Building Design and Construction Handbook - Merritt - Ventech!

11.40 SECTION ELEVEN
be applied with long dimensions horizontal and should be tied with 16-ga wire.
Edges should be lapped at least 1 in, ends 2 in.
The first, or scratch, coat should be forced through the interstices in the lath so
as to embed the metal completely. In three-coat applications, the coat should be at
least 1 ⁄2 in thick. Its surface should be scored to aid bond with the second, or brown,
coat. That coat should be applied as soon as the scratch coat has gained sufficient
strength to carry the weight of both coats, usually after about 4 or 5 hr from
completion of the scratch coat. The second coat should be at least 3 ⁄8 in thick. It
should be moist cured for at least 48 hr with fine sprays of water and then allowed
to dry for at least 1 week. The finish coat should be at least 3 ⁄8 in thick. (When
only two coats are used, for example, on a masonry base, the base coat should be
a minimum of 3 ⁄8 in thick and the finish coat, 1 ⁄4 in. Before application of the base
coat, a bond coat, consisting of one part portland cement and one to two parts sand,
should be dashed on the masonry with a stiff brush and allowed to set.)
For both the scratch and brown coats, the mix, by volume, may be 1 part portland
cement to 3 to 5 parts sand, plus hydrated lime in amount equal to 25% of the
volume of cement. Masonry cement may be used instead of portland cement, but
without addition of lime, inasmuch as masonry cement contains lime. The finish
coat may be a factory-prepared stucco-finish mix or a job mix of 1 part white
portland cement, not more than 1 ⁄4 part of hydrated lime, 2 to 3 parts of a lightcolored
sand, and mineral oxide pigment, if desired.
Ingredients should be thoroughly mixed dry. Then, water should be added and
the materials mixed for at least 5 mm in a power mixer. The first two coats usually
are applied with a trowel. The finish coat may be sprayed or manually applied.
(‘‘Plasterer’s Manual,’’ EBO49M, Portland Cement Association.)
11.20 PRECAST-CONCRETE OR METAL AND
GLASS FACINGS
In contrast to siding in which a single material forms the complete wall, precast
concrete or metal and glass are sometimes used as the facing, which is backed up
with insulation, fire-resistant material, and an interior finish. The glass usually is
tinted and is held in a light frame in the same manner as window glass. Metal
panels may be fastened similarly in a light frame, attached to mullions or other
secondary framing members, anchored to brackets at each floor level, or connected
to the structural frame of the building. The panels may be small and light enough
for one man to carry or one or two stories high, prefabricated with windows.
Provision for expansion and contraction should be made in the frames, when
they are used, and at connections with building members. Metal panels should be
shaped so that changes in surface appearance will not be noticeable as the metal
expands and contracts. Frequently, light-gage metal panels are given decorative
patterns, which also hide movements due to temperature variations (‘‘canning’’) and
stiffen the sheets. Flat sheets may be given a slight initial curvature and stiffened
on the rear side with ribs, so that temperature variations will only change the
curvature a little and not reverse it. Or flat sheets may be laminated to one or more
flat stiffening sheets, like mineral-fiber panels or mineral-fiber panels and a second
light-gage metal sheet, to prevent ‘‘canning.’’
It may be desirable in many cases to treat the metal to prevent passage of sound.
Usual practice is to apply a sound-absorbing coating on the inside surface of the