Jeff Bowlsby CCS, CCCA
Exterior Wall and Stucco Consultant
Licensed California Architect
Stucco Perimeter Movement Joint Subassembly (PMJS)
(“expansion joint” at internal corner)
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
PMJS are not mentioned in the building code or Minimum Stucco Industry Standards by name, but generically they are described as “perimeter relief” or simply as “expansion joints” at internal corner locations. In search of clarity I derived the term Perimeter Movement Joint Subassembly (PMJS) and use it on this website for easier recognition and intuitive understanding, and because it clearly describes the function of this stucco movement joint subassembly, and describes that it is a subassembly and not just a lath accessory. An understanding of the primary function of the PMJS, 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 PMJS, 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 plaster shrinkage movement during curing and thermal movements while in service and building substrate movements are significant factors that when combined, can contribute to stucco cracks. While shrinkage movement, thermal movement and building movement are important, it is recognized that other factors can contribute to cracking so addressing these movements alone, are not the only factors contributing to stucco cracking. This webpage focuses primarily on the issue of stucco shrinkage movement, thermal movement and building substrate movement and methods to minimize their contributory effects to stucco cracking.
The PMJS is a limited application, special condition stucco perimeter movement joint subassembly, not merely just the perimeter movement joint lath accessory component itself. While not as common as other stucco movement joints, the PMJS was the original form of the stucco “expansion joint” subassembly genre, and its development has taught us many things about stucco behavior towards minimizing stucco cracks. The PMJS creates an isolated stucco perimeter edge to accommodate stucco movement. A PMJS is required by Minimum Stucco Industry Standards to be provided for lath and stucco at internal corner planar transitions under the following conditions:
· When an internal corner is oriented vertically, horizontally or in any direction and,
· Where each adjacent planar substrate support is of different materials, or
· Where each adjacent planar substrate support is of different loadbearing characteristics
Stucco cracks related to internal corners at the transitions of substrate support materials or transitions in substrate material loadbearing conditions can be caused by stucco substrate support movement, stucco shrinkage movement and stucco thermal movement. More specifically, these conditions occur where continuous stucco cladding is installed at internal corners such as where a framed, furred or suspended ceiling adjoins a concrete wall or concrete column (transition of substrate support materials and transition of substrate support loadbearing/non-loadbearing conditions) or at a vertical wall internal corner where a framed wall adjoins a concrete or masonry wall (transition of substrate support materials). Minimum Stucco Industry Standards require that at substrate support material and loadbearing condition transitions that the lath is terminated and fastened to the substrate support and stucco is terminated at either side of the internal corner and both are discontinuous through the PMJS subassembly gap. Under these conditions, a PMJS accommodates the differential movements occurring at each adjacent plane at an internal corner and relieves stresses to minimize stucco cracks. This webpage explores the conditions that make the PMJS 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.
Patent research: No patented PMJS lath accessory components are known to be patented.
In the 1940’s the common stucco lathing practice was to install lath continuously over all substrates, which articulated continuously over and around all planar transitions. The belief was that continuous lath provided a continuously rigid, restrained lath structure for continuous stucco, conceptually similar in function to a solid base that was thought to minimize any potential movements and reinforce the stucco to minimize cracks. A typical example of this practice was stucco continuously and directly-applied onto a vertical load-bearing concrete wall which then transitioned through a wall-to-ceiling juncture, onto a horizontal suspended grillage/lath ceiling, where the ceiling lath was intentionally extended onto and secured to the adjacent wall surface.
From Metal Lath Specifications
Metal Lath Manufacturers Association, 1946
As described for us in the 1947 ACI Journal article Crack Control in Portland Cement Plaster Panels(2), a major stucco installation was constructed at the Grand Coulee Dam construction project that featured direct-applied stucco installed onto concrete walls which then continued uninterrupted onto adjacent suspended grillage/lath ceilings, where ceiling lath was extended onto and secured to the adjacent walls. The installation experienced significant and unacceptable cracking at the stucco on suspended metal lath ceilings. Extensive investigation and testing was performed by the Bureau of Reclamation to evaluate a long list of possible causes, to determine the primary cause of the ceiling cracking and to develop a solution. The primary conclusion was that the ceiling cracking occurred because the lath and stucco at the ceiling perimeter was attached to and restrained by the attachment at the perimeter concrete walls, and therefore the ceiling stucco subassembly was not able to accommodate the stucco shrinkage movement. Not mentioned in the article, other contributory causes of ceiling cracking for this condition likely included stucco thermal movement, differences in stucco behavior at the transition between different support substrate materials (grillage/lath to concrete), and differences in support substrate loadbearing conditions at the transition between loadbearing to non-loadbearing conditions (suspended ceiling to concrete wall) and potentially other contributory causes.
The primary solution derived a method of application that included eliminating “restraint at all edges of the plaster slab, allowing shrinkage to take place without stress development and the attendant cracking.” Discontinuing the ceiling lath at its perimeter and not securing it to the wall, detached the mechanical attachment of the ceiling lath from the perimeter concrete walls which eliminated the perimeter lath restraint condition, prevented the stucco from bridging across the transition at the ceiling to wall internal corner location, and created a visible joint line in the stucco at the transition, but the joint line was not objectionable. This isolation solution also addressed the potential causes of cracking from thermal movement, from the differences in the stucco behavior at the transition between different substrate support materials (grillage/lath to solid concrete), and from differences in substrate support loadbearing conditions at the transition from loadbearing to non-loadbearing conditions.
After reconstructing the suspended ceiling as an isolated subassembly from the perimeter concrete walls, no ceiling cracks occurred, and the ceiling perimeter shrinkage gaps measured 3/4 in. of total shrinkage over its 52 ft. length.
Grand Coulee Dam ceilings - Restrained lath and stucco perimeter
Condition created by continuous lath at ceiling-to-wall internal corner
that caused ceiling cracks
From Crack Control in Portland Cement Plaster Panels,
by Bert Hall, American Concrete Institute Journal, Vol. 19, No. 2, October 1947
Figure reproduced with permission from the American Concrete Institute
Perimeter Movement Joint Subassembly (PMJS) (lath and stucco not restrained at the perimeters) at a ceiling-to-wall internal corner, which eliminated ceiling cracks. Note the transition between different substrate support materials (suspended grillage/lath to concrete) and transition between different substrate support loadbearing conditions (non-loadbearing to loadbearing).
From Crack Control in Portland Cement Plaster Panels,
by Bert Hall, American Concrete Institute Journal, Vol. 19, No. 2, October 1947
Figure reproduced with permission from the American Concrete Institute
The Grand Coulee Dam ceiling shrinkage event was not an isolated occurrence. A similar circumstance was reported in a 1985 article in AWCI’s Construction Dimensions magazine, Controlling Shrinkage in Portland Cement Plaster(3), at a new Chicago parking garage ceiling at the first level of a new office tower. In a ceiling installation similar to Grand Coulee but on a much larger scale, a 45,000 SF suspended grillage/lath structure supporting stucco was installed. This time the ceiling was not attached (restrained) to the inside of perimeter concrete walls that formed the building perimeter and the garage ceiling was also penetrated by support columns and fire sprinkler penetrations. At this installation, the perimeter of the stucco ceiling subassembly was constructed of casing beads aligned and butted to the perimeter concrete walls, but the lath was not attached to the walls, just to the suspended ceiling lath and its support grillage. Wide gaps developed at the building stucco ceiling perimeter juncture with the walls, and at the columns and fire sprinkler penetrations through the ceiling, attributed to stucco shrinkage movement at the ceiling. It was Grand Coulee déjà vu.
Unrestrained stucco ceiling shrinkage
Chicago parking garage, 1985
From Controlling Shrinkage in Portland Cement Plaster,
Construction Dimensions Magazine, September 1985
ASTM C1063 Standard Specification Installation for Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster(1:
· (188.8.131.52) Provide PMJS with 3/8-in. (9.5-mm) separation gap where furred or suspended ceilings and soffits adjoin penetrating elements such as columns, walls or beams. Discontinue lath through PMJS, cornerite not allowed.
· (184.108.40.206) 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.
· (220.127.116.11) Provide BMJS or PMJS aligned with expansion joint in substrate support.
ASTM C926 Standard Specification for Application of Portland Cement-Based Plaster(1):
· (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 18.104.22.168.
· (A22.214.171.124) 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.
· (A126.96.36.199) 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.
We learned significant lessons about stucco behavior and performance from the Grand Coulee Dam and Chicago parking garage ceilings related to the specialized instance of stucco cracking related to continuous stucco at internal corners. One lesson was that “the shrinkage coefficient of cement mortar or plaster is the basic element for consideration in crack eliminations.” Other lessons learned involve the various restraint conditions regarding lath and stucco at stucco panel internal corners including restrained substrate movement, restrained lath and stucco shrinkage movement, restrained stucco thermal movement, and that restrained movements may manifest as stucco cracking:
· Lath and stucco applied continuously over framed support substrates (without dividing the lath and stucco into discrete isolated panels), is prone to crack.
· Lath and stucco where continuous and attached at stucco panel perimeters, is prone to crack
· Lath and stucco applied continuously over transitions between different support substrate materials, such as from wood framing to concrete or from suspended steel grillage to concrete, is prone to crack.
· Lath and stucco applied continuously over transitions between different support substrate loadbearing conditions such as from non-loadbearing to loadbearing, is prone to crack.
We also learned about how a PMJS addresses perimeter restrained lath and stucco conditions, by isolating adjacent stucco planes at internal corners to minimize stucco cracking.
· A PMJS isolates adjacent stucco panels, isolates and accommodates stucco substrate movement, stucco shrinkage movement and stucco thermal movement and allows these movements to occur unrestrained because of discontinuous lath and stucco at stucco panel perimeters and internal corners, to minimize stucco cracks.
· Stucco cracks occur from restrained transitions between different substrate support materials such as between suspended grillage/lath to concrete, or between framing to concrete. A PMJS isolates the different adjacent stucco behaviors at the transition location between different substrate support materials, to minimize stucco cracks.
· Stucco cracks occur from restrained transitions between different substrate support loadbearing conditions such as between non-loadbearing elements to loadbearing elements. A PMJS isolates the different adjacent stucco behaviors at the transition location between the different substrate support loadbearing conditions, to minimize stucco cracks.
Minimum Stucco Industry Standards ASTM C926 and C1063 are referenced standards in the building code which state minimum installation requirements for PMJS lath accessories and subassemblies.
Where the substrate support material and substrate support loadbearing conditions are the same, and where the substrate support condition is continuous on either side of an internal corner, a PMJS is not required, however when installed may be beneficial in accommodating shrinkage to minimizing stucco cracking.
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. Shrinkage and thermal movements are accommodated with a 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.
The PMJS is a specific configuration of stucco and its components at internal corners including the associated substrates, lath, lath accessories, lath fasteners, lath accessory attachments, and stucco mortar that allow the perimeter edges of adjacent stucco panels to accommodate substrate, shrinkage and thermal movements including at transitions of substrate support material or substrate support loadbearing conditions. PMJS for internal corners are “expansion joint” subassemblies to accommodate movements from differential substrate support conditions that need to be accommodated to minimize stucco cracking. The need for and development of PMJS at internal corners of planar transitions was recognized before and preceded the development of both SMJS and BMJS for continuous wall surfaces and yet share similarities with both in addressing stucco and substrate movement conditions that cause cracks.
Horizontal PMJS subassembly at building entry soffit at interface
with curved wall and round column
Horizontal PMJS subassembly at arcade soffit at interface with square columns
Minimum Stucco Industry Standards provide limited but important and useful basic information on PMJS. Minimum Stucco Industry Standards for PMJS are defined in the building code which includes reference standards ASTM C926 and C1063. In the codes, requirements for the PMJS are described generically as “expansion joints” but the term “PMJS” is not named.
The earliest and simplest form of a PMJS is jobsite fabricated from two adjacent casing bead lath accessories, separated by a gap to accommodate movements. The gap width dimension between the casing beads, should be 3/8 in minimum where sealant is required, and shall be determined by the amount of movement anticipated and may be filled with a resilient sealant for aesthetic or water management purposes. To function correctly, a PMJS requires discontinuous substrate support materials, discontinuous lath and discontinuous stucco to create an isolated, unrestrained condition, where the lath edges are fastened to framing or blocking on either side of the PMJS to allow for maximum movement potential at the PMJS. The conditions requiring the PMJS itself are described, although the PMJS is not specifically named, in industry reference documents since the 1971 ANSI A42.3 and is described in Minimum Stucco Industry Standard ASTM C1063 today, so the need for the PMJS and its requirements are not new.
Where the PMJS is located at a weather-exposed surface (WES), a flexible barrier membrane, continuous WRB and sealant in the gap are required. At non-weather-exposed surfaces the sealant may be optional, serving an aesthetic function only. PMJS lath accessories also function as stucco thickness control screeds and temporary work stoppage locations which prevent cracks caused by stucco cold joints.
Adjacent casing beads are the predominant PMJS lath accessory(s). One specialized PMJS lath accessory is manufactured but detailed PMJS lath accessory engineering technical and performance information is not available for this lath accessory. No industry product standard exists that specifies the engineering, performance and technical aspects of the PMJS lath accessory adding to the challenges for designers and installers to accurately be informed on how to use the lath accessory and integrate it into stucco wall cladding system. Lath accessory manufacturers are encouraged to either develop a generic industry product standard for PMJS and fabricate their lath accessories to conform to the standard or for their proprietary products at least, provide complete technical and performance information indicating physical dimensions, materials, fastening and attachment requirements, splicing methods, termination and intersection methods, movement capabilities and corrosion resistance properties of the lath accessory and installation, for design and installation reference.
The design authority must clearly describe the requirement for back-up framing or blocking and required fasteners in the construction documents, at PMJS locations, so that lath edges at discontinuous lath can be fastened to framing.
Isolating stucco wall cladding panels using PMJS located in certain locations such as where columns or loadbearing elements penetrate the stucco cladding or non-loadbearing elements, configured in a certain way to accommodate this differential movement, sealed at critical junctures to effectively manage the effects of water, create a Perimeter 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 PMJS lath accessory component or PMJS subassembly in isolation can accomplish the effect of the Perimeter Movement Joint Assembly as a whole on the stucco wall cladding system.
Follow product manufacturer’s additional recommendations when using their products (Manufacturer’s instructions for use of their products).
PMJS are not well understood and are an occasionally overlooked stucco “expansion joint” subassembly.
The real world testing performed and lessons learned from the Grand Coulee Dam and Chicago parking garage projects demonstrated that continuous lath and stucco, at the specialized condition of internal corner locations where bridging over transitions in substrate support materials and transitions in substrate support loadbearing conditions, creates restrained lath and stucco conditions that do not accommodate stucco shrinkage and thermal movements, which may result in stucco cracks. The Grand Coulee Dam ceilings forever made continuous lath and stucco, at internal corner conditions at transitions in substrate materials and transitions in loadbearing conditions, an obsolete practice. At the Chicago parking garage ceiling, where the perimeter lath and stucco at the perimeter wall juncture was not attached (not restrained) it was an illustration of the severity of effects of stucco shrinkage and thermal movements that are natural to the material and can cause cracks. The wide gaps that developed at the unrestrained perimeters and penetrations of the stucco ceiling, while unanticipated, displayed the nature of stucco on lath to shrink.
Minimum Standards of Care:
· Provide PMJS at locations and conditions where substrate movements occur at internal corners – column penetrations at soffits, substrate support material transitions, etc.
· PMJS lath accessories can be fabricated by installing casing beads adjacent to each other with a 3/8 in separation gap, or a specialty 2-piece prefabricated lath accessory. Either subassembly must be constructed with discontinuous lath and stucco at the PMJS location to isolate adjacent stucco panels, and accommodate movement to minimize stucco cracking.
· Where the PMJS occurs at weather-exposed surfaces, provide a flexible barrier membrane behind the PMJS and sealant in the separation gap and at exposed terminations and splices to prevent water entry.
· The design authority is required to determine PMJS locations, select lath accessories including determining ground depth, determine installation requirements and depict them on the construction documents. 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 that the stucco craftsman be responsible to design or locate these movement joint subassemblies. The following two example details are for conceptual information only, require additional supplemental details for conditions such as PMJS terminations, articulations and splices, and to meet specific application requirements.
Stucco Best Practice:
PMJS lath accessory manufacturers should provide engineering technical
documentation for dedicated PMJS lath accessories and for arrangements of
casing beads used as PMJS lath accessories, describing their products:
· Physical dimensions
· Material properties
· Detailed and comprehensive installation requirements if not explicitly stated in ASTM C1063 such as attachment, joinery and splicing methods, fastener requirements, conditions and requirements for sealant at terminations, joints and splices, etc.
· Performance characteristics: Movement capabilities and limitations when installed into stucco as a PMJS
The following detail drawings are diagrammatic depictions of PMJS 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 PMJS.
Weather-protected soffit condition
Weather-exposed wall condition
(1) ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
(2) Crack Control in Portland Cement Plaster Panels, Bert Hall, Journal of the American Concrete Institute, Vol. 19, No. 2, October 1947
(3) Controlling Shrinkage in Portland Cement Plaster, John Boland, Construction Dimensions Magazine, September 1985
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