Jeff Bowlsby CCS, CCCA
Exterior Wall and Stucco Consultant
Licensed California Architect
Architectural Aesthetics and Stucco Technology
Stucco is one of the few building exterior cladding materials with the innate capability of expressing the full range of architectural geometries and form. From linear and planar, to articulated and faceted, to dynamic, curvilinear, fluid building forms, and from visual solidity to visual segmentation. Stucco cannot be all things to all projects though, from a technical perspective, and attempts to do so are usually futile or at least less that completely successful. Use stucco in technologically appropriate ways, respecting stucco’s functional requirements to most effectively achieve aesthetic purposes.
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Masonry and concrete buildings throughout the world and throughout time into the present, have received exterior plaster cladding in part, as a functional means of weather protection. Stucco is most commonly directly-applied and continuously-bonded to masonry and concrete substrates creating a barrier wall, which eliminates the need for most joints, and creates a visual aesthetic of mass and solidity.
El Presidio de Santa Barbara
Santa Barbara, California, 1782
Ludwigsburg, Germany, 2005-2008.
Architect: Juergen Mayer H.
Framed building structures of wood and steel are preferred contemporary building structural systems in many parts of the world today, especially in the USA. Framed building structures are inherently more resilient than solid mass building structures. Greater flexibility from various loading conditions imposed result in more substrate support movement from a stucco cladding perspective, which obviously means more movement for stucco wall cladding systems which means more stucco movement joint subassemblies. Framed or framed and sheathed buildings must be protected with a concealed water resistive barrier, and then stucco as the outermost wall covering is applied over a mechanically-fastened lath as an exterior wall cladding, which provides a weather-exposed wall cladding assembly to protect the wall system. Stucco wall cladding systems installed over a water resistive barrier create a drainage wall system and are installed in panels defined by expressed joints to address the substrate support system and stucco membrane movements which creates a visual aesthetic with a panelized delineation. Stucco wall cladding system movement joints are a natural expression of the functional substrate support system and stucco membranes’ response to movement.
Whether wall substrate support systems are solid concrete or masonry barrier walls, or framed or framed and sheathed drainage walls, or a combination of both, sometimes architectural expressions requires curved, warped and fluid wall assemblies, and exterior stucco wall claddings are appropriate for those applications, as a result of the fluid nature of the portland cement-based plaster as it is applied.
Krzywy Domek (aka “Crooked Little House”)
Sopot, Poland, 2004.
Architects: Szotyńscy & Zaleski
A comparatively thin coating of stucco wall cladding is effective in protecting wall system substrate supports of masonry and concrete, or framing and sheathing. Stucco wall cladding is maintainable and renewable, and results in a desirable architectural aesthetic.
The characteristics of the aesthetic expression, primarily the inclusion or omission of stucco movement joints, are a function of the stucco substrate support characteristics which define the wall type as either a barrier wall or drainage wall system.
Concrete and masonry stucco buildings express a desirable aesthetic that is most often a continuous, seamless stucco surface that adheres to the solid substrate and conveys a sense of visual massiveness as a barrier wall. If the substrate support surface is irregular such as with rough masonry, the stucco can reflect that irregularity in the substrate support with integrity of expression.
Wall systems of framed, and framed and sheathed substrate supports and a water resistive barrier, create a drainage wall system, require an aesthetic expression that is most often segmented into an array of adjacent panels, defined by expressed joints, a desirable, but different aesthetic than when stucco is direct-applied to concrete or masonry. The joints accommodate substrate support movement, or movement in the stucco cladding itself. As a drainage wall, linear drainage flashing terminations are also expressed at the stucco cladding surface, as a drainage function of the concealed water resistive barrier system.
Because stucco is applied while in its plastic state, it easily conforms to whatever its substrate support geometry requires. Curves, spheres and distorted planarity are not significant challenges as they are with most prefabricated panelized claddings. Fantasy-themes, whimsy and irrational geometries can be realized for aesthetic effect.
Exposed concrete and mass masonry buildings of stone, adobe, brick are directly subject to the weather and due to their robust thickness and mass, water and air infiltration is of relative minor concern. The small dimensional size of masonry and stone units and the fluid nature of cast in place concrete, easily accommodate the full range of architectural geometries – from linear and planar, to curvilinear, even spherical. However, buildings constructed of these materials can develop cracking which can allow water intrusion, and because of direct weather exposure are subject to surface deterioration over time. Stucco cladding can resolve these potential issues.
Stucco cladding can be constructed without joints, but it needs to be direct-applied to a mass masonry or solid concrete building substrate supports as a barrier wall. No WRB is used in this assembly; the mass of the wall is the weather protection. Stucco applied in this manner is continuously bonded to its substrate support. Depending on the circumstances, this can be a durable method for applying stucco on a building. Not many buildings are constructed with mass masonry or solid concrete walls in the USA but they are common in other parts of the world.
Stucco on buildings of framed or framed and sheathed substrate support however is an entirely different circumstance. Stucco cracking at wall openings such as window and door corners has been problematic for stucco on framed buildings since the earliest use of stucco. At the time, stucco on wood lath and later metal lath, was applied continuously over all building substrate support surfaces without interruptions as if it were emulating stucco directly applied to masonry barrier wall buildings. The earliest causes of stucco cracking at wall opening corners on framed buildings were commonly regarded to be building movement and expansion of wood windows due to water absorption.
Framing components are linear and more easily used to produce linear and planar forms. Stucco on framed drainage wall buildings has behavioral characteristics profoundly different than stucco adhered to masonry and concrete barrier wall support substrates. For this reason expressed movement joints in the stucco cladding for minimizing the effects of movement in the substrate support and from shrinkage and thermal effects on the cladding, as well as expressing drainage flashings, are essential for stucco on framed and sheathed buildings to function properly, to minimize cracking and accommodate drainage from weather exposure.
Why is there such an aversion to expressed movement jointing in stucco? Virtually every other building cladding and surfacing material requires joints as an integral component of the system. Joints provide human scale, accommodate thermal movement, provide modularity, facilitate installation, provide locations for drainage, and more – all the same characteristics that benefit stucco cladding. Stone, tile, metal panels, wood siding, brick, CMU, precast concrete, GFRC, glass curtain walls…all have expressed jointing. By holding stucco cladding on framed drainage walls to the same aesthetic and performance expectations as stucco on concrete or masonry barrier wall buildings disregards what stucco needs and wants to be on frame buildings. We need a paradigm shift which requires expressing stucco movement joints as they need to be, and it is appropriate and beneficial that stucco movement joints be used for creative purposes and celebrated.
Attempts to intermix a panelized stucco application on a concrete or masonry barrier wall substrate support, or continuous stucco on a framed/sheathed drainage wall substrate support presents technical challenges and are best avoided. Stucco application to the same building with both concrete or masonry barrier wall substrate supports and framed/sheathed drainage wall substrate supports can be quite successful and the stucco cladding, although applied differently because of the different substrate supports, can provide a homogenous aesthetic if that is a desirable architectural intention. The two following images illustrate singular buildings with stucco both direct applied to a monolithic concrete barrier wall substrate support and onto framed/sheathed drainage wall substrate supports, resulting in a monolithic, homogenous aesthetic but each stucco wall cladding type functions differently and independently.
Marin County Civic Center
Marin County, CA 1959
Architect: Frank Lloyd Wright
450 South St. Parking Garage
San Francisco, CA, 2009.
Architect: WRNS Studio
(Photo used by permission of WRNS Studio)
Of critical importance, certain stucco finishes and jointing configurations demand special evaluation when stucco is used in non-vertical, non-planar or otherwise distorted geometries, and at weather-exposed low-slope conditions. Overlooking these limitations can reduce durability and increase maintenance. Stucco is best suited for vertical and near vertical exterior wall surfaces greater than 60 degrees from horizontal. Where stucco transitions into low slope, sky-facing surfaces…the functional equivalent of roofs…this stucco application should be avoided, or at the very least special considerations need to be made for waterproofing these conditions and integration with adjacent materials.
Stucco lath accessories and flashings are typically linear and rigid and may require special design and fabrication to accommodate curvilinear forms.
The essential stucco aesthetic question: Is the stucco cladding for your building intended to express a visually solid single mass or a system of discrete, adjacent panels with joints? The answer depends on and is determined by the wall system type based on how it functions, either as a barrier wall or drainage wall, onto which the stucco will be installed. Solid substrate support bases – concrete and masonry – are water-barrier (surface barrier or just ‘barrier’) walls to which stucco is preferably directly-applied, thereby eliminating stucco movement joints that are not in the solid substrate support base. Framed substrate support bases – studs and sheathing – are water-management (concealed drainage or just ‘drainage’) walls requiring lath over a water-resistive barrier, a stucco wall cladding system that requires expressed stucco movement joints at the stucco surface for drainage flashings, and joints to minimize the effects of substrate support movement, and stucco cladding shrinkage and thermal movements.
Stucco Best Practices:
· Stucco cladding materials should not determine or limit building form
· Understand, respect and address the technical considerations and challenges for using stucco cladding in every configuration
· For stucco direct-applied onto concrete or masonry barrier walls:
ü No concealed water-resistive barrier is required or suggested
ü No stucco movement joints are required except at structural substrate support movement joint locations (BMJS subassemblies)
ü Stucco is continuously bonded to concrete/masonry, minimizing the risk of cracks
ü Stucco can be installed onto any surface, vertical or low-slope, although slope is always important for drainage
ü Direct-applied stucco without high performance finish coats are best used at non-occupied spaces such as parking garages that can tolerate limited water intrusion
· For stucco applied to lath over a WRB on framed substrate support drainage walls, provide expressed stucco movement joints to accommodate:
ü For substrate support movement: BMJS and PMJS subassemblies (“expansion joints”)
ü For shrinkage/thermal stucco cladding movement: SMJS subassemblies (“control joints”)
ü For concealed water-resistive barrier drainage: Exposed drainage screed flashing terminations or drip edges
ü Although technically possible with special detailing, stucco on low-sloped sky facing, weather-exposed surfaces such as recessed windows, stucco wall caps should be avoided for performance and durability reasons.
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.