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Building Design and Construction Handbook - Merritt - Ventech!

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WALL, FLOOR, AND CEILING SYSTEMS 11.53<br />

Predecorated. A gypsumboard with a finished surface, such as paint, texturing<br />

material, vinyl film, or printed paper coverings, applied before the board is delivered<br />

to the building site.<br />

White Coat. A gaged lime-putty troweled-finish coat.<br />

11.25 PLASTER FINISHES<br />

Prepared on the building site, plaster finishes are classified as wet-type construction.<br />

They take longer to complete than dry-type construction, because they are field<br />

mixed with water <strong>and</strong> require curing before decorative materials, such as paint or<br />

wallpaper, can be applied. Plaster finishes are selected nevertheless because they<br />

are hard, abrasion resistant, rigid, incombustible, <strong>and</strong> provide a monolithic (unseamed)<br />

surface, even at corners. They are relatively brittle, however, <strong>and</strong> must be<br />

properly applied to avoid cracking when movements due to drying shrinkage or<br />

thermal changes are restrained.<br />

11.25.1 Components of Plaster<br />

A plaster finish consists of a supporting base, such as masonry or lath, <strong>and</strong> one or<br />

more coats of plaster or mix of plaster <strong>and</strong> other ingredients with water that is<br />

troweled or machine sprayed over the base.<br />

The principal ingredient of plaster usually is gypsum but may be portl<strong>and</strong> cement.<br />

(Portl<strong>and</strong>-cement plaster, or stucco, is discussed in Art. 11.19.) Gypsum plasters<br />

generally are formulated to meet the requirements of ‘‘St<strong>and</strong>ard Specification<br />

for Gypsum Plasters,’’ ASTM C28, or ‘‘St<strong>and</strong>ard Specification for Gypsum Veneer<br />

Plaster,’’ ASTM C587.<br />

C28 gypsum plasters include ready-mixed, neat, wood-fiber <strong>and</strong> gaging plasters.<br />

They may be applied over a masonry or lath base, generally in two or more<br />

coats, with a total thickness exceeding 1 ⁄2 in. They are required to contain 66% or<br />

more by weight of CaSO 4 � 1 ⁄2H 2O.<br />

Veneer plasters must be applied over a special gypsum base, meeting the requirements<br />

of ASTM C588, <strong>and</strong> are limited in thickness to a maximum of 1 ⁄4 in.<br />

They are usually selected because of low cost, rapid installation (permitting application<br />

of decorative materials 24 hr after plastering), <strong>and</strong> high resistance to cracking,<br />

nail popping, <strong>and</strong> impact <strong>and</strong> abrasion failure. The finishes are, however, not<br />

so rigid as conventional lath <strong>and</strong> plaster. Also, veneer plasters are more susceptible<br />

than gypsum plasters to ridging <strong>and</strong> cracking at joints when dried too rapidly because<br />

of low humidity, high temperature, or exposure to drafts.<br />

Application of gypsum <strong>and</strong> veneer plasters should meet the requirements of the<br />

following ASTM specifications:<br />

C841. Installation of Interior Lathing <strong>and</strong> Furring<br />

C842. Application of Interior Gypsum Plaster<br />

C843. Application of Gypsum Veneer Plaster<br />

C844. Application of Gypsum Base to Receive Gypsum Veneer Plaster<br />

See also Art. 11.27.

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