Wall Sheathing and Exterior Finishes

Exterior Cladding - Stucco Finishes

Stucco generally consists of a mixture of Portland cement and well-graded sand, with hydrated lime added to make the mixture more plastic. An alternative stucco mixture calls for replacing the lime with masonry cement. Table 37 lists the proportions for the preparation of these two stucco mixes. Other proprietary stucco mixes are available. Their formulations will vary depending on the manufacturer of the mix.

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 finely textured acrylic finishes. The “stone dash” finish is seldom used except in retrofit applications. Stucco reinforcing of 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 2 in. (50 mm). External corners are reinforced either by extending the mesh from one side 6 in. (150 mm) around the adjacent corner, or by vertical strips of reinforcing that extend 6 in. (150 mm) on either side of the corner. Stucco must be no closer than 8 in. (200 mm) from finished grade except where it is applied over concrete or masonry.

A well-detailed layer of heavy-weight building paper, lapped 4 in. (100 mm) at the edges, must be applied. It is very important to apply flashings around penetrations in the walls. The building paper must be carefully applied around window openings and lapped correctly to ensure that water does not enter at the window flanges. Tar-saturated felts or papers should not be used beneath the stucco. The tar can bleed through the stucco causing unsightly discolouration.

Galvanized steel fasteners should be used to hold the mesh in place. Suitable fasteners are 1/8 in. (3.2 mm) diameter nails with heads that are about 7/16 in. (11.1 mm) or alternative with 0.078 in. (1.98 mm) thick staples. Fasteners are spaced 6 in. (150 mm) vertically and 16 in. (400 mm) horizontally, or 4 in. (100 mm) vertically and 24 in. (600 mm) horizontally. Other fastening patterns may be used, provided there are at least two fasteners per square ft. (20 fasteners per square metre) of the wall surface. Where the sheathing is other than lumber, waferboard or plywood, the fasteners should penetrate the sheathing and go into the framing member (stud or plate) at least 1 in. (25 mm).

The base coat consists of two layers of stucco. The first layer (scratch coat) of stucco is applied to a thickness of ½ in. (12 mm) 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. It is not uncommon to allow 48 hours of cure time before the second coat is applied.

Just before putting on the second coat, the base is dampened to ensure a good bond between the coats. The second coat is applied at least ¼ in. (6 mm) thick and firmly trowelled into the scored surface of the base.

A wide variety of finishes are available today, from standard white or coloured cement to modified and acrylic finishes. Acrylic finish coats are often applied over conventional Portland cement, with good results. It is important that the chosen product has good weatherability and is vapour permeable. In cases where a wall requires a fire rating, the base coat thickness will need to be check to ensure it is sufficiently thick.

For finish coats, the second coat should be moist-cured for at least 48 hours and then left to dry for five days, preferably longer, before the finish coat is applied. The base should be dampened to ensure a good bond and the finish applied to a depth of at least 1/8 in. (3 mm).

In dry warm weather, fresh 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 50°F (10°C) for 48 hours after application.

 
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