204 CHAPTER SIXTEEN
The supporting construction may be wood, steel, con-
crete, gypsum, tile, masonry, or lath. Certain types of lath
used with the plaster are self-supporting. The plaster itself
may be two or three coats, with a variety of materials avail-
able for each coat.
Proper use of plasters and bases provides the secure
bond necessary to develop the required strength. A mechan-
ical bond is formed when the plaster is pressed through the
holes of the lath or mesh and forms keys on the backside. A
suction or chemical bond is formed when the plaster is
applied over masonry and gypsum bases, with the tiny needle-
like plaster crystals penetrating into the surface of the base.
Both mechanical and suction bonds are developed with per-
forated gypsum lath.
16–10 Plaster
In its plastic state, plaster can be troweled to form. When set,
it provides a hard covering for interior surfaces such as walls
and ceilings. Plaster is the final step in wetwall construction
(although other finishes may be applied over it). Together
with the supporting construction and some type of lath, the
plaster will complete the assembly. The type and thickness of
the plaster used will depend on the type of supporting con-
struction, the lath, and the intended use. Plaster is available
in one-coat, two-coat, and three-coat work, and it is gener-
ally classified according to the number of coats required. The
last and final coat applied is called the finish coat,while the
coat, or combination of coats, applied before the finish coat
is referred to as the base coat.
Base Coats. Base coat plasters provide a plastic working
material that conforms to the required design and serves as a
base over which the finish coats are applied. Base coats are
available mill-mixed and job-mixed. Mill-mixed base coats are
available with an aggregate added to the gypsum at the mill.
Aggregates used include wood fibers, sand, perlite, and vermi-
culite. For high-moisture conditions, a portland cement and
lime plaster base coat is available.
Three-coat plaster must be used on metal lath. The first
coat (scratch coat) must be of a thickness sufficient to form
keys on the back of the lath, fill it in completely, and cover the
front of the lath. The thickness may vary from 1/8 to 1/4 inch.
The second coat (brown coat) ranges from 1/4 to 3/8 inch
thick, and the finish coat ranges from 1/16 to 1/8 inch thick.
Two-coat plaster may be used over gypsum lath and
masonry. The first coat is the base coat (scratch or brown),
and the second coat is the finish coat. Base coats range
from 1/4 to 1/8 inch in thickness, and the finish coat ranges
from 1/16 to 1/8 inch. Perforated gypsum lath will require
enough material to form the mechanical keys on the back
of the sheets.
Finish Coats. Finish coats serve as leveling coats and pro-
vide either a base for decorations or the required resistance to
abrasion. Several types of gypsum-finish plasters are avail-
able, including those that require the addition of only water
and those that blend gypsum, lime, and water (or gypsum,
lime, sand, and water). The finish coat used must be compat-
ible with the base coat. Finishing materials may be classified
as prepared finishes, smooth trowel finishes, or sand float fin-
ishes. Finish coat thickness ranges from 1/16 to 1/8 inch.
Specialty finish coats are also available. One such spe-
cialty coat is radiant heat plaster for use with electric cable
ceilings. It is a high-density plaster that allows a higher oper-
ator temperature for the heating system, as it provides more
efficient heat transmission and greater resistance to heat
deterioration. Applied in two coats—the first to embed the
cable, the second a finish coat over the top—its total thick-
ness is about 1/4 to 1/8 inch. It is usually mill-prepared and
requires only the addition of water.
One-coat plaster is a thin-coat, interior product used
over large sheets of gypsum plaster lath in conjunction with
a glass fiber tape to finish the joints. The plaster coat is 1/16-
to 1/32-inch thick.
Keene cement plaster is used where a greater resistance
to moisture and surface abrasions is required. It is available
in a smooth and sand-float finish. It is a dead-burned gyp-
sum mixed with lime putty and is difficult to apply unless
sand is added to the mixture; with sand as an additive, it is
less resistant to abrasion.
Acoustical plasters,which absorb sound, are also avail-
able. Depending on the type used, they may be troweled or
machine-sprayed onto the wall. Trowel applications are usu-
ally stippled, floated, or darbied to a finish. Some plasters
may even be tinted various colors. Thickness ranges from 3/8
to 1/2 inch.
Special plasters for ornamental plastering work, such as
moldings and cornices, are also available. Molding and cast-
ing plasters are most commonly used for such work.Stucco. Stucco is used in its plastic state. It can be trow-
eled to form. When set, it provides a hard covering for exte-
rior walls and surfaces of a building or structure. Stucco is
generally manufactured with portland cement as its base
ingredient and with clean sand and sometimes lime added.
Generally applied as three-coat work, the base coats are
mixed about one part portland cement to three parts clean
sand. If lime is added, no more than 6 to 8 pounds per 100
pounds of portland cement should be used in the mix. The
lime tends to allow the mix to spread more easily. The finish
coat is usually mixed 1:2 (cement to sand), and no coat
should be less than 1/4 inch thick.
Stucco is usually applied to galvanized metal lath that is
furred out slightly from the wall, but it can also be applied
directly to masonry. Flashing is often required and must be
included in the estimate.
Various special finishes may also be required and will
affect the cost accordingly. Finishes may be stippled,
broomed, pebbled, swirled, or configured in other designs.
Synthetic stuccos are also available. They may be applied
by installing reinforcing wire over a wood wall and by
adding one or two base coats and a top coat. Alternately, they
may be applied over rigid insulating board by installing a