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Building Design and Construction Handbook - Merritt - Ventech!

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WALL, FLOOR, AND CEILING SYSTEMS 11.59<br />

Plaster should not be applied directly to exterior masonry walls because dampness<br />

may damage the plaster. It is advisable to fur the plaster at least 1 in in from<br />

the masonry.<br />

Properly aged concrete block may serve in walls as a plaster base, but for block<br />

ceilings, a bonding agent or a special bonding plaster should be applied first.<br />

For precast or cast-in-place concrete with smooth dense surfaces, a bonding<br />

agent or a special bonding plaster should be used first. But if a plaster thickness<br />

of more than 3 ⁄8 in is required for concrete ceilings, or 5 ⁄8 in for concrete walls,<br />

metal lath should be secured to the concrete before plastering, in which case s<strong>and</strong>ed<br />

plaster can be used.<br />

11.25.8 Plaster Base Coats<br />

The base coat is the portion of the plaster finish that is applied to masonry or lath<br />

bases <strong>and</strong> supports the finish coat (Fig. 11.22).<br />

Except for veneer plasters, plaster applications may be three-coat (Fig. 11.21)<br />

or two-coat (Fig. 11.20). The former consists of (1) a scratch coat, which is applied<br />

directly to the plaster base, cross-raked after it has ‘‘taken up,’’ <strong>and</strong> allowed to set<br />

<strong>and</strong> partly dry; (2) a brown coat, which is surfaced out to the proper grounds,<br />

darbied (float-finished), <strong>and</strong> allowed to set <strong>and</strong> partly dry; <strong>and</strong> (3) the finish coat.<br />

Three-coat plaster is required over metal lath, 1 ⁄2-in gypsum lath spanning horizontal<br />

supports more than 16 in c to c, all gypsum lath attached by clips providing only<br />

edge support, <strong>and</strong> 3 ⁄8-in perforated gypsum lath on ceilings.<br />

The two-coat application is similar, except that cross-raking of the scratch coat<br />

is omitted <strong>and</strong> the brown coat is applied within a few minutes to the unset scratch<br />

coat. Three coats are generally preferred, because the base coat thus produced is<br />

stronger <strong>and</strong> harder.<br />

FIGURE 11.22 Application of a base coat of plaster to gypsum lath. (Gold<br />

Bond <strong>Building</strong> Products, a National Gypsum Division.)

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