Stucco is a mixture of Portland cement and well-graded sand, with hydrated lime or masonry cement added to make the mixture more plastic. Other proprietary stucco mixes are available. Their formulations will vary depending on the manufacturer of the mix.
Usually applied in three coats (two base coats and one finish coat), the stucco is held in place by stucco mesh or wire lath. A variety of finish coats are available, from standard coloured cement finishes to textured acrylics. Acrylic finish coats are often applied over conventional Portland cement, with good results. It is important that the chosen product be durable and vapour-permeable. Provide a space between the stucco and the sheathing membrane. Some sheathing membranes such as spun-bonded polyolefin can be affected by surfactants (soapy residues). Surfactants can reduce the water repelling capability of a membrane by changing the viscosity of water and can be produced by a) certain types of wood species or b) additives mixed with the stucco to improve workability during installation.
Stucco must be kept at least 200 mm (8 in.) above finished grade except where it is applied over concrete or masonry. In high exposure areas (frequent high rainfall and/or strong wind-driven rains), a furred and drained airspace of no less than 10 mm (7⁄16 in.) must be left between the stucco and the sheathing membrane to create a rainscreen. The top of the cavity should be vented and baffled to keep rain from blowing into it. The bottom of the drainage cavity must be vented to the exterior and be flashed.
Stucco reinforcing (self-furring welded mesh or fully primed or galvanized woven mesh) is stretched horizontally over sheathing paper, with the joints in the mesh lapped at least 50 mm (2 in.). Reinforce external corners by extending the mesh from one side 150 mm (6 in.) around the adjacent corner, or by vertical strips of reinforcing that extend 150 mm (6 in.) on either side of the corner.
Galvanized steel fasteners should be used to hold the mesh in place such as 3.2 mm (1⁄8 in.) diameter nails with heads that are about 11.1 mm (7⁄16 in.), or 1.98 mm (0.078 in.) thick staples. Fasteners are spaced 150 mm (6 in.) vertically and 400 mm (16 in.) horizontally, or 100 mm (4 in.) vertically and 600 mm (24 in.) horizontally. Other fastening patterns may be used, provided there are at least 20 fasteners per square meter (2 fasteners per square foot) of the wall surface. Where the sheathing is other than plywood, OSB or lumber, the fasteners must penetrate the sheathing and go into the framing member (stud or plate) at least 25 mm (1 in.).
A layer of sheathing membrane, lapped 100 mm (4 in.) at the edges, must be applied over the sheathing, and all openings must be flashed. Apply the membrane carefully around window openings and lap them correctly to ensure that water does not enter at the window flanges. Tar-saturated felts or papers should not be used beneath the stucco but tar-impregnated felts or paper may be used. The tar can bleed through the stucco causing unsightly discolouration.
The base coat consists of two layers of stucco. The first layer or “scratch coat” is applied in a thickness of 6 mm (1⁄4 in.) that completely embeds the wire lath or mesh. The scratch coat surface must be scored or raked to provide a bonding key for the second coat. Curing time will depend on outdoor temperature and weather conditions. Up to 48 hours of cure time is recommended before the second coat is applied.
Just prior to adding the second coat, dampen the base to ensure a good bond between the coats. Apply the second coat at least 6 mm (1⁄4 in.) thick and firmly trowel it into the scored surface of the base.
The second coat should be moist-cured for at least 48 hours and then left to dry for at least five days before the finish coat is applied. The second coat should be dampened to ensure a good bond and the finish coat applied to a depth of at least 3 mm (1⁄8 in.).
In dry, warm weather, new stucco should be kept damp to ensure proper curing. In cold weather, each coat of stucco should be kept at a temperature of at least 10°C (50°F) for 48 hours after application.