December 2, 2015
By William Spilchen, P.Eng.
The combination of steel and portland cement stucco has been successfully used in various climates and in applications across the country. Properly specified and installed with a drainage plane, this combo provides a fire-resistant and non-combustible cladding, making it suited for multi-family and other high-rise structures.
Stucco cladding has many attributes and benefits for architectural design, including the versatility to create various shapes and forms as well as curved walls, arches, and other ornamental configurations. Additional benefits of cost-effective stucco wall systems include:
However, these positive attributes cannot be achieved without attention to the proper design and application of the stucco system. Stucco plaster is a mixture of approximately 80 per cent sand, portland cement, hydrated lime, and the appropriate amount of water. Volumetric standards have been developed that regulate the amounts of each ingredient—specifically ASTM C926, Standard Specification for Application of Portland Cement-based Plaster, and National Building Code of Canada (NBC) Article 9.28.6, “Stucco.” Additionally, the quality and cleanliness of the sand is important.
For improved control of the ingredients, some companies provide pre-bagged stucco mixes, even with the sand (dried). Therefore, quality and ratio of materials is controlled by a manufacturer rather than by jobsite mixing and blending that could vary for numerous reasons. On larger projects, silo systems are now available to be set up onsite. Each of the ingredients is automatically fed into a mixer as prescribed. Other additives such as fibres, plasticizers, and other proprietary admixtures are available for improved performance, along with reduced shrinkage and the resulting, decreased cracking.
Much has been learned over the last decade regarding the importance of water-resistive barriers (WRBs) and flashing. Stucco is the primary barrier to the elements, but careful attention is required to prevent water from entering in behind the stucco itself. Water will not readily pass through the stucco, but finds paths around it—into junctions where the stucco stops and abuts another wall element such as windows, doors, or control joints.
Behind the stucco is a secondary, hidden barrier (i.e. a drainage plane); this includes the proper installation of air barriers, vapour retarders, and flashings around penetrations such as windows or doors. Generally, these flashings are the various peel-and-stick membranes available. Detailed instructions are accessible on proper application and shingling, especially at the bottom with the WRB, which would be applied later. Correct application and shingling of the WRB is critical. Appropriate metal flashings are required specifically at the tops of parapet walls. On multi-family buildings, through-wall flashings at each floor level are also recommended.
The appropriate metal flashings would consist of combinations of:
These dimensions are similar to through-wall flashings. The other critical metal components are parapet cap flashings and the correct installation of required control and expansion joints.
The WRB should be applied in two layers as double layers or half laps. These may be asphalt paper—preferably 30- or 60-minute rating—or building wraps. With the latter, the first layer should be a wrap, while the second layer can be an asphalt paper. The purpose of the two layers is to provide a bond-breaker with the layer in contact with the stucco. This allows unimpeded passage of incidental moisture to drain downward between the two layers.
Steel and lath
In Canada, a rainscreen cavity is required—a 10-mm (2/5-in.) interior drainage plan (at minimum) per NBC requirements to dispose of any water migrating behind the stucco. This physically creates an open drainage space to provide for air and vapour movement to achieve drying. The reason for rainscreens is the result of leaky condominiums in British Columbia. Due to high rainfall, much of which could be wind-driven, drainage and drying was inadequate during the annual cycle, which has led to building deterioration, saturated insulation, and subsequent mould problems. The only assured design was to incorporate the rainscreen to offer drainage and drying.
The metal lath is the next critical element of a sound stucco wall. Being a cementitious material, stucco is strong in compression, but weak in tension. Therefore, similar to concrete, steel reinforcement is required to provide the tensile strength.
Lath generally consists of galvanized welded wire in either 51 x 51-mm (2 x 2-in.) grids formed by 16-ga. wires (i.e. 1.5 mm [0.06 in.] in diameter), or 38 x 38-mm (1.5 x 1.5-in.) grids formed by 17-ga. wires (i.e. 1.3 mm [0.05 in.] in diameter). The lath must be a self-furring type and hold the body of the lath not less than 6 mm (0.2 in.) away from the substrate.
There are expanded metal laths also available, but these are not as common in Canada as they are in the United States. In Canada, most stucco is installed with the use of welded wire laths. Expanded metal lath is more often specified for usage with tiles and manufactured stone. Proper installation of the lath requires it be pulled taut and lay flat. Fastening must be with code-specified, corrosion-free fasteners—this usually means roofing nails, but they may also be construction staples or screws. The fastener should be located at framing members and penetrate 25 mm (1 in.) into the framing. The fasteners’ spacing is required to be spaced 150 mm (6 in.) vertically when framing is 400 mm (15.7 in.) on centre (oc) horizontally, or 100 mm (3.9 in.) vertically when framing is 600 mm (23.6 in.) horizontally.
The lath normally undergoes a hot-galvanized process to prevent corrosion. It is also protected with embedment within the stucco that is highly alkaline—a pH in the 10 range. Metals will not corrode in a high-pH environment. There is a small exception to this rule where carbon dioxide (CO2) from the atmosphere will cause carbonation. This will slowly reduce the alkalinity in the stucco surface, but only to a depth of 3 mm (1/8 in.). This is the reason for instructions on lathing products, especially stucco corner beads indicating a minimum embedment of 3 mm is required.
There are numerous suppliers of each type of lath, including the welded wire laths and the expanded metal types. The former is normally available in rolls that are 1200 or 1350 mm (48 or 54 in.) in width; while length varies, they are 60 m2 (450 sf) per roll.
Some lath features longitudinal wires that are partially flattened. This provides an important safety feature for lathers where the lath will remain flat when unrolled, and have the risk of unexpectedly rolling back, causing potential injury. Further, the wires are strengthened by the flattening process, and the manufacturer claims a resulting decrease in cracking with their use.
Application of the plaster
The first coat is called the scratch coat and should be nominally 9 mm (3/8 in.) The second coat, called the brown coat, should also be nominal 9 mm for a total of approximately 19 mm (3/4 in.).
There are two accepted methods for applying the scratch and brown. The traditional method is to do the scratch on one day and then the brown either the next day or later. The second method is to scratch and double on the same day, and wait until the first coat sets up adequately to provide support for the second coat. The theory of this second method is less moisture is lost, resulting in less overall shrinkage.
Depending on weather conditions, moist curing is required. (This would generally be hot and windy conditions.) Moist curing enables the cement materials to hydrate properly and the stucco membrane to reach its desired physical properties and to reduce shrinkage, which in turn can lead to cracking.
The finish coat can be conventional cementitious coat or an acrylic finish. There are many manufacturers of these coatings and application recommendations should be followed. Normally a period of seven days is required before application of the finish.
The application of simulated or manufactured stone veneer has grown dramatically over recent years. Since the manufactured stone is not an approved cladding, the approved cladding is intended to be a 19-mm (3/4-in.) thick stucco layer behind the stone. However, this is not a case where the stucco can be the primary barrier since it does not have a finish to resist water passage. Therefore, as water enters through the stone work joints, it becomes trapped and starts to saturate the stucco and wall.
This is a best-case scenario—in many instances, there is not the 19-mm stucco thickness behind the stone. When expanded metal is used, there is a high density of openings that provide good ‘hang-on’ of the stone when mortar is wet. Therefore, there is a tendency to just butter the back side of the stone pieces and then attach directly to the lath. At best, there may be 9 mm of mortar, but it is not densified or totally continuous and does not stop water entry to come in contact with the building.
This scenario is a problem not only in wet climates, but also in areas with snow buildup. With heat passing from the structure, the snow melts and the resulting water passes down and in between the stonework. The issue of moisture in the building is only a problem when the water is trapped and is unable to pass through the manufactured stone, becoming trapped and causing building component failures.
Stone manufacturers provide installation recommendations and these must be followed. To attain the solid plaster base stipulated by these companies, there are welded wires that provide an enhanced plaster base because such a product is specifically engineered to provide a full scratch coat, making it difficult for applicators to ‘lick and stick’ the stone pieces on.
Using a traditional expanded metal lath proper embedment can be an issue and without embedment of the lath to the plaster stones can fall off when a first coat of plaster has not been allowed to properly set.
A common complaint with stucco is the potential of cracking. By its nature, the material is brittle and some degree of cracking may occur. This would be the result of building movement or settlement. Other cracks may form as a result of excessive shrinkage during curing. However, there are acrylic stucco coatings with various degrees of elasticity that can cover and bridge cracks.
The combination of steel and portland cement stucco has proven itself serviceable in various climates and been used successfully in all regions of Canada. Properly done, stucco in combination with the metal reinforcing is a successful coating for low- and high-rise multi-family buildings.
William Spilchen, P.Eng., has more than 40 years of experience in the construction and stucco industry. He is currently president of Spilchen & Associates Consulting Engineers and is involved in investigation of building envelope failures. Spilchen is member of numerous trade associations dealing with improvements of standards, including the ASTM C11 Stucco Work Group. He can be contacted by e-mail at email@example.com.
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