Gypsum is a cementitious material composed of at least 70% of CaSO4 – 2H2O by weight (Art. 4.6). It is a main ingredient of many building products.
This product consists of a core of set gypsum surfaced with specifically manufactured paper firmly bonded to the core. It is designed to be used without addition of plaster for walls, ceilings, or partitions and provides a surface suitable to receive either paint or paper (see also Sec. 11). Gypsum board is extensively used in ‘‘drywall’’ construction, where plaster is eliminated. It is also available with one surface covered with aluminium or other heat-reflecting type of foil, or with imitation woodgrain or other patterns on the exposed surface so that no additional decoration is required.
The types of gypsum board generally available include wallboard, backing board, core board, fire-resistant gypsum board, water-resistant gypsum board, gypsum sheathing, and gypsum formboard.
This type is used for the surface layer on interior walls and ceilings. Regular gypsum wallboard comes with gray liner paper on the back and a special paper covering, usually cream-colored, on facing side and edges. This covering provides a smooth surface suitable for decoration. Foil-backed gypsum wallboard has aluminium foil bonded to the liner paper to serve as a vapor barrier and, when contiguous to an airspace, as thermal insulation. Predecorated gypsum wallboard does not require decorative treatment after installation because it comes with a finished surface, often a decorative vinyl or paper sheet. Wallboard should conform with ASTM C36.
Wallboard usually is available 4 ft wide in the following thicknesses and lengths:
· 1 ⁄4 in—for covering and rehabilitating old walls and ceilings, 4 to 12 ft long
· 5 ⁄16 in—where thickness greater than 1 ⁄4 in is desired, 4 to 14 ft long.
· 3 ⁄8 in—mainly for the outer face in two-layer wall systems, 4 to 16 ft long
· 1 ⁄2 in—for single-layer new construction with supports 16 to 24 in c to c, 4 to 16 ft long
· 5 ⁄8 in—for better fire resistance and sound control than 1 ⁄2 in provides, 4 to 16 ft long
Standard edges are rounded, bevelled, tapered, or square.
This type is used as a base layer in multi-ply construction, where several layers of gypsum board are desired for high fire resistance, sound control, and strength in walls. It has gray liner paper on front and back faces. Also available is backing board with aluminium foil bonded to the back face. Gypsum backing board should conform with ASTM C442. The boards come 16 to 48 in wide, 4 to 16 ft long, and 1 ⁄4 to 1 in thick.
Gypsum core board.
To save space, this type is used as a base in multi-ply construction of self-supporting (stud less) gypsum walls. core board may be supplied as 1-in-thick, solid backing board or as two factory-laminated, 1 ⁄2-in-thick layers of backing board. core board too should conform with C442.
Type X Gypsum board.
For use in fire-rated assemblies, Type X may be gypsum wallboard, backing board, or core board with core made more fire resistant by addition of glass fibre or other reinforcing materials.
Water-Resistant Gypsum Backing Board.
This type comes with a water-resistant gypsum core and water-repellent face paper. It may be used as a base for wall tile in baths, showers, and other areas subject to wetting. The board should conform with ASTM C630.
This type is used as fire protection and bracing of exterior frame walls. It must be protected from the weather by an exterior facing. Sheathing should conform with ASTM C79. It comes 24 to 48 in wide, 6 to 12 ft long, and 3 ⁄8, 4 ⁄10, 1 ⁄2, and 5 ⁄8 in thick.
This type is used as a permanent form in the casting of gypsum-concrete roof decks.
Gypsum lath is similar to gypsum board in that it consists of a core of set gypsum surfaced with paper. The paper for gypsum board, however, is produced so that it is ready to receive paint or paper, while that for gypsum lath is specially designed or treated so that plaster will bond tightly to the paper. In addition, some lath provides perforations or other mechanical keying to assist in holding the plaster firmly on the lath. It is also available with reflective foil backing .
Gypsum lath should conform with ASTM C37. It comes in 16-, 161 ⁄2-, 24-, and 32-in widths, lengths of 32, 36, and 48 in, and 3 ⁄8- and 1 ⁄2-in widths. Veneers plasters, special proprietary compositions for thin plaster surfaces, are best applied over veneer plaster base, similar to gypsum lath, but produced to accommodate the veneer plaster compositions. Both gypsum lath and veneer base are made as regular, X-rated (fire-retardant), and insulating (foil-backed) types. These bases should conform with ASTM G588. They come 48 in wide, 6 to 16 ft long, and 3 ⁄8, 1 ⁄2, and 5 ⁄8 in thick.
Gypsum Sheathing Board
Gypsum sheathing boards are similar in construction to gypsum board (Art. 4.26), except that they are provided with a water-repellent paper surface. They are commonly made 3 ⁄4 to 5 ⁄8 in thick, 6 to 12 ft long, and with a nominal width of 24 or 48 in in conformance with ASTM C79. They are made with either square edges or with V tongue-and-groove edges. Sheathing boards also are available with a water-repellent core or fire-resistant Type X.
Gypsum Partition Tile Or Block
Gypsum tiles or blocks are used for non-load-bearing partition walls and for protection of columns, elevator shafts, etc., against fire. They have been essentially replaced by dry-wall systems.
A precast gypsum product used particularly for roof construction is composed of a core of gypsum cast in the form of a plank, with wire-fabric reinforcement and usually with tongue-and-groove metal edges and ends. The planks are available in two thicknesses—a 2-in plank, which is 15 in wide and 10 ft long, and a 3-in plank which is 12 in wide and 30 in long.