Ceiling and wall covering may be broadly divided into PLASTER and DRY-WALL covering. Dry-wall covering is a general term applied to sheets or panels of wood, plywood, gypsum, fiberboard, and the like. A plaster and/or ceiling covering requires a "plaster base" and a "plaster ground" before it is installed. The plaster base, such as gypsum, fiberboard, or metal lath, provides a plane-surface base to which the plaster can be applied. Wooden strips of the same thickness as the combined thickness of the lath and plaster, called plaster ground, are installed before the lath is applied to serve as guides for the plasterers to ensure uniform plaster thickness around doorframes and window frames and behind casings.

The use of dry wall over the lath-and-plaster finish is rapidly increasing. Installation or construction time is faster with the application of dry wall. Being wet, plaster requires drying time before other interior work can be started. Gypsum is one of the most widely used types of dry-wall finishes. It is made up of a gypsum filler faced with paper or with a foil back that serves as a vapor barrier on exterior walls. It is also available with vinyl or other prefinished surfaces. It comes in 4- by 8-ft sheets and in lengths of up to 16 ft for horizontal application. Notice in

Figure 6-61-Application of gypsum board finish: A. Strongback B. Vertical application; C. Horizontal application.

figure 6-61, view A, a "strongback" is usually used for aligning ceiling joists or studs to provide a smooth, even surface. Figures 6-62 and 6-63 show typical application of paneling using other types of dry-wall finishes. A variety of ceiling systems can also be used to change the appearance of a room, lower a ceiling, finish off exposed joints, or provide

Figure 6-63.-Application of tongued-and-grooved paneling over studs.

acoustical control. Suspended acoustical ceiling systems are designed to integrate the functions of lighting, air distribution, and fire protection. Acoustical tiles, available in 12-to 30-in. widths, 12- to 60-in. lengths, and 3/16- to 3/4-in. thicknesses, are used with the other grid system components (fig. 6-64). Depending on the type of ceiling or roof construction, ceiling tiles may be installed in various ways, such as with the use of wood strips nailed across the ceiling joists or roof trusses (fig. 6-65).