Jeff Bowlsby CCS, CCCA
Exterior Wall and Stucco Consultant
Licensed California Architect
Stucco Building Movement Joint Subassembly (BMJS)
(“expansion joint” at wall plane)
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
BMJS are not mentioned in the building code or Minimum Stucco Industry Standards by name, but generically they are described as “expansion joints”. The common term stucco “expansion joint” is unfortunately generic, nebulous, ambiguous, obscure, does not clearly describe that this is a subassembly and not just a lath accessory, or the function of this joint subassembly and is rampantly misunderstood throughout the stucco industry. In search of clarity I derived and have used the term Building Movement Joint Subassembly (BMJS) as used on this website for easier recognition and intuitive understanding, and because it more clearly describes the purpose and function of this stucco movement joint subassembly. An understanding of the primary function of the BMJS, the purposes it serves and how it is to function as presented here, will resolve any lingering debate about its function and how and where to install it.
The BMJS, its intended purpose, function, and installation configuration can be misunderstood amongst building owners, architects and stucco craftsmen. 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.
Building structure substrate support movement conditions are significant factors that if not accommodated, can contribute to stucco cracking. While building structure substrate movement is important, it is recognized that other factors can contribute to cracking so addressing building substrate movement alone, can be considered only one of many factors contributing to stucco cracking. This webpage focuses primarily on the issue of building structure substrate movement and methods to minimize its contributory effects to stucco cracking.
Portland cement-based plaster when cured is a brittle wall cladding material which is not resilient enough to accommodate building structure substrate support movement. Building structure substrate support movement joints are designed joints and located to resolve wind, seismic and other dynamic loading conditions at structural building movement isolation locations such as seismic joints for large buildings, and at adjacent property line walls where separate, isolated structural systems adjoin. Building structure substrate support movement can also occur under normal loading conditions such as at deflecting beams, at inter-story drift joints of multi-story buildings (using slip tracks) and sawn lumber floor framing used with wood platform framing, where the floor framing experiences cross-grain shrinkage. These significant building structure substrate support movement conditions can contribute to stucco cracks localized to the area of movement. While building structure substrate support movement is a significant condition, it is recognized that many other factors can contribute to cracking. This webpage focuses on building structure substrate support movements and methods to mitigate them. Building Movement Joint Subassemblies (BMJS) developed from the Perimeter Movement Joint Subassembly (PMJS), so a complete understanding of the principles presented on the PMJS webpage is important to understand along with the additional information presented here.
Building structures as a substrate support for exterior stucco wall cladding systems experience significant structural system movements that are real and are known causes of stucco cracking. If building structures as a substrate support did not experience movements such as beam and floor edge deflections, wind and seismic forces, story drift and similar movements, then there would be no essential purpose for the BMJS. This webpage explores the conditions that make the BMJS 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.
Mass masonry and concrete buildings typically do not require building movement joints in the stucco wall cladding system except where their structure requires movement joints to isolate known or anticipated building movements. Where mass masonry or concrete buildings receive stucco wall cladding, it is often directly applied and fully bonded to the mass masonry/concrete substrate support. Building movement joints in a mass masonry/concrete building structure must be accommodated with a similar movement joint in the stucco wall cladding system to minimize cracking at substrate support movement conditions.
Framed buildings have experienced stucco cracking from the earliest days when stucco began to be installed on framed buildings. Sheathed framing rigidified buildings but cracking continues to occur. The effects of portland cement-based plaster shrinkage and thermal movement became better understood as contributory to cracks in the 1950s and shrinkage and thermal movement jointing solutions were developed to address those movements.
Contemporary framed buildings are often continuously sheathed. The sheathing, when it performs a building structural function stiffens the building to help resist building structural movement related to wind and earthquake forces. The sheathing, while also performing a building structural function related to lateral building movement, provides a more uniform substrate support surface for the stucco wall cladding that is helpful towards achieving a uniform stucco thickness, which helps minimize cracking.
Framed building structures whether of metal or wood framing, are often a support substrate for stucco wall cladding on contemporary buildings. A framed substrate support can contain components that move, movement that if translated directly to the exterior stucco wall cladding system, will most certainly cause cracking of the stucco cladding system. Movements such as spanning horizontal floor slab edges deflecting over lower walls by way of a slip track, and sawn lumber floor joints experiencing cross-grain shrinkage, or building seismic joints separating building sections – all require isolation from the stucco wall cladding system. In these conditions the stucco substrate support of the building is divided into separate adjacent building substrate supports, usually with a gap, where the gap provides a zone for movement to occur. The stucco wall cladding system is physically terminated on either side of this gap to accommodate the movement and to minimize cracking that would otherwise occur if the stucco wall cladding system were not isolated from the movement zone.
Patent research: Not all BMJS lath accessory components and subassemblies are patented and not all patented BMJS lath accessory components were produced or are currently available. Select BMJS lath accessory components and subassemblies are discussed.
The 1959 Barr design lath accessory and subassembly is on the market today, and marketed as a “control joint”. In reality though, it is a BMJS lath accessory and subassembly given the way it is depicted in its patent illustration, shown with both flanges fastened to a discontinuous substrate support, although it is not commonly used and its movement range is limited for this purpose.
1965 Barr design BMJS lath accessory component and subassembly
A few years after the Clark “control joint” (SMJS lath accessory component and subassembly) appeared on the market in the mid-1950s, Ross Washam brought another stucco movement joint to market in the early 1960s as both a two-piece stucco lath accessory component and subassembly or Building Movement Joint Subassembly (BMJS), reportedly to accommodate movements of greater magnitude than what the SMJS lath accessory and subassembly could accommodate. While Washam’s patent is dated 1967, it appeared in product catalogs a few years earlier.
Illustrations from Washam Patent 3,331,176
(Note the patent illustrates both a BMJS lath accessory component and BMJS subassembly. The BMJS lath accessory is depicted as wire-tied
over discontinuous lath or fastened to the building substrate support)
The Clark design and Washam design stucco movement joint lath accessory components and subassemblies share many similarities:
· Both were devised and promoted to minimize stucco cracking of walls and ceilings, at interior and exterior building locations.
· Both were exclusively manufactured and introduced to the market by the Penn Metal Company (and later by Keene)
· Both were illustrated as wire-tied to lath in their patents
· Both feature paired ground screeds with a non-plastered gap in between
· Both patents are identically titled Building Construction and Expansion Joint Therefor even though filed 10 years apart and by different inventors.
· Both patents describe the lath accessory component as an “expansion joint”
· Both patents are assigned to Penn Metal Company Inc. and drafted by the same attorneys
· Both are viable solutions to minimizing stucco cracking, although from movements of different origins
· Both are in common use today
The Washam BMJS lath accessory component and subassembly has a familial resemblance to the Clark design SMJS lath accessory component and subassembly in that the plaster is terminated at opposite grounds of the lath accessory, to allow movement between the grounds. The fact that both the Clark design SMJS and the Washam BMJS were invented to address portland cement plaster shrinkage and thermal movement in cement plaster claddings, just with different approaches and solutions, is interesting and indicative of the significance of the issue of stucco cracking. With these commonalities it is not surprising why both the Clark design and Washam stucco movement joint lath accessory components were referred to interchangeably as “expansion” joints, “expansion/control” joints, and “expansion and contraction” joints in product catalogs and by everyone that used them, a blurred terminology which has contributed to the persistent controversies with stucco movement joints.
Take note of the differences between the Washam lath accessory component from Clark, which includes discontinuous lath, a variable dimension, determinable separation gap and grip edge flanges which engage the stucco panel edges to avoid the separation gap that the Clark design SMJS suffered from then and to this day. Washam can accommodate movement in two-directions, whereas Clark can only accommodate one-directional movement. Note that the patent illustrations of both Clark and some of the Washam illustrations are not graphically indicative of any relationship to framing members, the building structure substrate support (continuous or discontinuous?), fasteners or a WRB, the lath accessories are simply shown as just wire-tied over the lath and portrayed in an idealistic free floating condition. This idealism has contributed to the persistent controversies with stucco movement joints.
Since the mid-1950’s, the Clark design and Washam design stucco movement joint lath accessory components and subassemblies were the primary stucco movement jointing solutions available. In 2008 a variation of the Washam lath accessory was invented and brought to market by Don Pilz, which bears a resemblance to the 1972 Conway design “Joint Construction”. The Pilz design BMJS lath accessory and subassembly includes a dimensionally-extended vertical flange for integrating with the WRB and a sloped drainage plane which is intended to facilitate water drainage for horizontally-oriented installations on weather exposed building exterior walls. The Pilz two-piece design BMJS lath accessory and subassembly is intended for horizontally-oriented exterior wall weather-exposed stucco wall claddings, to provide drainage as well as significant movement capacity.
Pilz design two-piece BMJS lath accessory and subassembly
US Patent 2008/00168080A1
ASTM C1063 Standard Specification Installation for Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster(1):
· 3.2.7 BMJS or PMJS, noun: A movement joint subassembly accommodating stucco movements that are greater than those associated with plaster shrinkage and curing. Reference Specification C926, Annex A126.96.36.199
· (7.5.4) 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.
· (188.8.131.52) Provide BMJS or PMJS with 3/8-in. (9.5-mm) minimum separation gap where load bearing walls or partitions adjoin structural walls, columns or floor or roof slabs. Discontinue lath through BMJS or PMJS, cornerite not allowed.
· (184.108.40.206) Provide BMJS aligned with expansion joint in substrate support.
· (A1.2) A1.2 Provide BMJS, PMJS to accommodate building substrate movement and to minimize movement related stucco and WRB damage.
ASTM C926 Standard Specification for Application of Portland Cement-Based Plaster(1:
· (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.
· (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.
· (X1.1.6) Indicate the type, location depth and orientations of BMJS, PMJS and SMJS in the construction documents.
· (X22.214.171.124) Application of Plaster Basecoats: (1)…lathing accessories used as plaster thickness screeds include casing beads at stucco panel end and edge terminations at dissimilar material, BMJS, PMJS and SMJS installed following Specification C1063,...
The need for “expansion joints” (BMJS lath accessories and subassemblies) has developed from stucco wall cladding system cracking conditions at transitions of perimeter substrate support conditions. In large scale building contemporary construction with substrate support movement conditions such as multiple stories and drift-deflection joints, long spans of deflecting floors, and seismic joints, BMJS are an essential tool for minimizing cracking.
Buildings and parts of buildings experience movement from static and dynamic loading conditions, movements which require isolation from stucco cladding systems to minimize cracking. Movements of this type are localized movements between buildings such as wind and seismic movements, and portions of buildings such as floor slab edge deflections, story drift deflections, and sawn lumber cross-grain shrinkage movements. These movements occur where they occur and can be locally addressed with BMJS lath accessories and subassemblies to minimize stucco wall cladding system cracking.
As a point of interest, the Washam stucco movement joint lath accessory component and subassembly were initially devised and intended to accommodate stucco shrinkage and thermal movements that were greater than what the Clark design stucco movement joint lath accessory component could accommodate. The Clark design and Washam solutions are conceptually similar in their approach of defining separate isolated adjacent stucco wall cladding panels, discontinuing the lath and accommodating movement at the location of the movement joint subassembly.
As a two-piece lath accessory component, Washam accommodates movement by the use of two nested and overlapping sheet metal flanges that are exposed at the finish surface of the stucco wall cladding system. This configuration creates an exposed “weather shield” (terminology from the patent) to cover the separation gap between the adjacent stucco panel edge grounds, which also protects the concealed WRB from UV exposure.
In contrast with the Clark design, the Washam design includes solid sheet metal flanges, not foraminous flanges, to integrate with the stucco wall cladding system. Where lath is used with Washam, the lath accessory component is depicted as overlaying the lath, over its separate discontinuous lath edges, and is wire tied to the lath edges. With the Pilz design, lath overlays the solid sheet metal flanges and fasteners for both the lath accessory and lath are not depicted. The depicted substrate support for both Washam (lath) and Pilz (wall panel) is depicted as discontinuous.
Because of the comparatively large dimensional movement requirements for substrate support related movement joints, as compared to stucco shrinkage movement, a single-piece building movement joint lath accessory and subassembly is not a feasible solution. Two-piece BMJS lath accessory and subassembly joint designs featuring overlapping and nested sections of a range of dimensions to accommodate substrate support movements. Each piece of the two-piece BMJS lath accessory includes a stucco panel edge ground which functions not only as a stucco panel edge termination, but because the lath terminates at the stucco ground the panel edge function as ½ of a SMJS to accommodate stucco shrinkage and thermal movements.
The dimensional separation gap between the stucco panel edge grounds is variable and can be determined based on the amount of anticipated movement required. This separation gap creates a recess in the plane of stucco wall cladding that creates an aesthetic shadow line.
The Washam stucco movement joint lath accessory component and subassembly is configured with a pair of folded and nested plates that slip against each other to accommodate movement. The configuration is not inherently watertight.
The Pilz design stucco movement joint lath accessory component and subassembly is configured conceptually and functionally similar to Washam but also includes sloped drainage planar surfaces for facilitating drainage. The Pilz design stucco movement joint lath accessory component and subassembly are intended for horizontally-oriented installations where drainage is required.
The BMJS lath accessory with integral grounds in either case adds convenience as an integral stucco thickness screed and location to stop plastering work to prevent cold joints in the plaster.
Minimum Stucco Industry Standards ASTM C926 and C1063 are referenced standards in the building code which state minimum installation requirements for BMJS lath accessories and subassemblies.
It is essential to understand that any stucco wall cladding location with discontinuous lath or where lath terminates at the edge of a stucco panel, meets the essential functional requirement of a stucco SMJS, a designated location within the stucco wall cladding system where stucco shrinkage and thermal movements can occur. The BMJS broadens that functionality by adding discontinuity of the substrate support to address building structure substrate support movements. Both the Washam design and Pilz design patent illustrations depict discontinuity of the substrate support.
Washam depicts that where the BMJS lath accessory component is used with lath that the solid flanges of the lath accessory overlap and are wire tied to the face of the lath. Where used at solid bases such as masonry (see the patent document), the lath accessory is mechanically fastened to the masonry substrate. Washam therefore illustrates two different idealized subassembly configurations where the BMJS lath accessory component is used to both accommodate movement of the lath and accommodate movement of the substrate support. Pilz depicts the lath overlapping the flanges of the lath accessory component.
The term used to describe the stucco “expansion joint” as a lath accessory and subassembly, has been used, misused, misunderstood and misapplied throughout the industry. Even its originators did not understand its applicability in isolating building substrate support movement, although they did understand its purpose in minimizing stucco wall cladding system cracking. This website preferentially uses the term Building Movement Joint Subassembly (BMJS) and BMJS lath accessory. The two-piece BMJS lath accessory is used in a BMJS which includes the substrate support (separated and discontinuous), framing/blocking to fasten both sides of the BMJS lath accessory and lath edge fasteners, the WRB, the BMJS lath accessory and its fasteners, the configuration of the lath (discontinuous), and of course the portland cement-based plaster.
By the mid 1950’s initial stucco shrinkage during curing and thermal movements while in service were recognized causes of cracks and the Clark design one-piece stucco SMJS component and subassembly was brought to market as a more convenient form of the jobsite fabricated, two-casing bead SMJS. The Washam design BMJS lath accessory and subassembly was devised and effective for the same purposes, but its greater movement potential has been found to be more beneficial for accommodating building structure substrate support movements. Framed building structure substrate supports especially with regards to larger scale buildings, often include substrate support movement conditions that must be managed where occurring under stucco wall claddings, or cracking will occur. Spanning beams and floor slab edges deflect relative to the walls below them, seismic joints vertically and horizontally allow movement to occur between sections of building structures and story drift joints at building stories slide past each other to accommodate lateral movement building deflections to occur whose dimensional movement can be significant. Only two-piece BMJS are currently capable of the required dimensional extent and directionality of movement these conditions require.
As the SMJS is related to movement of the lath, the BMJS is related to movement of the substrate support. It is interesting to note that the lath terminates at both SMJS and BMJS, and as a result both are capable of accommodating stucco wall cladding shrinkage and thermal movements, even though that is not the primary purpose of a BMJS. To accommodate stucco wall cladding shrinkage and thermal movement the SMJS lath accessory is wire-tied only to the lath which is discontinuous to allow the greatest stucco wall cladding movement, and the substrate support is continuous because the movement is related to the stucco cladding, not the substrate. In contrast, to accommodate building structure substrate support movement the BMJS lath accessory component is attached only to the separate building structure substrate supports which are separated and discontinuous to allow the greatest movement between the separate building structure substrate supports.
Building structures and materials are exposed to and must accommodate various structural and physical forces and their related deflections or movements to function as expected. Gravity loads cause vertical building movements such as beam and floor slab edge deflections. Wind and seismic loads cause lateral building movements such as inter-story drift. Stucco, as one of the few exterior wall cladding materials applied as a wet material, experiences shrinkage movement as the stucco cures. Daily and seasonal ambient thermal variations cause expansion and contraction movements within all building construction materials. Each of these movements must be accommodated in some way or the building and its cladding material will not function as anticipated.
Building structural movements, expressed either vertically or laterally, as a substrate support for stucco, are accommodated with BMJS or PMJS. Portland cement-based shrinkage and thermal movements are accommodated with SMJS. BMJS and PMJS also accommodate shrinkage and thermal movements because the lath is not continuous through these joints, but shrinkage movements and thermal movements are not the primary function of BMJS and PMJS.
Isolating stucco wall cladding panels using BMJS located in certain locations where substrate support movement occurs such as where floor slab edges deflect or where the substrate support systems change, configured in a certain way to accommodate this differential movement, sealed at critical junctures to effectively manage the effects of water, create a Building 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 BMJS lath accessory component or BMJS subassembly in isolation can accomplish the effect of the Assembly as a whole on the stucco wall cladding system.
Building Movement Joint Subassemblies (BMJS) serve these purposes:
· 1st Purpose: Accommodates building structure substrate support movements because the substrate support is discontinuous. Accommodates portland cement-based plaster shrinkage movement, because the lath is discontinuous.
· 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.
Bulk water can potentially intrude into and behind the exterior stucco wall cladding 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.
A concern with BMJS 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 BMJS 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. BMJS 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 BMJS 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 BMJS 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 BMJS 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 Washam design BMJS lath accessory has been developed with a drainage surface (Pilz, 2008) to provide drainage capability for horizontally-oriented BMJS 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 Pilz design Horizontal Drainage BMJS lath accessory at intersections with vertical stucco movement joint subassemblies, to avoid perpendicular butted termination splice cracking.
A continuous framing or blocking substrate support member is required at BMJS for fastening the BMJS lath accessory component to the building structure substrate support and the adjacent discontinuous lath edges at the BMJS location. Often the typical framing already present is sufficient.
BMJS accommodate building substrate support movements. A gap or discontinuity in the substrate support to accommodate movement is required and essential for a BMJS to function.
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 BMJS, in their function as screeds for thickness control, be shimmed to achieve acceptable finish surface alignment? Succinctly no, for multiple reasons. BMJS 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 BMJS 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 BMJS lath accessories is not recognized or discussed in SMJS lath accessory manufacturer’s product literature.
· Shimming BMJS lath accessories may create excessive differential stucco thickness variations and may allow wet portland cement-based plaster mortar migration behind the BMJS lath accessory, which can cause cracking.
· Shimming BMJS lath accessories causes fastener penetrations through the WRB that are often not in compression to the WRB, potentially allowing water intrusion.
BMJS Substrate Support condition exceeding planarity tolerance
(Install framing within planarity tolerance
or shim lower sheathing panel to meet planarity tolerance)
The two BMJS lath accessories available on the market today share many similarities in that they are of a two-piece design of rigid materials and require a physical gap to perform their function.
The Washam design BMJS lath accessory and subassembly is only appropriate for vertically-oriented applications on walls because its drainage capability is along its length.
The Pilz design BMJS lath accessory and subassembly is most appropriate for horizontally-oriented applications on weather-exposed exterior walls because of its sloped drainage surface. This configuration requires the WRB to overlap the Pilz PMJS lath accessory to function as a drainage screed flashing.
Materials: BMJS lath accessories are available in galvanized steel, solid zinc alloy, stainless steel sheet metals and extruded PVC. Not all BMJS lath accessories are available in all materials from every manufacturer. BMJS 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 BMJS lath accessory material used in most regions throughout the USA. Typically G60 galvanization but G90 may be available from select manufacturers.
· Solid zinc alloy sheet metal: Several manufacturers recommend solid zinc alloy BMJS 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 BMJS lath accessories cost more than galvanized or PVC to both purchase and install as a result of its inherent flexibility. Solid zinc and PVC lath accessories with larger dimensions of material exposure are subject to extreme thermal movements.
Solid zinc alloy BMJS subassemblies with larger dimensions
of material exposure
· Extruded Polyvinyl Chloride (PVC): Lowest cost and offering excellent corrosion protection, PVC BMJS lath accessories are primarily used at coastal and corrosive environments where corrosion protection is essential. PVC BMJS lath accessories. 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.
Two different generic BMJS 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 BMJS lath accessory products available on the market are ideal solutions, although if utilized correctly they 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 BMJS lath accessory profile from any manufacturer, setting the condition in a sealant bed is the primary solution to seal these conditions watertight. Hollow-profile BMJS lath accessories can function as water channels and drainage must be considered in their use and installation.
No stucco industry product standard exists that specifies the engineering technical aspects of the BMJS lath accessory and no two manufacturers make them identically, so a BMJS lath accessory is technically not a generic lath accessory conforming to a generic industry standard. BMJS lath accessory engineering technical information, including performance testing characteristics when assembled as BMJS subassemblies, are not typically published or available from their manufacturers. The lack of complete engineering technical information about BMJS lath accessory products, adds to the challenges for designers and installers to accurately know how and where to use the BMJS lath accessory or how and where to integrate the lath accessory into exterior stucco wall cladding subassemblies for greatest effectiveness. BMJS 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 BMJS lath accessory as installed in a BMJS subassembly, and the correlating installation requirements for design and installation reference.
Mass masonry or solid concrete buildings from previous eras often do not have building structure substrate support joints of any kind because they are solid and monolithic and that results in a certain architectural aesthetic that many find attractive. Contemporary mass masonry or solid concrete buildings use masonry and solid concrete differently than in earlier times and most if not all do require and have substrate support joints to accommodate building structure movements that are the substrate support for stucco wall claddings. Framed building structures also include building structure substrate support joints to accommodate movement at conditions such as floor slab edges of multi-story buildings, seismic joints, inter-story drift joints etc. Minimum Stucco Industry Standard ASTM C1063 requires a BMJS to be located within the stucco wall cladding system proximate to building structure substrate support movement joints to minimize cracks in the stucco wall cladding system. Omission of BMJS in the stucco wall cladding system at building structure substrate support movement joints promotes cracking localized to the omitted substrate support joint condition and does not comply with ASTM C1063.
Complete omission of BMJS to achieve a monolithic aesthetic also deprives the stucco craftsman of the thickness screed (gauge) and work stoppage functions that BMJS provide. Without BMJS 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.
The Washam design and Pilz design BMJS lath accessory components include solid sheet metal flanges for mounting. The Washam patent illustration depicts the solid flanges of the lath accessory component overlapping the lath, a condition that is well-known to cause a linear stucco crack from the differential substrate as the stucco sees it. In both the Washam design and Pilz design lath accessories, lath must overlap the solid flanges of these subassemblies to minimize stucco cracking at the edge of the solid flange.
BMJS lath accessory components include integral stucco termination grounds which create stucco panel perimeter edges. Lath terminates at these termination grounds and is discontinuous through the BMJS. While a BMJS exists to address building structure substrate movements, it also minimizes portland cement-based plaster shrinkage and thermal movements, to minimize stucco cracking.
The BMJS is related to movement of the substrate support, and the BMJS lath accessory has solid flanges. Potential fastener types used for BMJS include nails, screws, staples, wire ties, and concrete fasteners. Fastener type selection is based on BMJS functional requirements for materials to be joined, and substrate support material characteristics.
· Joining BMJS lath accessory flanges to suspended steel grillage substrate support without sheathing requires wire ties or screws.
· Joining BMJS lath accessory flanges to wood framed or furred substrate supports with or without sheathing requires nails, screws, or staples.
· Joining BMJS lath accessory flanges to steel framed or furred substrate supports with or without sheathing requires screws.
· Joining BMJS lath accessory flanges to solid plaster base substrate support, mass masonry or solid concrete, requires concrete fasteners.
BMJS lath accessory fasteners may impact the function of BMJS, yet fastener type, sizes, spacing etc. are seldom indicated in lath accessory manufacturer’s literature, or on construction documents.
Because the BMJS is subservient to substrate support movement, it needs to be located proximate to the substrate support movement condition. Movement conditions at the building structure substrate support determine the location for BMJS. A BMJS has no functional purpose to accommodate movement when used in other locations.
BMJS in horizontal orientations on weather-exposed exterior building walls are subject to wind driven water exposure. The Pilz BMJS lath accessory and subassembly includes an extended flashing flange to integrate with the WRB, and a sloped drainage surface, where the Washam design does not.
BMJS lath accessories are manufactured in 10 feet lengths and may be required and located continuously around buildings with long wall lengths such as for interstory drift joints or floor slab edge deflection joints on multistory buildings. BMJS lath accessory component splices along panel edges can contribute to localized splice termination cracking and water intrusion. Splice plates are not available for BMJS lath accessories. Butt splices of horizontally-oriented BMJS can be embedded in sealant and located at sealed intersections with other exposed lath accessories to minimize splice termination cracking and to minimize water penetration.
BMJS in vertical orientations on weather-exposed exterior building walls are subject to wind driven water exposure. The Washam design BMJS lath accessory and subassembly can simply be placed over the WRB and does not require to integrate with it for drainage which can occur vertically down its length.
BMJS lath accessories are manufactured in 10 feet lengths and may be required and located to extend vertically up buildings such as for seismic joints between adjacent sections of large buildings. BMJS lath accessory component splices along panel edges can contribute to localized splice termination cracking and water intrusion. Splice plates are not available for BMJS lath accessories. Butt splices of horizontally-oriented BMJS can be embedded in sealant and located at sealed intersections with other exposed lath accessories to minimize splice termination cracking and minimize water penetration.
BMJS accommodate building structure substrate support movements. SMJS accommodate stucco wall cladding shrinkage and thermal movement joints that are significantly smaller than the BMJS movements.
At intersections of BMJS and SMJS, the BMJS must be continuous through the intersection because the potential movement is greater. (2)
The Washam design BMJS when installed on ceilings and soffits presents one usage condition, often non-weather exposed conditions. But what about when this lath accessory component and subassembly is on a weather-exposed building exterior wall? How does it function when it is vertically oriented on a wall or when it is horizontally-oriented on a wall and are the two conditions the same or different?
One of the objectives of the Washam design BMJS lath accessory and subassembly as stated in the patent was to “maintain the integrity of the weather shield while still permitting variations in the widths of the gaps between the plaster sections over wide ranges.” The method to accomplish that uses folded, nested, exposed sheet metal plates that protected the concealed WRB from ultraviolet exposure. The folded and nested plate configuration of the BMJS lath accessory however is not watertight if not sealed, especially where mounted in a horizontally-oriented position on an exterior building weather-exposed wall. Water can readily bypass around the articulated and nested folds and can penetrate behind the lath accessory if the separation gap is not sealed. Further, the BMJS lath accessory component like most lath accessory components is only produced in 10 ft long sections, which requires splices when longer lengths are required and it must often be installed at building interior and exterior corners. Splice and corner conditions are often simply square cut or mitered and end butted together. Though potentially sealed with sealant they are difficult to make water tight, in part because splice plates and factory manufactured prefabricated corners for two-piece joint accessories are not often available. Where the Washam design BMJS is installed in a horizontally-oriented position on a building exterior weather-exposed wall, it must address water drainage either over its mounting flanges or behind them, either of which problematic. Vertically-oriented installations are not as concerning in terms of a water management function, as long as the separation gap is sealed watertight.
Sealing the separation gap of the Washam design BMJS lath accessory is essential. Sealing the separation gap of the Pilz design BMJS lath accessory is advisable. Sealing BMJS splices, terminations and intersections is essential to minimizing water penetration into the stucco wall cladding assembly which is the most effective method of managing wind driven water exposure – minimize its entrance into the wall assembly. Sealant at vertical BMJS above and intersecting drainage screeds or flashings must be installed carefully to not impede drainage.
Exterior stucco wall cladding systems installed on low slope surfaces are not an optimum solution from a stucco durability and waterproofing perspective. Weather-exposed, stucco clad surfaces less than 60 degrees from horizontal can be considered as roofs in the building code by reasonable inference and it is recognized that roof surfaces are exposed to environmental conditions much more extreme than vertical and near vertical walls. Stucco is a wall cladding system, not a roofing system or material, and is not an optimum solution for low slope, weather-exposed surface conditions.
Wall cap surfaces such as at parapets, balcony railing walls and fences are oftentimes long narrow surfaces that exceed stucco panel area geometry proportion limitations, and cracking is more prevalent in this condition, which are potential water intrusion conditions. Stucco cladding movement joints that continue up from the wall below are especially problematic from a water intrusion perspective. The best technical solution is to provide durable materials appropriate for these conditions such as sheet metal copings, drainage and a concealed waterproofing membrane, which eliminates the need for BMJS on low-slope weather-exposed conditions.
Decorative ornamental feature elements as components of a stucco wall cladding system are often adhered to the brown coat for ornamental purposes. BMJS open and close with building structure substrate support movements and restricting these movements with continuous overlaying ornamental features may result in cracking in the less forgiving material, usually in the element spanning over and attached to both sides of the BMJS, or occasionally the adjacent localized stucco cladding and finish coat.
Terminating decorative ornamental feature materials at either side of the BMJS so that movement is free to occur preserves the functionality of the BMJS. Where visual continuity is desired consider using resilient sealant of appropriate color, over backer rod to span the gap over the BMJS.
Use of BMJS, even conservatively, cannot guarantee against cracking.
Restraints of the BMJS lath accessory itself: Even when correctly installed, the BMJS lath accessory may restrain building structure substrate support movement somewhat, especially when sealed. Sealant-based joints may be limited in range of movement by the characteristics of the sealant and configuration of the sealant joint.
BMJS lath accessories, subassemblies and assemblies using their current one-piece design, subassembly and assembly configuration, as with all things, have limitations in performance even when configured optimally. In spite of these limitations, they are the best BMJS lath accessory, subassembly and assembly solution we have available and we simply need to understand their capabilities and limitations and use them appropriately. BMJS lath accessory, subassembly and assembly limitations include:
· No range of ground depth adjustability to resolve substrate support planarity tolerance conditions, without using shims
· Nested and folded (hollow) BMJS lath accessory profile functions as water conduit
· Profiled splice plates are not available and BMJS lath accessory material in section is cumbersome to seal watertight at terminations, splices, intersections without embedment using a thick sealant daub as setting bed.
The term “expansion joint” is too nebulous to be meaningful, does not describe its function, does not make the important distinction between the joint subassembly and the lath accessory. Building Movement Joint Subassembly (BMJS) and BMJS lath accessory are more descriptive and useful and are used on this website.
Building structure substrate support movements are significant and if not effectively accommodated, can cause cracking.
The BMJS functions to accommodate building structure substrate support movements, functions as a plaster thickness gauge and convenient working edge to minimize cracking.
Wrinkling of a WRB caused by moisture absorption can cause cracking. Bulk water intrusion at BMJS is a concern of building enclosure performance that can be minimized with sealants applied in strategic locations and conditions before plastering. Drainage flashings provide a means of drainage to manage bulk water that does penetrate into the exterior stucco wall cladding system.
Framing/blocking at BMJS on framed or framed and sheathed walls is required for fasteners.
Sheathing panels such as wood-based plywood and OSB, and gypsum-based sheathing panels are common and beneficial to exterior stucco wall cladding to assist with attaining uniform thickness. Panelized sheathing as a substrate support for exterior stucco wall cladding is required to be discontinuous at BMJS.
Substrate support planarity tolerances are important if not critical, to providing an acceptable substrate for lathing and plastering, and to avoid shimming lath accessories. Localized offsets in planarity should be eliminated or minimized to minimize stucco cracking.
Two BMJS lath accessories exist to accommodate building structure substrate support movements, and one functions as a horizontal drainage screed. Galvanized steel BMJS lath accessories may corrode under corrosive environmental conditions, which may require solid zinc alloy, stainless steel or PVC BMJS lath accessory materials at corrosive environments. Larger dimension solid zinc and PVC BMJS lath accessories are subject to extreme thermal movements. Manufacturers of BMJS lath accessory components do not publish complete technical specifications of their BMJS products
The desirable monolithic aesthetic of stucco on a mass masonry or solid concrete substrate is functionally incompatible with a stucco and lath composite on a framed or framed and sheathed substrate support. BMJS provide important quality control functions related to minimizing stucco cracking, to a stucco wall cladding system on a framed or framed and sheathed substrate support.
Discontinuous lath at BMJS is required by ASTM C1063, to accommodate building structure substrate support movements, which minimizes stucco cracking.
Fasteners appropriate to the substrate support are used to fasten BMJS to the substrate support. BMJS lath accessories are fastened to the building structure substrate support to maximize BMJS movement and minimize stucco cracking.
BMJS location requirements provide minimum accommodation for minimizing stucco cracking related to building structure substrate support movement.
Horizontally-oriented BMJS at exterior building weather-exposed walls require a drainage function. Butt splice termination cracking must be considered when determining BMJS splice locations.
Vertically-oriented BMJS at exterior building weather-exposed locations, may be placed over the continuous WRB. Butt splice termination cracking must be considered when determining BMJS splice locations.
Both BMJS and SMJS are stucco movement joints with the BMJS accommodating greater dimensional movement. At intersections of BMJS and SMJS, the BMJS must be continuous through the intersection.
Seal BMJS butt splices, terminations and intersections watertight by setting the condition in a sealant daub during installation, to minimize bulk water intrusion behind the stucco wall cladding and protect the sealant from ultraviolet light deterioration. Omit sealant at the base of vertically-oriented BMJS, where above horizontal drainage screeds or flashings, to promote drainage.
Providing stucco cladding and BMJS on low-slope weather-exposed surfaces (less than 60 degrees from vertical) without specialized materials and detailing is not an optimum solution from a stucco durability and waterproofing perspective. The best technical solution is to avoid stucco wall cladding systems at this condition and provide roofing materials and concealed waterproofing at low-slope weather-exposed exterior surfaces.
Avoid installing ornamental features that span over both sides of a BMJS to avoid localized stucco cracking.
Recognize and accommodate the performance limitations of the BMJS lath accessory and BMJS subassembly in stucco wall cladding systems, for maximum performance towards minimizing stucco cracking and bulk water intrusion.
Minimum Stucco Industry Standard: “Expansion joint” is the term currently used in Minimum Stucco Industry Standards.
Stucco Best Practice: The terms Building Movement Joint Subassembly (BMJS) and Building Movement Joint lath accessory make an important distinction between the two similar terms, and should be used to replace “expansion joint”, as more accurate and descriptive terms, in their appropriate contexts.
Minimum Stucco Industry Standard: Recognize that building structure substrate support movements are significant and if not effectively accommodated, can cause cracking. BMJS effectively specified, located, detailed and installed can minimize stucco cracking.
Minimum Stucco Industry Standard: Recognize that the BMJS is useful to accommodate building structure substrate support movements, is a convenient exterior stucco wall cladding subassembly to assist in achieving required plaster thickness and planarity within tolerance, and a convenient location to stop plastering work to avoid cold joints and related cracking.
Minimum Stucco Industry Standard: Recognize that the building code and Minimum Stucco Industry Standards, as well as a long list of voluntary industry standards and references from throughout the stucco industry, require and recommend sealant or caulking at unsealed gaps such as at exposed lath accessory perimeter panel edges, mitered corners, intersections, end terminations and butted termination splices, to prevent entry of water.
Stucco Best Practice: Provide WRB materials that provide drainage that do not absorb moisture to avoid WRB wrinkles that may cause cracking.
Minimum Stucco Industry Standard: Framing or blocking in addition to regular framing members may be needed at BMJS locations on walls to receive fasteners. ASTM C1007 requires framing or blocking for steel stud framed assemblies. Necessary framing or blocking to receive lath edge fasteners at BMJS should be indicated in the construction documents by the design authority for all substrate support conditions.
Minimum Stucco Industry Standard: Panelized sheathing as a substrate support for exterior stucco wall cladding is required to be discontinuous at BMJS.
Minimum Stucco Industry Standard: Substrate support framing and sheathing must be within planar tolerance before the stucco wall cladding system is installed. 1/8 in. in 10 feet for steel stud framing, ¼ in. in 10 feet for wood framing and solid concrete or masonry support substrates. Use of shims with BMJS is not a recognized installation method in ASTM C1063 and not allowed. Avoid shimming BMJS lath accessories to avoid stucco performance discrepancies. Correct substrate support framing and sheathing that exceeds allowable planarity tolerance before installing lathing and lath accessories.
Stucco Best Practice: Eliminate or minimize localized planarity offsets in the substrate support to minimize portland cement-based plaster and stucco thickness variations and stress concentrations that may cause cracking.
Minimum Stucco Industry Standard:
· Two different BMJS lath accessories are produced, each with their own purpose and characteristics. The Washam design is suitable for vertically-oriented conditions on walls. The Pilz design is suitable for horizontally-oriented conditions on wall to provide a drainage function.
· Use BMJS lath accessories made of materials that will not corrode under normal ambient environmental conditions of the exterior stucco wall cladding installation, which may require solid zinc alloy, stainless steel or PVC BMJS lath accessory materials at corrosive conditions.
· Avoid larger dimension solid zinc and PVC BMJS lath accessories to avoid distortions from thermal movements.
· Manufacturers of BMJS lath accessory components are encouraged to publish complete technical specifications of their BMJS products, installation requirements, dimensions, movement capacity, fastening, splicing, joinery and sealing requirements and any usage restrictions or performance limitations.
Minimum Stucco Industry Standard: ASTM C926 and C1063 require BMJS at all stucco wall cladding systems as referenced in the IBC building code, at building structure substrate support movement conditions without exception.
Minimum Stucco Industry Standard: Discontinuous lath at BMJS is required by ASTM C1063, to accommodate building structure substrate support movements, and is a Minimum Stucco Standard of Care. The adjacent lath edges of discontinuous lath at BMJS must be nailed, screwed or stapled to the substrate support, framing or blocking, to minimize stucco panel edge curling.
Minimum Stucco Industry Standard: Use the appropriate fasteners to attach the BMJS to the substrate support.
Stucco Best Practice: Indicate required fastener types, sizes spacing locations etc. in construction documents.
Minimum Stucco Standard of Care:
· The design authority is responsible to graphically depict locations for BMJS in the contract documents as required by Minimum Stucco Industry Standards. Provide complete details of the subassembly installation requirements, including fasteners, framing and blocking members, terminations, splices and intersection conditions. Minimum Stucco Industry Standards do not require or suggest that stucco craftsman be responsible to locate these joint subassemblies.
· Typical building structure substrate support movement locations can be anticipated such as at beam and floor deflection slip tracks, beam deflections, interstory drift joints, seismic joints, etc.
Stucco Best Practice: Strategically locate BMJS at the exterior stucco wall cladding system at the following locations:
· Terminate and butt splice BMJS only at intersections with other stucco movement joints or exposed lath accessories.
· Avoid dead end terminations and butt splices along stucco panel edges away from stucco panel corners.
Stucco Best Practice:
· At horizontally-oriented BMJS locations on exterior weather exposed walls, use the Pilz design BMJS lath accessory with a sloped drainage surface, and subassembly and integrate the extended flashing flange with the WRB.
· Locate butt splices at intersections with other lath accessories and avoid splices along stucco panel edges
Stucco Best Practice:
· At vertically-oriented BMJS locations on exterior weather exposed walls, use the Washam design BMJS lath accessory and subassembly.
· Locate butt splices at intersections with other lath accessories and avoid splices along stucco panel edges
Minimum Stucco Industry Standard: At intersections of BMJS and SMJS, install the BMJS continuous through the intersection.
Minimum Stucco Industry Standard: Follow Sealant and Stucco webpage suggestions.
Stucco Best Practice: Follow Sealant and Stucco webpage suggestions.
Minimum Stucco Industry Standard: Recognize that stucco is a wall cladding system, not a roofing material, and stucco cladding with BMJS on low-slope, weather-exposed conditions is not an optimum solution.
Stucco Best Practice: Avoid stucco cladding at low-slope, weather-exposed surfaces. At low-slope weather-exposed conditions, provide durable materials appropriate for roofing conditions such as sheet metal copings and concealed waterproofing.
Stucco Best Practice: Terminate ornamental materials at either side of the BMJS so that building structure substrate movement is free to occur to minimize damage to the ornamental features and stucco wall cladding system. If visual continuity is desired, use resilient sealant of appropriate color, over backer rod to span the gap over the BMJS.
Stucco Best Practice: Recognize that the BMJS lath accessory and subassembly are not perfect solutions to portland cement-based plaster and stucco cracking, but they are the best solutions available. Use BMJS correctly, recognizing and accommodating their limitations for maximum effectiveness. Seal the separation gap, splices, intersection and transitions watertight.
The following detail drawings are diagrammatic depictions of BMJS configurations, in horizontal and vertical orientations on an exterior building wall, but are not sufficient enough in detail to be construction documents. Their purpose is merely to diagrammatically illustrate the relationships of various essential components of a functional BMJS.
Weather-exposed wall condition
Weather-exposed wall condition
(1) ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
(2) NAAMM Standard EMLA 920-09, Guide Specifications for Metal Lathing and Furring, Fifth Edition, National Association of Architectural Metal Manufacturers, 800 Roosevelt Road, Glen Ellyn, IL 60137
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