so an SMJS lath accessory is technically not a generic lath accessory conforming to a generic industry standard. SMJS lath accessory engineering technical information, including performance testing characteristics when assembled as SMJS, are not published or available from their manufacturers. The lack of complete engineering technical information about SMJS lath accessory products, adds to the challenges for designers and installers to accurately know how and where to use the SMJS lath accessory or how and where to integrate the lath accessory into exterior stucco wall cladding subassemblies for greatest effectiveness. SMJS lath accessory manufacturers are encouraged to either develop a common industry product and installation standard and conform to it, or at least provide complete engineering documentation indicating all physical dimensions, materials, splicing methods, termination and intersection methods, movement capabilities and limitations of the SMJS lath accessory as installed in a SMJS subassembly, and the correlating installation requirements for design and installation reference.
Paired casing beads: The earliest SMJS was site-fabricated from two back-to-back casing bead lath accessories, separated by a gap that is sealed with a resilient sealant. This SMJS also requires a flexible barrier membrane (SAF), a continuous WRB and discontinuous lath installed behind the paired casing beads. This SMJS has been in stucco industry standards since the 1971 ANSI A42.3 and is still described as a SMJS lath accessory in ASTM C1063 today. This subassembly is not common because it is has been replaced in the market by one-piece SMJS lath accessories, which are more time efficient to install. Depending on the separation gap dimension and sealant selected, this subassembly should provide reasonable SMJS function, but admittedly I have not seen one. Manufacturers may want to consider prefabricating these and bringing them to market.
Double-V: The original one-piece SMJS lath accessory design, patented in 1962 by Raymond Clark. It has a Double-V configuration in section. When made of sheet metal, the mounting flanges are expanded sheet metal. When made of PVC the extruded mounting flanges are perforated. Some designers prefer this profile because the narrow shadow line is visually minimalistic. While this product meets minimum SMJS lath accessory requirements, other SMJS lath accessory products perform better.
Double-V Internal Corner: At internal corners, other than using opposed casing bead lath accessories, the Double-V Internal Corner lath accessory is the only one-piece lath accessory specifically made for a SMJS lath accessory at internal corners on the market today. The Double-V Internal Corner SMJS lath accessory and subassembly, is useful and appropriate at stucco internal corners of similar substrate support conditions, such as two abutting framed or framed/sheathed walls, where a SMJS is required due to larger adjacent stucco panel area sizes, where a SMJS would normally be required. For smaller (4 feet dimension or less) stucco wall panel sizes abutting internal corners, the Double-V Internal Corner SMJS are not necessary and the lath should be installed continuously through the internal corner or discontinuously and lapped with cornerite.
Double-V, Horizontal Drainage: This one-piece SMJS lath accessory appeared on the market in approximately 2008. It is the Cemco Solid Leg #15 and at this writing is a proprietary lath accessory produced only by a Cemco. It is intended only for horizontally-oriented SMJS on a wall. It is a dual function lath accessory serving as both a horizontal drainage screed flashing and an SMJS lath accessory. Its solid sheet metal flange is oriented upwards on the wall and must be integrated with the WRB to facilitate drainage and its downward oriented expanded sheet metal flange is wire-tied to the lath beneath it to accommodate shrinkage movement of the stucco panel below. This lath accessory is not a perfect solution but it does address conditions that no other SMJS lath accessory does. This is the only solution for horizontal SMJS that require drainage on the market today.
Double-J: This one-piece SMJS lath accessory was introduced to the market in about 1978. It has a Double-J configuration in section and features locking edge flanges that engage the edges of adjacent stucco panels. As this profile opens during shrinkage and thermal contraction, the locking edge flanges move with the stucco panels and parallel separation gaps at one or both edges of the SMJS lath accessory are less likely to occur than with the Double-V SMJS lath accessory. When made of sheet metal, the mounting flanges are expanded sheet metal. When made of PVC the extruded flanges are perforated. More metal is visible with this lath accessory than with the Double-V, but the potential for parallel edge cracking is much reduced if not completely eliminated. This is the optimum functional solution for vertical SMJS on the market today.
The classic historic aesthetic for stucco-clad buildings is a continuous plaster parge coat over mass masonry or solid concrete, without joints to achieve a monolithic expression. Think Mission style or Miami Beach Art Deco on solid masonry or concrete substrate support. Omission of SMJS is acceptable for direct-applied stucco onto solid masonry or concrete substrate support. While this aesthetic is desirable for all stucco installations, it is functionally incompatible with a stucco and lath composite over a WRB on a framed/sheathed substrate support. Random cracking is the unfortunate result of this stucco cladding assembly without movement joints as the portland cement-based plaster experiences initial shrinkage and thermal movements. Complete omission of SMJS to achieve a monolithic aesthetic also deprives the stucco craftsman of the thickness screed (gauge) and work stoppage functions that SMJS provide. Without SMJS the stucco craftsman loses an important quality control tool that is useful in applying plaster to a uniform nominal thickness and a place to stop work for the day without creating a cold joint, both conditions of which may cause cracking.
A stucco wall cladding system without SMJS may benefit from or require high performance finish systems and other mitigating solutions such as is often used remedially to address cracking, but this system is unconventional and changes the drainage wall to a barrier wall.
Compliance with Minimum Stucco Industry Standards ASTM C926 and C1063 are required of all stucco wall cladding systems that are subject to ASTM C926 and C1063 as referenced in the ICC building code. As it specifically pertains to SMJS, no exception exists for certain specific building occupancies or for regional preferences unless local revisions are adopted by the local AHJ which is unlikely. Omission of SMJS from a stucco wall cladding system is not justified on the basis of building occupancy such as if the building is a residence, a warehouse or a roof penthouse. Omission of SMJS from a stucco wall cladding system is not justified on the basis of region such as if the installation is in Albuquerque or Boise. The point to remember is that the material and functional properties of stucco do not change based on regional location or building occupancy where it is applied, and the requirements are the same, making exceptions to SMJS requirements based on regional location or building occupancy or any other similar reason not relevant.
‘Dead ends’ describe SMJS assemblies that just end, terminate, or are discontinuous within a stucco panel area and are not contiguous to completely define a panel area. ‘Dead ends’ terminate within the panel area, at internal corners or external corners and can be the result of simple oversight by the installer or design professional; we all understand that people are subject to human error. Dead end SMJS may cause cracking at the end termination, may allow water intrusion into the wall cladding if the termination is not sealed and potentially other maladies such as localized spalling or staining.
Omitted SMJS resulting in a ‘dead end’
A formed groove or cut in the stucco is a Decorative Joint Subassembly (DJS), not a SMJS. A Decorative Joint Subassembly can be mistaken for a SMJS and is mentioned here to provide clarification. A DJS may include a decorative lath accessory or tooled finish coat plaster for decorative effect, which in some resource references is called an ‘architectural joint’. A DJS is decorative only because it has no movement capability as evidenced by continuous lath and substrate support through the joint. DJS are not recognized as SMJS in Minimum Stucco Industry Standards, are not effective as SMJS and do not accommodate movement because of their continuous lath. Because a DJS is decorative only, it must be located within the boundaries of a stucco panel area defined by SMJS and other stucco movement joints. Various lath accessories can be used to assemble DJS such as the SMJS lath accessory, reveal screeds etc., with continuous lath.
Formed or cut grooves in the stucco are not allowed in the stucco scratch and brown coats when they reduce the required minimum nominal stucco thickness under any condition. Formed or cut grooves are acceptable in the finish coat and may be useful for decorative purposes only. Formed grooving in the finish coat is distinguishing characteristic of a sgraffitto finish coat.
SMJS lath accessories used decoratively,
not strategically located to minimize cracking
What is restrained construction and how does it relate to SMJS? In the 1940’s at the Grand Coulee Dam project, the term restrained construction was coined in the context of it being the cause of significant cracking in suspended stucco ceilings. Lath had been wire-tied to suspended ceiling grillage in several rooms, and was attached at the perimeters of the rooms to fixed concrete perimeter walls as was common practice at the time. Attachment of the lath to the perimeter walls as described created a restrained construction configuration, or more accurately a restrained lath configuration, restraining lath movement by attaching it to perimeter walls, that should be allowed to move to accommodate portland cement-based plaster shrinkage and thermal movements as the plaster cures and hardens. The damaging effect of restrained lath was confirmed by significant testing, and was determined to directly cause significant stucco cracking in the Grand Coulee Dam ceilings. The solution was derived to detach the lath, creating a discontinuous lath condition at the ceiling perimeters, and was proven to eliminate the cracking by subsequent reconstruction of the same suspended stucco ceilings using unrestrained lath, or better, discontinuous lath.
Another, completely different usage of the terms restrained construction and restrained lath have circulated in the industry in recent years in an attempt to posture them as a beneficial method for lath and lath accessory installation, but it is a flawed theory, a misinterpretation of the terminology and does nothing to minimize cracking. In this theory as in the previous discussion, the terms restrained construction and restrained lath, are also described to mean continuous lath that does not include provisions for accommodating shrinkage or thermal movements. In this second theory, restrained construction and restrained lath are promoted in an effort to legitimize the omission of functioning SMJS that require discontinuous lath to accommodate shrinkage and thermal movements. Let’s consider that approach and explore that for a moment. The theory is rationalized along these lines: Stucco substrate support assemblies such as framed and sheathed wall assemblies with bracing and gypsum board, create a relatively rigid and robust stucco substrate support system creating a restrained construction condition presumed to perform similarly to mass masonry or solid concrete walls as a stucco substrate support. [Note that the term restrained construction as used in this theory has mutated to now refer to the substrate support assembly (the wall), whereas in the original Grand Coulee Dam example it referred to the lath. This transition in meaning is a major flaw with this theory.] The theory promoted is that continuous lath intermittently attached (with nails, screws or staples) to a framed/sheathed restrained construction substrate support assembly, provides a rigid, robust plaster base to receive portland cement-based plaster, with similar substrate support characteristics for stucco as mass masonry or solid concrete, and therefore does not require provisions for shrinkage and thermal movements such as SMJS that require discontinuous lath. SMJS lath accessories if included at all in this assembly function as thickness screeds or for decorative purposes only and continuous lath is an essential characteristic. The code requirement for discontinuous lath at SMJS is ignored. This usage of the term restrained construction suggests that if stucco continuously-adhered to a rigid, robust solid masonry or concrete substrate support functions acceptably, then stucco as a lath and plaster composite intermittently attached to a similar rigid, robust framed/sheathed substrate support system with restrained continuous lath (without provisions for shrinkage and thermal movement) should also perform in a similar manner.
The fatal flaw with this theory is that it defies basic science. Undeniably, a portland cement-based plaster and lath composite shrinks as it cures and hardens, and as stucco it is subject to thermal movements while in service. Stucco movement occurs, plain and simple. The plaster and stucco movements are inherent with the material and the movement is restrained by its mechanical lath fastener attachments to the substrate support system which can be considered for this discussion as ‘intermittent’, and the plaster is applied over a WRB membrane that functions to isolate it so the plaster is not in continuous bond with its substrate support. In contrast, stucco directly-applied to a mass masonry or solid concrete support substrate is continuously-bonded to its substrate which minimizes cracking by continuous uniform adhesion to the substrate support, whereas on a framed/sheathed wall with continuous lath configuration, stucco is only adhered to the lath which is not continuously adhered to the substrate support because it is placed over a WRB which isolates it from the substrate support. In this configuration of stucco over continuous lath isolated from its substrate support by the WRB, the continuous, restrained lath has no capacity to accommodate inherent portland cement-based plaster shrinkage movement or stucco thermal movements, and cracking is the result. This condition of portland cement-based plaster on continuous lath intermittently attached to framed substrate supports can be loosely compared to the situation where cement-based plaster is applied to a solid plaster base like mass masonry or solid concrete – where the stucco is continuously adhered it performs well, where it is not continuously adhered, it cracking. The countless stucco applications over continuous lath without functioning SMJS, that rely on continuous lath or where the SMJS lath accessories are screwed, nailed or stapled over continuous lath, and the resulting cracking, are witness to the ineffectiveness of this approach. Portland cement-based plaster and stucco require accommodations for the inherent shrinkage and thermal movements they experience or cracking occurs. Whereas stucco on framed/sheathed substrate supports, applied over lath with strategically and correctly located, functional SMJS which include discontinuous lath to accommodate shrinkage and thermal movements, minimizes cracking.
From the earliest applications of portland cement-based plaster over lath on framed or framed/sheathed buildings, the lath was installed continuously over framed substrate supports, and stucco movement jointing of any kind did not exist. Significant efforts were spent testing and evaluating stucco with continuous lath to address the cracking conditions that occurred. The practice of segmenting lath and creating smaller stucco panel areas defined at their perimeters by stucco movement joints that accommodated shrinkage and thermal movements to minimize cracking began, as far as the historical record reflects, with the Grand Coulee Dam ceilings in the late 1940’s.
Some in the stucco industry have the perspective that the SMJS does not move, is not supposed to allow movement and is merely a break in the otherwise continuous plane of stucco to diffuse stresses that can accumulate. A challenge to this perspective is if there is no movement then what stresses exist that need to be diffused? A material that is inert develops no internal stresses. While this is the condition at a Decorative Joint Subassembly (DJS) that accommodates no movement, continuous lath does not define a SMJS. Shrinkage and thermal movements in an exterior stucco wall cladding system accumulating at a SMJS is a numerically small dimension, occurs slowly and is not easily observed, but that does not mean that it does not occur. No one can see the wind but most people believe wind occurs because they see the effects of the wind – tree branches that sway, ripples on lake water – indications of movement. Cracking in stucco applied over continuous lath without stucco movement joints is an indication of stucco movement. Stucco shrinkage and thermal movements are real, not imagined. When SMJS movement is restrained such as when the lath is continuous or when the SMJS lath accessory flanges are nailed, screwed or stapled to the substrate support, the cracking that occur are a visible manifestation of that inability to accommodate shrinkage and thermal movement.
Continuous lath at SMJS restrains and effectively negates the movement capability of the SMJS. Continuous lath at SMJS does not create a functioning SMJS that accommodates movement; its functionality is reduced to no more than that of a Decorative Joint Subassembly (DJS), a plaster thickness screed or termination screed to prevent cold joints to facilitate plastering application. If SMJS have no movement capability, they have no ability to minimize the effects of stucco shrinkage movement or thermal movement, and avoidable stucco cracking can occur. With that understanding, it should be obvious then why continuous lath at stucco control SMJS violates the Minimum Stucco Industry Standards of the building code and ASTM C1063 and can result in excessive stucco cracking.
Stucco wall panels with continuous lath at “control joints”
Metal studs, continuous woven wire lath at Double-V “control joints, acrylic finish
Cracking outlined in dashed red lines
Various attempts to justify continuous lath at SMJS include the following:
One attempt to justify continuous lath at SMJS ignores or overlooks their role in minimizing stucco cracking and in how SMJS are required to function. ‘When the construction document details do not indicate the framing or blocking required at SMJS to install lath edge fasteners with discontinuous lath, then the lath should be installed continuously at SMJS to avoid lath edge fastener placement not located at framing members and to prevent stucco panel edge curling’, or similar excuses related to financial implications, project scheduling delays etc. that are used to avoid providing the necessary framing or blocking members. On the surface one or more of these arguments may seem like a reasonable justification for continuous lath at SMJS, but it is a flawed conclusion because the lath needs to be discontinuous for a SMJS to perform its function and accommodate shrinkage and thermal movement, and for the no less important reason that continuous lath at SMJS is a building code violation, as required by Minimum Stucco Industry Standard ASTM C1063. The issue of back-up framing or blocking not indicated, financial implications or scheduling delays to provide the necessary framing or blocking is a separate matter from the C1063 requirement for discontinuous lath at SMJS, just because it may not be indicated does not mean that it is not required. Necessary framing or blocking should be indicated on construction documents as a Minimum Standard of Care to comply with ASTM C926 A188.8.131.52, to satisfy the requirement of the design authority to indicate the ‘method of installation’ for SMJS. The Minimum Standard of Care is to comply with the building code which requires discontinuous lath at SMJS. A situation like this potential conflict between a building code requirement and a potential oversight on a construction document should be brought to the attention of the design authority for clarification and resolution during bidding, as the traditional party to interpret construction document requirements. A licensed design or construction professional whose legal obligation is to comply with the minimum requirements of the building code, will want to comply with the building code. Additional framing or blocking is not required for horizontally-oriented SMJS on vertical walls, the lath side edges are simply fastened to the vertical studs and the SMJS lath accessory is wire-tied to the face of the lath. The majority of vertical SMJS locations should be located at already occurring framing member locations – at wall opening corners where grouped studs already occur (king-jack-cripple studs), so additional vertical framing or blocking members for wood stud framing for many building designs should be minimal. For steel stud framing ASTM C1007 requires allowances for this framing or blocking for SMJS as described in the standard so ‘additional’ framing or blocking is not the case for steel stud framing substrate supports. Some of the already occurring framing locations may only need an additional stud or block to create a wider bearing surface for the adjacent lath edge fasteners. Discontinuous lath at SMJS is the most critical characteristic of the subassembly that allows it to accommodate shrinkage and thermal movement. Whatever the situation, discontinuous lath at stucco SMJS is a minimum code requirement, for functionality reasons, to minimize stucco cracking.
Another attempt to justify continuous lath at SMJS over a framed wall substrate support, where continuous sheathing and interior wallboard are installed, the “restrained construction” or better “restrained lath” theory, is misinformed. While on its surface this restrained construction theory may seem credible, it ignores basic physics, ignores the properties of wet portland cement-based plaster mortar to shrink and be subjected to thermal movements after it hardens into stucco, violates the building code, and does nothing to minimize cracking; it actually increases cracking due to restraining the lath from accommodating these movements. SMJS address initial shrinkage and thermal expansion/contraction movements only, not movement in the substrate support wall subassembly (substrate support movement is accommodated by BMJS or PMJS). Once the wet plaster mortar is applied onto the lath which is fastened to the wall framing and hardens, the stucco and lath become a single new composite material, with combined properties reflecting its individual components. The wet plaster mortar initially shrinks and that shrinkage movement is resisted by the lath and lath fasteners which are secured to the wall framing. In a perfect world, the plaster mortar would freely shrink and the lath would not resist plaster mortar shrinkage movement, but the material properties of the lath and lath fasteners are such that the lath and lath fasteners may restrict shrinkage movement. This is not to say that all stucco cladding movement related to initial shrinkage and thermal expansion and contraction is completely prevented by the lath or lath fasteners; movement capability is only reduced by these conditions. Movements from initial stucco shrinkage and thermal expansion and contraction do occur and are visible and measureable in installed stucco systems. Obviously, functional SMJS that accommodate movements are more beneficial towards minimizing cracking than are SMJS that do not accommodate movements. It should be intuitive to anyone with basic mechanical knowledge, that if a theoretically rigid, non-shrinking and non-expanding structure is constructed (a perfectly inert building) and a plastic material (stucco) that shrinks as it cures is wrapped around it, that the dimensional differences can only express themselves as a shortening of the outer wrapping material, which with stucco is manifestation as cracking. Continuous lath does nothing to minimize stucco cracking under this restrained lath theory it only renders SMJS nonfunctional in performing their intended service. SMJS are a primary method of minimizing stucco cracking by providing a location for shrinkage and thermal movements to occur. The utilization of continuous lath at SMJS effectively allows no relief from these movement forces to the stucco and excessive cracking results.
The singular industry voice advocating continuous lath at SMJS comes from many of the contractor-based plaster bureaus through their published technical bulletins and job specific opinion letters. Continuous lath advocates sometimes qualify their position based on certain conditions, other times the position is promoted unconditionally as preferable and beneficial to stucco. Either way, these factual observations for the purpose of rational evaluation and intellectual discussion should not be misconstrued as an attack on stucco bureaus or their representatives as our industry colleagues; they are respectfully entitled to their opinion. The remaining plaster bureaus either support discontinuous lath at SMJS or do not publish their position on this issue. One or more of the following rationale are the common basis for that position, and in response, further suggested considerations are noted:
· Conditional statement advocating continuous lath at SMJS: ‘Where construction documents do not indicate framing or blocking at SMJS for lath edge fasteners as required for discontinuous lath, continuous lath at SMJS is acceptable practice to mitigate stucco panel edge curling.’
è Consider: In a perfect world additional framing/blocking for vertically-oriented (parallel to stud) SMJS are indicated in the construction documents. But no design or construction professional or construction document is perfect. Whether indicated or not, the ASTM C1063 requirement for discontinuous lath at SMJS is a building code requirement, a Minimum Stucco Industry Standard. Two wrongs do not make a right. Additional framing or blocking can and should be provided at vertical SMJS, by change order if necessary, for minimum code compliance and SMJS performance reasons.
è Consider: Additional framing or blocking is not required for horizontally-oriented (perpendicular to stud) SMJS, just like it is not required between studs for horizontal lath side laps. Simply fasten the non-overlapped lath sides to the studs and wire-tie the SMJS lath accessory to the face of the lath, similar to the way that lath horizontal side laps are wire-tied together between studs.
è Consider: For steel stud framed walls, any required framing or blocking should be provided for SMJS as required in ASTM C1007, regardless if it is indicated on construction documents or not.
· Conditional statement advocating continuous lath at SMJS under former building code: ‘The code required that lath be fastened to framing within two inches of the lath edge, presumably to minimize panel edge curling. Where a vertical “control joint” occurs between studs, located greater than 2 inches from a framing member, and an additional framing or blocking member is not provided for the lath edges fasteners for discontinuous lath at “control joints” then continuous lath at the “control joint” is an acceptable method of installation.’
è Consider: Continuous lath at SMJS does not accommodate shrinkage and thermal movement, promotes cracking, is a building code violation and does not comply with Minimum Stucco Industry Standards.
· Unconditional anecdotal statements advocating continuous lath at SMJS: “Continuous lath at a “control joints” works well and is an acceptable “control joint” installation method and has been done for decades”.
è Consider: No stucco assembly installation where “control joint” subassemblies were installed with continuous lath has demonstrated reduced stucco cracking comparative to discontinuous lath installations, has been documented or is known to exist. But the opposite is true (Grand Coulee Dam)
è Consider: The reality is that SMJS performance testing proved this myth to be unsubstantiated and false. This myth is in direct violation of the building code and Minimum Stucco Industry Standards which require discontinuous lath at SMJS. With the adoption of the 2006 and later International Building Code (most jurisdictions in the USA), or when ASTM C1063 is a specified contract requirement, this myth is a non-issue and should be disregarded.
è Before the 2006 IBC, the continuous lath at SMJS issue was debated in the industry; the debate is meaningless and obsolete now on every level. Discontinuous lath at SMJS has been the recommendation in the industry since the 1971 ANSI A42.3 which became ASTM C1063 in 1986 and often specified as a contract requirement, then it became the required Minimum Stucco Industry Standard as early as 2006 when ASTM C1063 became a reference standard in the 2006 IBC when that code was adopted in local jurisdictions. Today, any reference resource promoting continuous lath at SMJS is irrelevant, unsubstantiated misinformation, violates the building code and Minimum Stucco Industry Standards, ignores basic behavioral characteristics of portland cement-based plaster shrinkage and stucco thermal movements and the performance requirements and expectations of SMJS, promotes portland cement-based plaster and stucco cracking and should be disregarded without exception.
· Unconditional statement advocating continuous lath at SMJS: SMJS lath accessories installed over continuous lath are easier to install, than constructing a SMJS with discontinuous lath. No additional framing or blocking is required if the SMJS lath accessory is wire-tied to the face of continuous lath.
è Consider: Easier perhaps, but SMJS with continuous lath are not functional, merely Decorative Joint Subassemblies (DJS), defy their very purpose to relieve stress, only restrain movement and do nothing to minimize stucco shrinkage and thermal movements that cause cracking.
· Unconditional statement advocating continuous lath at SMJS: SMJS lath accessories installed over continuous lath require fewer numbers of lath fasteners - nails, screws or staples that penetrate the WRB or SAF - than with discontinuous lath which increases the risk for water intrusion into the wall assembly.
è Consider: Counterintuitively, at SMJS (horizontally and vertically-oriented SMJS are similar) where the SMJS lath accessory is not wire-tied over the face of continuous lath but is mechanically fastened with nails, screws or staples, more fasteners are required with continuous lath, than with discontinuous lath.
Basis of comparison: Only 2 rows of fasteners are needed with discontinuous lath (1 row at each lath edge), where the SMJS lath accessory is wire-tied to the face of lath.
- 3 rows of fasteners are needed (1 for the lath and 2 for the SMJS lath accessory flanges) where the SMJS is installed over continuous lath.
- 4 rows of fasteners are needed (2 for the lath edges and 2 for the SMJS lath accessory flanges) where the SMJS is installed with discontinuous lath, where the SMJS lath accessory is installed either under or over lath, with nails, screws or staples.
è Consider: Most design authorities require SAF at the SMJS lath accessory, in part to gasket fasteners, and to minimize concerns over the effects of water intrusion.
· No SMJS performance testing with results to substantiate the performance of continuous lath has been performed by the proponents of continuous lath, especially large scale wall assemblies.
è Consider: Reference the SMJS performance testing completed by Bowlsby in 2008, which evaluated both continuous and discontinuous lath configurations of otherwise identical SMJS, and concluded that continuous lath at SMJS, regardless of lath type, does not accommodate movement. Excessive, preventable, stucco cracking occurs in stucco wall cladding systems with a continuous lath configuration at SMJS.
è Consider: No documented performance history of existing stucco applications supports continuous lath as reducing cracking.
è Consider: No SMJS lath accessory manufacturer’s information exists supporting continuous lath as reducing cracking.
· Advisory publications are circulated that promote continuous lath at “control joint” subassemblies.
è Consider: Just because something is published does not mean it has any validity or authority, especially where no justification is provided. Advisory publications are opinion pieces only, do not preempt the authority of Minimum Stucco Industry Standards including minimum building code requirements, its referenced standards, or code evaluation reports and may not speak for the industry as a whole.
· Misinformation circulated by these publications is a disservice to licensed design and construction professionals who are required by license laws to comply with Minimum Stucco Industry Standards, a disservice to building owners who are entitled to minimum code compliant stucco wall cladding Systems, and ultimately a disservice to the stucco industry by promoting a practice that promotes rather than minimizes stucco cracking.
Continuous lath advocates marginalize the opportunity that correct usage of SMJS provides towards making stucco better, and sacrifices SMJS functional requirements and performance for the sole benefit of installation convenience.
Finally, ASTM C1063 requires the SMJS lath to be “tied” at each “side” of the SMJS. An obvious dilemma is that if lath is continuous at the SMJS, it has no ‘side’ to tie to. Continuous lath at SMJS lath accessories has no reasonable and certainly no functional justification.
ASTM C1063 @ 184.108.40.206 states “Lath shall not be continuous through control joints but shall be stopped and tied at each side.” To understand this text we must put it into context and understand that this is antiquated language harkening back to the era when lath was wire-tied to open framed substrate supports, when stucco wall cladding systems did not include sheathing much less a WRB as they do today. This identical language has been a requirement in ASTM C1063 since the initial draft in 1981. The language simply was not updated when sheathed framing became a common stucco substrate support system. This text states two requirements for lath installation at SMJS:
· “Lath shall not be continuous through control joints but shall be stopped…”. Indisputably, the lath must be discontinuous at SMJS, regardless of whether the “control joint” term refers to a SMJS subassembly or SMJS lath accessory. This requirement for discontinuous lath at SMJS is stated clearly. In the standard before the 1981 C1063 draft, ANSI A42.3-1971, the text said “…the lath shall terminate at a…control joint…designed to keep the lath and plaster free from adjoining materials.”
· “Lath shall…be tied at each side”. In the days when wire-tying lath to substrate supports was common, this text was clearly understood because it was the common practice of the day. But in today’s construction where sheathing often exists, wire-tying lath to the substrate supports is not only not common, but not possible. With sheathed substrate supports, the adjacent lath edges of the discontinuous lath at SMJS must be nailed, screwed or stapled to the substrate support framing members.
This text is an example in ASTM C1063 where the term “control joint” can have two interpretations, where sometimes the term “control joint” refers to a lath accessory and other times it refers to a subassembly, which may cause confusion. As used here, the term “control joint” is referring to a subassembly. It is describing a configuration of lath (discontinuous), and lath edge fastener (wire-tie) that is the essence of what a SMJS requires to be functional. Further, it would not be possible to ‘discontinue lath through a “control joint” lath accessory.’
This text could be modified to accommodate sheathed substrate supports, open framing and suspended grillage and all types of appropriate fasteners as follows:
‘Discontinue lath at SMJS. Terminate lath at each side of the SMJS and fasten or wire-tie lath edges to substrate support framing.’
Some in the industry struggle to understand what that phrase “Lath shall not be continuous through control joints but shall be stopped…” means, but it is stated in simple, unambiguous language. The subject of the requirement is the lath, not the SMJS lath accessory. “Not continuous” means ‘discontinuous’, in any language or context and no conditions or exceptions are indicated. The lath “shall be stopped” at SMJS …means terminated, ended, finis. Terminating the lath at SMJS creates an exterior stucco wall cladding system describing a panelized configuration of lath, bordered by perimeter SMJS, which as we know are intended for the purpose of accommodating portland cement-based plaster shrinkage and thermal movements.
This text is almost as famous for what it does not say as for what it does say, because it is text only about a lath condition and lath edge fastening at SMJS, not any other stucco wall cladding assembly component. We must be careful to not deduce other false ‘requirements’ for other stucco wall cladding system components in the SMJS from this text:
· This text does not address, literally or impliedly, the configuration of the WRB – continuous or discontinuous? The WRB should be continuous behind the stucco wall cladding system to perform its water management function.
· This text does not address, literally or impliedly, the condition of sheathing or substrate support – continuous or discontinuous? The fundamental purpose of the SMJS is to accommodate shrinkage and thermal movements of the portland cement-based plaster and stucco while in service. Continuity of sheathing and substrate support has nothing to do with these movements. Further, discontinuity of the sheathing or substrate support indicates substrate movement, for which BMJS or PMJS are required.
· This text does not address any other lath edge fasteners other than wire-ties given its historical context. The current text is antiquated and needs to be updated to include appropriate fasteners for all lath to substrate support substrate support fastening conditions - sheathed and non-sheathed conditions, as well as all substrate support materials – wood, steel, framing and suspended grillage.
The critical functional requirement for a SMJS which defines it is discontinuous lath, to provide a location for the release of shrinkage and thermal movements within the stucco cladding system, which will manifest as a crack to occur proximate to the edge of discontinuous lath unless a SMJS lath accessory is provided. At discontinuous lath, with or without a SMJS lath accessory the adjacent lath edges require fastening to the substrate support to transfer stucco gravity and lateral loads to the framing, and importantly, to minimize stucco panel edge curling. Sheathing and the WRB should be continuous through the SMJS. As a reference from the concrete slab on grade industry, the NRMCA recommends that wire mesh in the slab be discontinuous through what that industry calls ‘contraction joints’, joints that are intended to minimize cracks caused by dimensional changes.
These behaviors with discontinuous lath (unrestrained) at SMJS were observed and proven by SMJS performance testing of multiple prepared test specimens.
Stucco panel edge curling condition resulting from
discontinuous lath edges that are not fastened to framing.
(Photo courtesy of Mark Fowler, WWCCA)
Does discontinuous lath at SMJS negate the structural shear capacity of stucco? Stucco is primarily intended as a protective wall cladding; it protects the water resistive barrier from weather and physical damage and is otherwise decorative. While stucco may have incidental structural property capabilities, they are relatively minor in capacity, not completely predictable in value or very useful as a structural diaphragm and its structural properties are best considered as incidental.
ASTM C1063 as a Minimum Stucco Industry Standard requires discontinuous lath at SMJS, and conforming to ASTM C1063 requirements is a Minimum Stucco Standard of Care.
Discontinuous lath at SMJS is the most critical functional requirement of an SMJS that allows it to function correctly to accommodate stucco shrinkage and thermal movements. SMJS movement occurs at the SMJS, where the lath edge strands (even though the lath edges are fastened to framing/blocking) allow movement by distortion around lath edge fasteners, as the stucco shrinkage movement and thermal movement pull the lath away from the SMJS to open the SMJS lath accessory. Where lath is continuous through SMJS, the continuous lath restrains movement of the stucco cladding system (the lath and plaster composite) and allows no movement, and hence no stress relief at the SMJS. Where the SMJS lath accessory flanges are fastened to framing or blocking with nails, screws or staples, the accessory can accommodate no movement, and excessive cracking can occur – the SMJS lath accessory must not be fastened to the substrate, only wire-tied to the discontinuous lath edges. These behaviors (restrained and unrestrained lath) have been demonstrated and proven by SMJS performance testing. As a result, excessive stucco cracking occurs in stucco cladding with SMJS with continuous lath and SMJS lath accessories fastened to framing or blocking.
The collective list of proponents of discontinuous lath at SMJS with published documentation is extensive and represents a wide range of stucco industry interests. Note the breadth and diverse authority of the proponents – the building code and Minimum Stucco Industry Standards organizations, authorities having jurisdiction, portland cement product manufacturers and their industry organization, stucco, lath and SMJS lath accessory manufacturers and their organizations, and stucco industry professionals:
Building code and Minimum Stucco Industry Standards organizations
· ICC International Code Council (International Building Code organization)
· ANSI American National Standards Institute (former standards organization that held stucco Industry Standards before ASTM)
· ASTM American Society for Testing and Materials, Committee C11 (current holder of stucco industry standards)
Portland Cement Manufacturers and their organizations
· PCA Portland Cement Association
· ACI American Concrete Institute
Authorities Having Jurisdiction (governments)
· Bureau of Reclamation (Grand Coulee Dam project)
· Johnson County Kansas Building Officials
· Lenexa Kansas, Department of Community Development
· All AHJ that adopt the IBC which includes ASTM C1063 by reference
Stucco, Lath and SMJS Lath Accessory manufacturers and their organization
· Penn Metal Products/ Keene Corporation / Metalex (original manufacturers of SMJS lath accessory products)
· EMLA Expanded Metal Lath Association (previously the ML/SFA)
· StructaWire Corporation (StructaLath wire lath manufacturer)
· Tree Island Steel Limited (K-Lath wire lath manufacturer)
· Davis Wire Corporation (wire lath products manufacturer)
· AMICO (metal lath and lath accessory product manufacturer)
· Masterwall Inc. (stucco product manufacturers)
Stucco Industry Professionals
· Raymond Clark (inventor of the SMJS lath accessory and SMJS subassembly, by reasonable inference of his assumption of movement at “control joints”)
· Jacob Ribar, author How to Avoid Deficiencies in Portland-Cement Plaster Construction, Construction Technology Laboratories(7)
· James Rose, author of Stucco and Plaster Guide (Flintkote)
· Kansas City Homebuilders Association
· Florida Lath and Plaster Bureau
· Walter Pruter, Technical Director, Information Bureau for Drywall Lath and Plaster, California Wall and Ceiling Contractors Association (currently TSIB and WWCCA), author Crack Control in Portland Cement Plaster, and Portland Cement Plaster Crack Analysis and Repair
· Gary Maylon, author of The Metal Lath Handbook, The 8 Deadly Sins: Expanded Metal Lath Installation for the Application of Portland Cement Stucco, long time chairman of ASTM C1063 and former technical director at AMICO)
· Jeff Bowlsby, Architect, Simpson Gumpertz & Heger, author of Scratching the Surface with Stucco Control Joints, and Cement Plaster Metrics: Quantifying Stucco Shrinkage and Other Movements; Crack Acceptability Criteria for Evaluating Stucco
· Lee Cope, PE, Wiss Janney Elstner Associates Inc., author of Common Sources of Distress in Stucco Façade
The following is a timeline synopsis of the references in support of discontinuous lath at SMJS:
· 1947: Crack Control in Portland Cement Plaster Panels, Bert Hall, Bureau of Reclamation, Journal of the American Concrete Institute, Vol 19. No. 2, October 1947. Grand Coulee Dam ceilings where continuous lath at a change of substrate support caused shrinkage cracking, and terminating the lath at a change of substrate support eliminated shrinkage cracking. This event preceded and appears to have been the basis for the text in the 1971 ANSI 42.3 requiring discontinuous lath at perimeter movement joints, which were named “control joints” in ANSI 42.3.
· 1962: US Patent #3015194, Building Construction and Expansion Joint Therefor, filed June 6, 1955. Raymond Clark invented the SMJS lath accessory and SMJS subassembly. While the patent graphically depicts continuous lath behind the SMJS lath accessory it also clearly describes in multiple locations throughout the patent, the expectation of the lath accessory to accommodate shrinkage and thermal movement, and for the lath to be “expansible” to accommodate movement of the SMJS lath accessory.
· 1971: ANSI 42.3 Lathing and Furring for Portland Cement and Portland Cement-Lime Plastering, Exterior (Stucco) and Interior. Paragraph 5.7.3, 5.7.4, 5.7.6, regarding “control joints” describes lath as terminating at “control joints”.
· 1977: Specifications for Metal Lathing and Furring, Metal Lath/Steel Framing Association. Section 10.2 requires discontinuous lath at “control joints”.
· c.1980: Technical Manual, Keene Corporation/Penn Metal Products. Original mfr. of “control joint” lath accessory. 10-page manual dedicated to “control joints”, requires discontinuous lath at “control joints”.
· 1980: Portland Cement Plaster (Stucco) Manual, EB049.03, Third Edition, Portland Cement Association (PCA). Pages 16 and 25, illustrates a “control joint” with discontinuous lath installed on a wall and recommends discontinuous lath at “control joints”.
· 1981: Specifications for Metal Lathing and Furring, November 1981, Metal Lath/Steel Framing Association. Section 10.2 requires discontinuous lath at “control joints”.
· 1981-present: ASTM C1063 has required discontinuous lath at “control joints” from its initial draft in 1981, then its initial issuance in 1986 to the present. The 1971 ANSI 42.3 described the lath as “terminating at control joints”, which means the same thing as the term ‘discontinuous’ in ASTM C1063.
· 1984: How to Avoid Deficiencies in Portland-Cement Plaster Construction, Jacob Ribar, Construction Technology Laboratories, Skokie, IL. A report of significant field observations of the performance of installed stucco Assemblies. “Lath must be discontinued behind “control joints””.
· 1988: Specifications for Metal Lathing and Furring, Metal Lath/Steel Framing Division, NAAMM. Section 10.2 requires discontinuous lath at “control joints”.
· 1991: Guide Specifications for Metal Lathing and Furring, Fourth Edition, ML/SFA 920-91, NAAMM. Section 19 and Appendix A requires discontinuous lath at “control joints”.
· 1993: Guide to Portland Cement Plastering, ACI 524R-93, American Concrete Institute. Section 7.3.2. Describes discontinuous lath at “control joints” as the most effective “control joint” solution.
· 1993: Stucco and Plaster Guide, Tom C. Geary, Calaveras Cement Division of the Flintkote Company. Recommends discontinuous lath at “control joints”.
· 1995: Portland Cement Plastering Crack Analysis and Repair, Walt Pruter, September/October 1995, ICBO Building Standards Magazine. Reprint of same article in Walls and Ceilings Magazine, June 1993. Recommends discontinuous lath at “control joints”.
· 1996: Portland Cement Plaster (Stucco) Manual, EB049.04, Fourth edition, Portland Cement Association (PCA). Page 21, recommends discontinuous lath at “control joints”.
· 2003: The Metal Lath Handbook, Gary Maylon. Recommends discontinuous lath at “control joints”.
· 2003: Portland Cement Plaster/Stucco Manual, EB049.05, Fifth edition, Portland Cement Association (PCA). Page 26, recommends discontinuous lath at “control joints”.
· 2004: Guide to Portland Cement-Based Plaster, ACI 524R-04, American Concrete Institute. Page 8, recommends discontinuous lath at “control joints”.
· 2005: Best Practices for Stucco Applications, Report of the Task Force to Review Stucco, A Joint Project of Johnson County Building Officials and the Homebuilders Association of Greater Kansas City, January 2005. Section VI, recommends discontinuous lath at “control joints”.
· 2005: Technical Bulletin –MW#157-060105 Stucco Cracking, Masterwall (proprietary stucco manufacturer). Page 3, recommends discontinuous lath at “control joints”.
· 2005: Crack Control in Portland Cement Plaster, The Construction Specifier, Construction Specifications Institute (CSI), Walter Pruter, April 2005. “Until a testing program can be initiated to prove unequivocally the preferred method [continuous or discontinuous lath], it is this author’s recommendation unrestrained lath and plaster panels be built and detailed to the best degree possible, which includes cutting the lath and wire-tying the [SMJS] accessories to the lath edges.”
· Various wire lath manufacturers through their code evaluation reports and technical data require installation compliance with ASTM C1063, which requires discontinuous lath at “control joints”.
ü ICC/ES Report ESR-2317 Davis Wire Corporation
ü ICC/ES Report ESR-2017 Structa Wire Corporation
ü K-Lath, Reinforcement for Stucco Walls, Product Information and Installation Guide, 2013
Discontinuous lath at SMJS is a building code requirement wherever the 2006 International Building Code or later editions, are adopted. Licensed design and construction professionals are obligated to comply with Minimum Stucco Industry Standards, which is the building code and its referenced standards ASTM C926 and C1063:
· 2006-present: International Building Code. This IBC was the first building code to reference C1063 as a minimum code requirement. Discontinuous lath at “control joints” becomes a code requirement for the jurisdictions that adopt this code and its subsequent versions.
· 2008: Guide to Portland Cement-Based Plaster, ACI 524R-08, American Concrete Institute. Page 6, definition of “control joint” requires discontinuous lath. Sections 5.5 and 8.2 requires discontinuous lath at “control joints”.
· 2008-present: AMICO (metal lath and lath accessories manufacturer) product literature recommends discontinuous lath at “control joints”.
· 2008: Stucco Applications Guide, Lenexa, Kansas, requires discontinuous lath at “control joints”.
· 2009: Guide Specifications for Metal Lathing and Furring, Fifth Edition, Expanded Metal Lath Association (EMLA) 920-09, NAAMM. Paragraph 19.1.2 and Appendix A requires discontinuous lath at “control joints”.
· 2009: Scratching the Surface with Stucco Control Joints, Jeff Bowlsby, The Construction Specifier, April 2009. “Control joint” subassembly performance testing, as suggested by Walt Pruter in 2005. The conclusion of the performance testing suggests discontinuous lath at “control joints”.
· 2010: Cement Plaster Metrics: Quantifying Stucco Shrinkage and Other Movements; Crack Acceptability Criteria for Evaluating Stucco, Jeff Bowlsby, November 2010, RCI Building Envelope Technology Symposium. Stucco shrinkage and thermal movements are evaluated and discussed.
The second most critical of two functional requirements of an SMJS that accommodates stucco shrinkage movement and thermal movement, is for the flanges of the SMJS lath accessory to be attached to the discontinuous lath edges only, and not fastened to framing, blocking or the substrate support. The SMJS lath accessory flanges must not be fastened to framing, blocking or the substrate support to remain free to allow the SMJS lath accessory to accommodate movement, as a function of the movement experienced by the adjacent lath edges where the lath is discontinuous. Where the flanges of the SMJS lath accessory are fastened to framing, blocking or the substrate support with nails, screws, staples or concrete fasteners, the SMJS lath accessory is restrained from movement, and hence no stress relief by the SMJS subassembly occurs.
Potential fastener types used for SMJS include nails, screws, staples, wire ties, and concrete fasteners. Fastener type selection is based on SMJS functional requirements for materials to be joined, and substrate support material characteristics.
· Joining SMJS lath accessory flanges to separate, adjacent lath edges requires wire ties.
· Joining lath to suspended steel grillage substrate support without sheathing requires wire ties.
· Joining lath to wood framed or furred substrate supports with or without sheathing requires nails, screws, or staples.
· Joining lath to steel framed or furred substrate supports with or without sheathing requires screws.
· Joining lath to solid plaster base substrate support, mass masonry or solid concrete, requires concrete fasteners.
As should be evident from the preceding discussion, wire ties are the only acceptable fasteners to join SMJS lath accessories to lath to accommodate movement at the SMJS subassembly. Rumors that some lathers may use zip ties, hot glue or other means may potentially work as long as the SMJS lath accessory is not dislodged during plaster application and that the plaster keys with both the SMJS lath accessory flange and lath, but these are not recognized fastening devices or methods by the Minimum Stucco Industry Standards.
The practice of using nail, screw, staple and concrete fasteners to install SMJS lath accessories impedes SMJS movement and promotes cracking. Inaccurate and misleading information from certain stucco industry sources is circulated in the form of technical bulletins and letters that SMJS with SMJS lath accessories installed with nail, screw, staple or concrete fasteners (in lieu of wire ties) are acceptable practices in certain regions. The movement characteristics and requirements of portland cement-based plaster and stucco do not vary from one region to another. From the discussion above, this practice of using nail, screw, staple or concrete fasteners to install SMJS lath accessories to the substrate support results in marginal stucco performance manifested by excessive cracking.
Other potential restrictions to stucco shrinkage and thermal movements that may contribute to cracking:
· Lath on vertical walls must be attached to framing members at not greater than 7 in. on center into studs. That’s all. Any additional lath fasteners unnecessarily restrict movement of the lath and plaster composite and are exc essive. Each unnecessary nail, screw or staple used to fasten lath or lath accessories to the wall is an unnecessary restriction to stucco shrinkage and thermal movement.
· It is possible that different lath types may also create different behaviors of restraint to stucco shrinkage and thermal movement which may cause different behaviors of the lath and plaster composite, as related to shrinkage and thermal movements, and ultimately contribute to cracking. It is purely theoretical at this point but for example, expanded sheet metal lath has more metal volume per square foot than does wire lath. Holding a piece of expanded sheet metal lath, it is more rigid and is not easily compressed in plane (simulating shrinkage movement), just the opposite of wire laths that are comparatively flexible. Woven wire fabric lath is more easily compressed in plane than is welded wire lath which is more rigid due to its geometry and welded intersections. Testing could and should be performed to evaluate the effects of these various laths on shrinkage and cracking potential.
ASTM C1063 @ 220.127.116.11 indicates “Lath shall …be stopped and tied at each side.” Essential to understanding 18.104.22.168 is to realize that it only states the lath requirements at a SMJS subassembly, and does not specify requirements for attaching the SMJS lath accessory. Much tribulation and knashing of teeth has transpired about what the word “tied” means. To put this wording into context, it is antiquated language harkening back to the era when lath was wire-tied to open framed truss studs and suspended steel grillage substrate supports, and when stucco clad wall and ceiling assemblies were not sheathed as they commonly are today. This identical language has been the requirement in ASTM C1063 since the initial draft in 1981. Some misinterpret 22.214.171.124 as specifying SMJS lath accessory attachment requirements, which is clearly in error. Others are convinced that the word “tied” is broad and generic enough to allow fastening the discontinuous lath edges at SMJS subassemblies with screws, nails or staples to framing, just like one installs lath. But that interpretation conflicts with the significant precedent in C1063 which clearly indicate what “tied” means, we just need to look. Performing a word search on C1063 reveals that, of the seventeen uses of the words “tie”, “tied” or ‘ties”, sixteen are literally and unambiguously associated with the use of wire for the process of tying, so it is a reasonable conclusion that “tied” n ASTM C1063 involves the use of wire, and the process of tying and that the word in C1063 has no relationship to screws, nails or staples. There is nothing technically wrong with the word “tied” or wire-tying as understood in this context so for ASTM C1063, “tied” or “to tie” means “wire-tied”, nothing else, nothing more. Enough of this dead horse already, lets end the madness and stop trying to make the word ‘tied’ into something it is not, it is what it is.
The real issue is that ASTM C1063 I@ 126.96.36.199 does not recognize other common and acceptable fasteners which may be used with or without sheathing. It is unfortunate after all this time that the language has not been updated to reflect when nails, screws and staples became commonly used and sheathed framing became a common stucco substrate support system. ASTM C1063 @ 188.8.131.52 needs to be updated to recognize other common and acceptable lath edge fasteners such as nails, screws and staples and concrete fasteners as appropriate for fastening lath to the substrate support, and selection is based on the substrate support material characteristics.
Indication of fastening requirements for lath and SMJS in the construction documents is required by ASTM C926 A184.108.40.206.
Minimum Stucco Industry Standards ASTM 926 and C1063 provide us with prescriptive-based requirements for locating SMJS. Prescriptive-based locations are minimum requirements stated as generalized rules that apply in all circumstances, are usually intended to be conservative to provide a factor of safety and often are limited in design flexibility. Conservative engineering practice will derive prescriptive-based requirements from performance-based data that will include safety factors. Prescriptive-based requirements are useful throughout the construction industry for many other typical systems and components beyond stucco wall cladding systems, such as energy conservation-related systems (minimum wall insulation and U-values, maximum glazing area), minimum fireproofing thickness dimensions on structural steel, minimum roof membrane slope and perimeter turn-up dimension, minimum dimensions for accessibility features, etc.
ASTM C926 SMJS prescriptive-based location requirements:
· Where dissimilar base materials abut and receive a continuous coat of plaster
Because this is substrate support movement, a BMJS or PMJS is the appropriate stucco movement joint subassembly for this condition.
· Where “determined [necessary] by the characteristics of the substrate”
SMJS accommodate portland cement-based shrinkage and thermal movements, and are indirectly related to “characteristics of the substrate”. This ASTM C926 text needs correction.
· The design authority must indicate SMJS locations on project contract documents.
ASTM C1063 SMJS prescriptive-based location requirements:
· Where main runners or cross runners are spliced at suspended grillage ceilings
Because this is substrate support movement, a BMJS or PMJS is appropriate for this condition.
· A “control joint” shall be installed where the ceiling framing or furring changes direction. Because this is substrate support movement, a BMJS or PMJS is appropriate for this condition.
As should be obvious, stated requirements in Minimum Stucco Industry Standards for locating SMJS are not sufficient for minimizing most stucco cracking. The substrate movement conditions described as requiring SMJS more appropriately require BMJS or PMJS. Stated requirements for stucco panel areas, maximum stucco panel dimensions describe SMJS locations relative to each other which are helpful criteria, but these criteria are insufficient at accommodating stucco wall cladding conditions that commonly experience significant cracking. Specific conditions of greatest importance include reentrant wall opening corners, and three-plane panel intersections. Without identifying specific locations that are the most critical for functional purposes to minimize cracking, SMJS are often located where they may look good for aesthetic reasons, but serve little if any purpose such as vertically centered on windows/doors, or as picture framing around windows to suggest trim, or to create visual geometric patterns such trapezoids with pie-shaped corners – all of which crack.