Cladding - Stucco Finishes
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
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
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.
or papers should not be used beneath the stucco. The tar can bleed through
the stucco causing unsightly discolouration.
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
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.