Jeff Bowlsby CCS, CCCA
Exterior Wall and Stucco Consultant
Licensed California Architect
Essential Stucco Principles
Stucco Principle 1: Stucco wall cladding is a System consisting of Assemblies, Subassemblies and Components
Stucco Principle 2: Stucco wall cladding is a complete and separate system from the WRB and drainage cavity, flashings, thermal insulation, and substrate support systems
Stucco Principle 3: All stucco is not the same
Stucco Principle 4: Stucco is for walls, not roofs
Stucco Principle 5: Stucco is dynamic not static
Stucco Principle 6: Architecture, construction type, occupancy, aesthetics, or geographic location do not determine stucco requirements
Stucco Principle 7: A stucco movement condition determines the stucco movement joint assembly, subassembly and component required
Stucco Principle 8: Stucco lath accessories are not stucco subassemblies
Stucco Principle 9: Use of stucco movement joint subassemblies are no guarantee against cracking
Stucco Principle 10: Understanding the distinction and importance of a Weather-Exposed Surface is critical to designing, constructing and inspecting stucco wall cladding Systems, Assemblies Subassemblies and Components
Stucco Principle 11: Form and Function should be complementary, they are not adversaries
Stucco Principle 12: Stucco wall cladding does not exist in isolation
This webpage describes a few essential, generally applicable stucco principles that are not specifically identified as principles in stucco industry resources, and as such are not well understood or implemented. Essential stucco principles are not absolutes but are generally true in most circumstances.
Visit the StuccoMetrics Reference Archives webpage for cited references and further information.
Everyone one who has experienced, specified, constructed, inspected or evaluated stucco has an opinion about stucco – what it is and should be, how it is designed and constructed, how it should perform, how it should look, etc. Independent of the multitude of factors and considerations that affect and determine an opinion regarding stucco, what are the basic and essential principles that are universally true about stucco, in most conditions and circumstances that we can use to guide us in how stucco is designed, how it relates with other exterior wall cladding systems, assemblies, subassemblies and components, how it is constructed, how it performs and is evaluated?
Without a mutual baseline understanding and agreement on a set of basic essential stucco principles, general consensus within the stucco industry will be difficult to achieve. The stucco industry needs general consensus on essential stucco principles as a common ground, to have the capacity of moving forward to address the new and increasingly challenging stucco issues we face daily, and into the future.
Without a mutually agreed upon basis of essential stucco principles, we could make decisions and form opinions based on false conclusions such as:
· A “stucco cladding system” is an entire assemblage of all components outboard of the framing and sheathing
· Stucco stands on its own and is independent of other building conditions
· Because stucco variants look the same, behave the same, feel the same … they are the same
· Stucco can be used anywhere on a building exterior with the same results
· Stucco is inactive and inert
· Stucco functional requirements and performance characteristics are variable and determined by the buildings architectural vocabulary, the use or occupancy of the building, the desired aesthetics, or the buildings regional location
· Stucco joints are mostly decorative because all stucco cracks anyway
· Stucco accessories can be used in different ways with little impact on stucco performance
An understanding of essential stucco principles can be a guide for determining how stucco is specified, detailed and constructed, how stucco performs, and how stucco is evaluated when scrutinized. Stucco principles are an important baseline for knowledge about stucco.
Stucco Principle 1: Stucco wall cladding is a System consisting of Assemblies, Subassemblies and Components
The good ole days when stucco was just a plaster trowelled onto a masonry base are long gone especially when it comes to stucco on lath on a framed substrate support. It is helpful to consider contemporary stucco wall cladding as an overall System, segregated and modularized into multiple, separate but interrelated Assemblies, Subassemblies, and Components functioning together that when combined, resolve to create a singular stucco wall cladding System, as follows:
· A Component is a separate individual part in isolation. Components have no function apart from their contributing function when they are integrated into a Subassembly.
è Components are items such as a weep screed drainage flashing, a corner bead lath accessory or a reinforcement fabric.
· A Subassembly is a separate individual configuration of multiple Components that when combined together create a functional Subassembly. Subassemblies perform functions such as a foundation drainage function, an external corner plaster reinforcement function or a crack minimizing function.
è Individual Subassemblies can perform several different functions simultaneously. As examples, a Soffit Drainage Subassembly (SDS) provides both a wall drainage function and stucco thickness control function. A Shrinkage Movement Joint Subassembly (SMJS) provides a shrinkage and thermal movement control function, a stucco thickness control function and a plaster cold joint elimination function.
· An Assembly is the configuration of multiple individual and related Subassemblies combined together to perform a function. Assemblies perform functions such as a wall drainage assembly, a building movement control assembly, a shrinkage and thermal movement control assembly, a finish assembly, etc.
è For example, a wall drainage assembly may include various sealed trim subassemblies, a continuous fabric lamina basecoat subassembly, a concealed WRB and drainage cavity subassembly and a Foundation Drainage Subassembly. In this example each subassembly either contributes to minimizing water entry into the stucco cladding system, managing water once entered or diverting water to drain out from behind the stucco cladding system to create an effective wall drainage assembly.
è Another example is that an effective shrinkage and thermal movement control assembly is not based on a singular Shrinkage Movement Joint Subassembly (SMJS). Its effectiveness to the stucco wall cladding system is based on the combined functionality of multiple Shrinkage Movement Joint (SMJS) subassemblies working together, which depends on the range of individual SMJS locations, materials, geometric configuration and panel sizes created between adjacent SMJS.
· A stucco wall cladding system consists of the combination of various Assemblies, Subassemblies and Components, where each performs separate functions, and which combine to function together to create an overall stucco wall cladding system.
A stucco exterior wall cladding assembly consists of lath and lath accessories (when stucco is on a framed substrate support), portland cement-based stucco base coats and a finish coat.
Although related to the stucco wall cladding system, the WRB and flashings, air barrier, drainage provisions, and thermal insulation serve different functions in a stucco wall cladding system, are separate systems from the stucco wall cladding system and may be designed, specified, installed and evaluated by different entities other than the stucco craftsman. The WRB and its flashings, drainage provisions and insulation should be envisioned, designed, specified, installed, inspected and evaluated as continuous and discrete systems from, but selectively integrated with, the stucco wall cladding system.
Broadly speaking an exterior stucco wall cladding system is, depending on a variety of conditions, a series of different materials, configured as assemblies, subassemblies and components that serve different purposes, over a substrate support. Each layer, material, assembly, subassembly and component has something beneficial to offer the greater stucco wall cladding system. The characteristics of each layer, material assembly, subassembly and component in the stucco wall cladding system, determines the performance and aesthetics of the specific stucco wall cladding system. Stucco wall cladding systems can be assembled with almost innumerable variations to create a variety of different systems that function in different ways, with different aesthetic characteristics.
Stucco consists of a broad family of assemblies, subassemblies and components that can be combined in various ways to create a range of stucco wall cladding systems, each with different functional and aesthetic characteristics. With stucco there are many alternatives to creating effective stucco wall cladding systems, some with definite absolutes, and an exception or two can usually be found for every condition. With no other qualifications indicated, to remain objective, general discussions about stucco can only assume minimum quality stucco systems and conditions such as non-corrosive environments, cement finish coats or basic portland cement-lime/sand mixtures without fibers or admixtures. Commentaries offered are generally applicable suggestions and not recommendations which can only be made in the context of a specific stucco work, are not absolutes, and may or may not be applicable to all circumstances. Consultation with licensed and experienced stucco professionals is recommended in any stucco endeavor for recommendations most applicable to a given circumstance.
Stucco is a durable portland cement-based exterior wall cladding material and system but every construction material and system has limitations. Stucco is most appropriate for vertical and near vertical wall surfaces that are not subject to hydrostatic pressures or significant dirt accumulations. Roofs include weather exposed surfaces that are below 60 degrees from horizontal, are sky facing and subject to far more extreme environmental conditions than vertical and near vertical surfaces, due primarily to greater water, thermal and ultraviolet exposure along with the additional effects of water accumulation and gravity forces. Walls are not roofs and the use of a stucco system should be avoided on roofs or roof like surfaces such as the tops of parapet walls or recessed window sills for optimum stucco performance, durability reasons. The portland cement stucco binder erodes over time in the presence of flowing water and durability is reduced with stucco finishes and joints in low-slope conditions.
Portland cement binder erosion, aggregates exposed.
This is not to say that stucco has not been used on low-slope, weather exposed surfaces and when it has, special considerations, conditions and accommodations have been made.
Stucco cladding as office building roof
Santa Clara, CA
Stucco cladding with “control joint”, at parapet wall cap.
Parapet wall below is solid concrete, framed parapet cap is steeply sloped
San Francisco, CA
Stucco cladding as parapet wall cap
A stucco wall cladding system is, as is the case for all building materials, a dynamic cladding from the moment it is applied. Portland cement shrinkage causes volume loss resulting in stucco movement. Ambient temperature variations cause thermal expansion and contraction resulting in stucco movement. Building substrate support segments may include dynamic joints that segment the building for building movement purposes, resulting in stucco movements. Stucco movements need to be accommodated or cracks will occur.
To understand stucco behaviors, one must understand its inherent dynamic behavioral characteristic of shrinkage as it cures and hardens, its behavioral characteristic of expansion and contraction resulting from variations in ambient temperatures while in service, and building substrate support movements that affect the stucco wall cladding system.
Stucco consists of three primary components when on framed substrates: stucco mortar made primarily of sand aggregate and portland cement (and water), and lath and lath fasteners. For a typical portland cement-based plaster mixture consisting of a 1:4 cement:sand ratio, that plaster is 80% sand and 20% cement. Combining the components creates a new composite material, forming a monolithic, homogenous, cementitous membrane with an embedded foraminous lath that is mechanically fastened to the support substrate. Shrinkage occurs with water volume loss in the cement paste only, the aggregate and lath does not shrink, they are just along for the ride. The lath, lath fasteners and aggregate are restrictions to shrinkage and thermal movements. The new composite material possesses a new set of physical and performance characteristics which is the resultant combination of the physical and performance characteristics of its individual interrelated components.
As a brittle material, a stucco wall cladding system is subject to either comparatively small or large movements. All movements affecting stucco must be accommodated or cracking will occur. Small movements include shrinkage and thermal movements, and are accommodated with any stucco movement joint (SMJS, PMJS or BMJS) subassemblies because the lath terminates at these joint subassemblies. Larger movements can only be accommodated by PMJS or BMJS subassemblies located at the substrate support movement location, because the substrate support is separated and discontinuous at these stucco movement joints.
At weather-exposed soffit corners some would rather not use a Soffit Drainage Subassembly (SDS) because it exposes a metal drip edge which some may consider aesthetically objectionable. Instead they will just use external corner reinforcement which creates a stucco bucket or water trap condition that can cause long term concealed deterioration of the substrate support. An aesthetic victory at the expense of function and serviceability. Others suggest that stucco movement joint subassemblies are not needed on residences or small buildings or in certain geographic locations, or that they are only needed on commercial and larger buildings, as if the inherent characteristic of portland cement-based plaster shrinkage and stucco thermal movement is somehow determined by the building occupancy, or by the city the stucco installation is located in. Various factors such as the architectural style, commercial vs. residential, warehouse vs. hospital, English cottage style vs. contemporary are in some venues suggested criteria used to determine when/where or even if stucco movement joint subassemblies are appropriate. Still others question the need for Foundation Drainage Subassemblies (FDS) in arid locations. In reality the functional requirements for stucco movement joint subassemblies, and drainage subassemblies etc., as defined in ASTM C1063 wherever it is in effect, applies to all stucco wall cladding systems on framed building structures over a WRB, should not be considered as optional because they provide functional solutions to potential functional problems. Architectural style, construction type, occupancy, aesthetics, or geographic location is for the most part, irrelevant criteria for determining stucco functional requirements.
Stucco movement joint assemblies, subassemblies and components are arguably the most misunderstood subjects in stucco since the time of their inception in the 1950’s. In part the difficulties relating to stucco movement joint assemblies, subassemblies and components are related to terminology, but also to a misunderstanding or lack of knowledge about the purpose and functional requirements and capabilities of stucco movement joint assemblies, subassemblies and components.
Some struggle to understand the various different stucco movement joint subassemblies - if, when and why they are needed, and their requirements for the assembly to function correctly. Stucco movement joint subassemblies and their requirements to function correctly, are not difficult to understand if one understands the various mechanisms of stucco movement. The key to understanding stucco movement joint subassemblies is in understanding a buildings’ stucco substrate support movement and the stucco/lath composite material movement characteristics. Without a clear understanding of why and how these movements occur, and how they relate to and affect stucco performance, confusion, misunderstanding and disagreements will persist, and stucco performance will suffer. And stucco cracking will continue. And building owners will be disappointed. And stucco wall cladding may actually disappear from the marketplace if we as an industry do not address and resolve the issues, and design and construct stucco correctly. Let us get on the same page, and agree that stucco movement subassemblies can be a useful counteraction to stucco loading conditions.
To understand the resultant movements, a review of building support substrate and stucco material loading conditions causing those movements is in order. Solid mass masonry or mass concrete building structures as a stucco substrate support imparts movements of very small dimensional magnitude. Solid mass masonry or concrete buildings are relatively stable as a substrate for stucco wall cladding, do not deflect significantly with vertical or lateral loads, shrinkage has already occurred when the stucco is applied and they do not move significantly with ambient thermal variations. Portland cement-based stucco has thermal coefficients similar to or identical to solid mass masonry or concrete support substrates, so when stucco is directly-adhered to solid mass masonry or concrete building substrate supports, the stucco continuously bonds with the substrate support and has similar or identical behavior as the substrate support. As a result stucco cracking in these conditions is generally atypical. No substrate support movement in these conditions means no stucco movement joint subassemblies are required. If and when installed on solid mass masonry or mass concrete building support substrates, stucco movement joint lath accessories only perform a plaster ground screed (thickness) function, decorative function or work termination function and do not accommodate building substrate support movement, because there is no movement that occurs. Stucco movement joint subassemblies on solid mass masonry or mass concrete building substrate supports are only functionally necessary to accommodate substrate support movement, where an expansion joint in the building substrate support occurs.
Framed building structures as a substrate support for stucco experience movement and behave entirely differently than solid mass masonry or mass concrete buildings. Building structures and materials are exposed to and must accommodate various structural and physical forces (loading conditions) and their related deflections or movements to function as intended. 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 and wall panel racking. Portland cement-based plaster as one of the few exterior wall cladding materials applied as a wet material, experiences shrinkage movement as the plaster cures and hardens. Daily and seasonal ambient thermal variations cause expansion and contraction movements within all building construction materials including stucco that is exposed to daily and seasonal temperature variations. Each of these movements must be anticipated (designed) and accommodated (constructed) in some way or the building and its cladding material will not function as anticipated.
Building structural substrate support movements, resulting from loading conditions occurring either vertically or laterally, as a substrate support for stucco, are accommodated with BMJS or PMJS subassemblies. Stucco shrinkage movements and thermal movements are accommodated with SMJS subassemblies. BMJS and PMJS subassemblies also accommodate stucco shrinkage movements and thermal movements because the lath terminates at and is not continuous through these joint subassemblies, but shrinkage movements and thermal movements are not the primary function of BMJS and PMJS subassemblies.
Conflictingly, in the stucco industry the same term is often used confusingly to describe both a lath accessory component and a subassembly. If we learn to think in terms of creating subassemblies and assemblies and not just bolting component parts onto a wall, stucco quality, performance and durability will improve. Some examples include:
· A stucco “expansion joint” lath accessory is not a stucco BMJS subassembly
· A base of wall weep screed accessory is not a Foundation Drainage Subassembly
· An external corner reinforcement lath accessory is not an External Corner Subassembly ECS)
On Stuccometrics.com the terminology used refers to the overall stucco Assembly as a default, unless specifically indicating a lath accessory item. So the term a “control joint” is a subassembly, and a “control joint” lath accessory is the lath accessory component of a “control joint” subassembly. An unfortunate tendency in the stucco industry today is to view lath accessories as just Component parts to be bolted onto a wall, often without a complete understanding that lath accessories are one of several Components of functioning stucco subassemblies, and how those subassemblies are intended to function.
The number and diverse types and variables in stucco wall cladding systems, stucco movement joint subassemblies and installed conditions is large enough that it is impossible for design and construction professionals to design or assemble a perfect, flawless stucco wall cladding system. Notwithstanding that realistic fact, material and installation standards exist that define an achievable, minimum standard of quality for an exterior stucco wall cladding system. Where the characteristics of a given stucco wall cladding system satisfy Minimum Stucco Industry Standards, then the resulting system will perform acceptably, although perhaps not perfectly. Where the characteristics of a given stucco wall cladding system do not satisfy Minimum Stucco Industry Standards, then the resulting system may not perform acceptably. Exceeding Minimum Stucco Industry Standard requirements whether in design, materials or installation can be beneficial to an exterior stucco wall cladding system.
Stucco Principle 10: Understanding the distinction and importance of a Weather-Exposed Surface is critical to designing, constructing and inspecting stucco wall cladding Systems, Assemblies and Subassemblies and Components
The term Weather-Exposed Surface (WES) has been defined in the building codes as a guiding principle since 1967, and has remained virtually unchanged ever since. Those involved with designing, constructing and inspecting stucco wall cladding systems need to be intimately familiar with the definition as it applies to exterior building stucco wall cladding systems. WES informs decisions on where WRB membranes and drainage screeds are required and where they can be omitted. It implies the in-service weather conditions that the stucco wall cladding system may be subjected to, dependent on its location on a building.
Form and Function in architecture are eternally debated including in the context of a stucco wall cladding system. Perceived justifications that might seem to allow Form to trump Function are not advisable and short sighted. Fundamentally, a stucco wall cladding system on framed substrate support buildings require a range of characteristics such as drainage assemblies, movement joint assemblies, small dimension stucco panel areas and sealant-filled perimeter joints with adjacent dissimilar materials expressed at the surface. Stucco wall cladding systems installed on low-slope sky-facing Weather Exposed Surfaces is not a best technical solution for a stucco wall cladding system to be durable and serviceable long term. Stucco is one of the most versatile exterior wall cladding solutions for exploring aesthetic expression using the wide range of finish coat colors, textures and embeddable materials available, none of which preclude satisfying the stucco wall cladding systems functional requirements. If these and other functional necessities for the stucco wall cladding system are not acceptable, the best technical solution is to construct the building walls from solid masonry or concrete where these conditions are not essential. When using Form to express Function, the best design will view them as complementary - res ipsa loquitur - let the stucco speak for itself, let it be what it needs to be.
At the same time a stucco wall cladding system is both a subassembly of a larger building assembly (a building or building enclosure system), and a composite system of interrelated, multiple, and different smaller assemblies, subassemblies and components, each serving a specific purpose, assembled in specific configurations that function together to make a complete, functional, aesthetically pleasing exterior wall cladding system.
To understand a stucco wall cladding system on a framed substrate support building as a subassembly of a building and its enclosure system, one must understand the relationship of the stucco wall cladding Assembly to the building and its enclosure system – how do the adjacent systems interface? A stucco wall cladding system is stucco mortar on a lath that is fastened to the building substrate support by way of mechanical fasteners. Wind loads are imparted from the stucco wall cladding system to the substrate support framing. Each fastener functions to secure the stucco wall cladding system to the wall, but also contributes a risk of water intrusion, and restrains shrinkage and thermal movement which may cause cracks. At stucco panel perimeters, the stucco wall cladding system terminates and then another enclosure assembly, system or component continues on, which requires them to interface to create a continuous building enclosure. Continuity occurs primarily through overlapping waterproofing elements for continuity, integrated drainage flashings with the WRB and sealant-filled joints. Continuity of the building enclosure from one system to another is essential to the long term serviceability of the building enclosure system, which does not depend upon the termination of one system or another.
To understand stucco as a composite system of interrelated, multiple, and different smaller assemblies, one must understand the purpose and function of each assembly and what each contributes to the greater stucco system to make a whole. Stucco subassemblies include stucco movement joints (BMJS, PMJS and SMJS) stucco drainage (foundation, soffit and designated), stucco panel edges and corners, decorative joints, etc. Taken together, stucco assemblies need to both function independently and work together make a complete stucco system.
Stucco subassemblies can be deconstructed further. A stucco subassembly consists of interrelated, multiple and different components, but is not just a discrete lath accessory component itself. For example the Soffit Drainage Subassembly (SDS) is not just the soffit drainage screed lath accessory in isolation by itself. A Soffit Drainage Subassembly includes not only the soffit drainage screed lath accessory and its fasteners, but also the WRB, often SAF, lath and its fasteners, the adjacent stucco base coats and finish coat components – and most importantly, how they are interrelated and configured to fully function together as a stucco subassembly. Assembling all the correct components but putting them in the wrong relationship with each other may create a dysfunctional subassembly. For example if the lath in this subassembly does not overlap the solid vertical flange of the soffit drainage screed component but is located behind the screed flange, then cracking may occur, drainage over the screed may not occur, and this soffit drainage subassembly may not function as required. Stucco lath accessory components include items such as the foundation weep screed, external corner reinforcement, SMJ component lath accessory, casing bead, etc., each a component of a larger stucco subassembly.
These few essential, generally applicable stucco principles are easy to comprehend and implement, and if followed will benefit exterior stucco wall cladding systems.
Stucco Best Practices:
As manifestations of Stucco Principles:
· Stucco Principle 1: Recognize the interrelationships of individual Components, Subassemblies and Assemblies, and their functional contributions to the overall stucco wall cladding System.
· Stucco Principle 2: Recognize that a stucco cladding system is a complete and separate system from, but related to and selectively integrated with adjacent systems as WRB, flashings, drainage, insulation, and substrate support. Envision, design, construct, inspect and evaluate the exterior stucco wall cladding system as a combined system of discrete, but interrelated separate systems which includes the substrate support system and additional separate systems for the WRB and flashings, drainage, thermal insulation purposes as well as the stucco cladding system. These separate systems must function independently and to the extent they are interrelated, must function collectively as an entire exterior wall system.
· Stucco Principle 3: Recognize that all stucco is not the same. Envision, design, construct, inspect and evaluate stucco for what it needs to be or is, based on the particulars of a given circumstance.
· Stucco Principle 4: Recognize that stucco is for walls, not roofs. Stucco performs best on exterior vertical or near vertical walls and exterior soffit surfaces. Avoid locating stucco on weather exposed surfaces (WES) sloped less than 60 degrees from vertical.
· Stucco Principle 5: Recognize that stucco is dynamic and not static. Envision, design, construct, inspect and evaluate stucco in consideration of its inherent need to accommodate building substrate support movement, shrinkage and thermal movement.
· Stucco Principle 6: Recognize that stucco functional requirements are not determined by architectural style, occupancy, aesthetics, or geographic location. A stucco wall cladding system needs to accommodate various physical conditions and functions such as drainage, shrinkage and thermal movement, edge terminations, etc., which requires drainage flashings, movement joints, casing beads and sealant, each with aesthetic implications. A functional stucco wall cladding system will accommodate these functional requirements and make them an aesthetic feature rather than avoid as a detriment.
· Stucco Principle 7: Recognize that a stucco movement condition determines the stucco movement joint subassembly required. Understand the movement conditions at a specific stucco wall cladding system condition and implement the appropriate stucco movement joint subassembly to accommodate the anticipated movement(s).
· Stucco Principle 8: Recognize that a stucco wall cladding subassembly consists of more than just a stucco lath accessory component. Think of a stucco wall cladding assembly as a series of interrelated stucco subassemblies, rather than stucco lath accessory components. Lath accessory components can be mis-specified, mis-detailed, mis-constructed and if so, the stucco subassembly may mis-perform.
· Stucco Principle 9: Recognize that Minimum Stucco Industry Standards are minimum requirements that may not perform perfectly. Exceeding Minimum Stucco Industry Standard requirements can be beneficial to an exterior stucco wall cladding system.
· Stucco Principle 10: Recognize the in-service performance requirements of an exterior stucco wall cladding system, as determined by the definition of Weather Exposed Surface.
· Stucco Principle 11: Use Form and Function in the context of stucco wall cladding system, as complementary. Explore aesthetics but never at the expense of functional requirements.
· Stucco Principle 12: Recognize that a stucco wall cladding system does not exist in isolation. Be cognizant of how the stucco wall cladding system interfaces and integrates with other exterior wall claddings, fenestrations, penetrations, components, etc., and preserves the functional and aesthetic integrity of the greater building enclosure and exterior wall cladding system.
Consultation with licensed and experienced stucco professionals is recommended for stucco-related endeavors. No liability is accepted for any reason or circumstance, specifically including personal or professional negligence, consequential damages or third-party claims, based on any legal theory, from the use, misuse or reliance upon information presented or in any way connected with StuccoMetrics.com.