Building Design and Construction Handbook - Merritt - Ventech!

11.60 SECTION ELEVEN
Veneer plaster applications, 1 ⁄16 in thick, may be one-coat or two-coat, both
applied to special gypsum base (Arts. 11.25.1 and 11.25.5). The single coat is
composed of a scratch coat without cross-raking and a double-up coat immediately
applied, then worked to a smooth or textured finish.
See also Art. 11.25.9 and ‘‘Manual of Gypsum Lathing and Plastering,’’ Gypsum
Association.
Plaster Grounds. Except for veneer plasters, thickness of base-coat plaster should
be controlled with grounds—wood or metal strips applied at the perimeter of all
openings and at baseboards or continuous strips of plaster applied at intervals along
a wall or ceiling, to serve as screeds. Plaster screeds should be used on all plaster
surfaces of large area.
Minimum Thicknesses. Grounds should be set to provide a minimum plaster
thickness of 1 ⁄2 in over gypsum lath and gypsum partition tile; 5 ⁄8 in over brick,
clay tile, or other masonry; and 5 ⁄8 in from the face of metal lath. A thickness of
1 ⁄16 in is included for the finish coat.
Gypsum Base-Coat Plasters. Three types of gypsum base-coat plasters are in
general use: gypsum heat plaster, gypsum ready-mixed plaster, and veneer plasters,
which may be used in thin one-coat or two-coat systems. In two-coat veneer-plaster
systems, the base-coat veneer plaster should be applied to a thickness of 1 ⁄16 to 3 ⁄32
in, and left with a rough surface to receive the finish coat. Veneer plasters should
meet the requirements of ASTM C587 and application should be in accordance
with ASTM C843.
Gypsum neat plaster, sometimes called hardwall or gypsum-cement plaster, is
sold in powder form and mixed with an aggregate and water at the construction
site. Mixed with no more than 3 parts sand by weight, it makes a strong base coat
at low cost. Scratch coats generally consist of 1 part plaster powder to 2 parts sand
by weight, fibered or unfibered; the base coat in two-coat work usually is a 1:2 1 ⁄2
mix; brown coats are 1:3 mixes. With perlite or vermiculite instead of sand, a 1:2
mix may be used.
Gypsum ready-mixed plaster requires the addition only of water at the site,
since it is sold in bags containing the proper proportions of aggregate and plaster.
It is specified when good plastering sand is high cost or not available, or to avoid
the possibility of oversanding. It costs a little more than neat plaster because of the
extra cost of transporting the sand.
The water ratio for base coat neat and ready-mixed plasters should be such that
slump does not exceed 4 in when tested with a 2 � 4 � 6-in cone at the mixer,
for mixes with sand proportions not exceeding those given for gypsum neat plaster.
Application of gypsum plaster should meet the requirements of ASTM C842.
The scratch coat (Figs. 11.20 and 11.21) applied to lath should be laid on with
enough pressure to form a strong clinch or key. The coat should cover the lath to
a thickness of 1 ⁄4 in. For two-coat systems, the double-up brown coat is applied
immediately. For three-coat systems, after the surface has been trued, the scratch
coat should be scratched horizontally and vertically with a toothed tool to form a
good bonding surface, then left to dry partly. When the surface is so hard that the
edges of the scoring do not yield easily under the pressure of a thumbnail, the
brown coat may be applied. Hardening may take at least 1 day, and sometimes as
long as 1 week, depending on drying conditions.