Figure 7-3.—Keys formed with perforated gypsum board.There is also a special fire-retardant gypsum lath,called type X. It has a specially formulated core,containing minerals giving it additional fire protection.Use only one manufacturer’s materials for aspecified job or area. This ensures compatibility. Alwaysstrictly follow the manufacturer’s specifications formaterials and conditions of application.Plain gypsum lath plaster base is used in severalsituations: for applying nails and staples to wood andnailable steel framing; for attaching clips to woodframing, steel studs, and suspended metal grillage; andfor attaching screws to metal studs and furring channels.Common sizes include 16 by 48 inches, 3/8 or 1/2 inchthick, and 16 by 96 inches, 3/8 inch thick.Perforated gypsum lath plaster base is the same asplain gypsum lath except that 3/4-inch round holes arepunched through the lath 4 inches on center (OC) in eachdirection. This gives one 3/4-inch hole for each16 square inches of lath area. This provides mechanicalkeys in addition to the natural plaster bond and obtainshigher fire ratings. Figure 7-3 shows back and sideviews of a completed application.Insulating gypsum lath plaster base is the same asplain gypsum lath, but with bright aluminum foillaminated to the back. This creates an effective vaporbarrier at no additional labor cost. In addition, itprovides positive insulation when installed with the foilfacing a 3/4-inch minimum air space. When insulatinggypsum lath plaster is used as a ceiling, and under winterheating conditions, its heat-resistance value isapproximately the same as that for 1/2-inch insulationboard.Long lengths of gypsum lath are primarily used forfurring the interior side of exterior masonry walls. It isavailable in sizes 24 inches wide, 3/8 inch thick, and upto 12 feet in length.Figure 7-4.-Types of metal lath.Gypsum lath is easily cut by scoring one or bothsides with a utility knife. Break the lath along the scoredline. Be sure to make neatly fitted cutouts for utilityopenings, such as plumbing pipes and electrical outlets.Metal LathMetal lath is perhaps the most versatile of all plasterbases. Essentially a metal screen, the bond is created bykeys formed by plaster forced through the openings. Asthe plaster hardens, it becomes rigidly interlocked withthe metal lath.Three types of metal lath are commonly used:diamond mesh (expanded metal), expanded rib, andwire mesh (woven wire). These are shown in figure 7-4.7-5
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