Masonry Bases for Plaster
Gypsum partition tile has scored faces to provide a mechanical bond as well as the natural bond of gypsum to gypsum plaster. The 12 30-in faces of the tile present an unwarped plastering surface because the tile is dried without burning. This is done so that a mechanic can lay a straighter wall than with other types of units.
The Gypsum Associations Manual of Gypsum Lathing and Plastering recommends that only gypsum plaster be applied to gypsum partition tile, since lime and portland cement do not bond adequately. Also, only gypsum mortar should be used for laying tile.
Brick and clay tile can be used as a plaster base if they are not smooth-surfaced or of a nonporous type. If the surface does not provide sufficient suction, it should offer a means for developing a mechanical bond, such as does scored tile.
Plaster should not be applied directly to exterior masonry walls because dampness may damage the plaster. It is advisable to fur the plaster at least 1 in in from the masonry.
Properly aged concrete block may serve in walls as a plaster base, but for block ceilings, a bonding agent or a special bonding plaster should be applied first.
For precast or cast-in-place concrete with smooth dense surfaces, a bonding agent or a special bonding plaster should be used first. But if a plaster thickness of more than 3â„8 in is required for concrete ceilings, or 5â„8 in for concrete walls, metal lath should be secured to the concrete before plastering, in which case sanded plaster can be used.
Plaster Base Coats
The base coat is the portion of the plaster finish that is applied to masonry or lath bases and supports the finish coat (Fig. 11.22).
Except for veneer plasters, plaster applications may be three-coat (Fig. 11.21) or two-coat (Fig. 11.20). The former consists of (1) a scratch coat, which is applied directly to the plaster base, cross-raked after it has taken up, and allowed to set and partly dry; (2) a brown coat, which is surfaced out to the proper grounds, darbied (float-finished), and allowed to set and partly dry; and (3) the finish coat.
Three-coat plaster is required over metal lath, 1â„2-in gypsum lath spanning horizontal supports more than 16 in c to c, all gypsum lath attached by clips providing only edge support, and 3â„8-in perforated gypsum lath on ceilings.
The two-coat application is similar, except that cross-raking of the scratch coat is omitted and the brown coat is applied within a few minutes to the unset scratch coat. Three coats are generally preferred, because the base coat thus produced is stronger and harder.
Veneer plaster applications, 1â„16 in thick, may be one-coat or two-coat, both applied to special gypsum base (Arts. 11.25.1 and 11.25.5). The single coat is composed of a scratch coat without cross-raking and a double-up coat immediately applied, then worked to a smooth or textured finish.
See also Art. 11.25.9 and Manual of Gypsum Lathing and Plastering, Gypsum Association.
Plaster Grounds. Except for veneer plasters, thickness of base-coat plaster should be controlled with grounds wood or metal strips applied at the perimeter of all openings and at baseboards or continuous strips of plaster applied at intervals along a wall or ceiling, to serve as screeds. Plaster screeds should be used on all plaster surfaces of large area.
Minimum Thicknesses. Grounds should be set to provide a minimum plaster thickness of 1â„2 in over gypsum lath and gypsum partition tile; 5â„8 in over brick, clay tile, or other masonry; and 5â„8 in from the face of metal lath. A thickness of 1â„16 in is included for the finish coat.
Gypsum Base-Coat Plasters. Three types of gypsum base-coat plasters are in general use: gypsum heat plaster, gypsum ready-mixed plaster, and veneer plasters, which may be used in thin one-coat or two-coat systems. In two-coat veneer-plaster systems, the base-coat veneer plaster should be applied to a thickness of 1â„16 to 3â„32 in, and left with a rough surface to receive the finish coat. Veneer plasters should meet the requirements of ASTM C587 and application should be in accordance with ASTM C843.
Gypsum neat plaster, sometimes called hardwall or gypsum-cement plaster, is sold in powder form and mixed with an aggregate and water at the construction site. Mixed with no more than 3 parts sand by weight, it makes a strong base coat at low cost. Scratch coats generally consist of 1 part plaster powder to 2 parts sand by weight, fibered or unfibered; the base coat in two-coat work usually is a 1:21â„2 mix; brown coats are 1:3 mixes. With perlite or vermiculite instead of sand, a 1:2 mix may be used.
Gypsum ready-mixed plaster requires the addition only of water at the site, since it is sold in bags containing the proper proportions of aggregate and plaster.
It is specified when good plastering sand is high cost or not available, or to avoid the possibility of oversanding. It costs a little more than neat plaster because of the extra cost of transporting the sand.
The water ratio for base coat neat and ready-mixed plasters should be such that slump does not exceed 4 in when tested with a 2 4 6-in cone at the mixer, for mixes with sand proportions not exceeding those given for gypsum neat plaster.
Application of gypsum plaster should meet the requirements of ASTM C842.
The scratch coat (Figs. 11.20 and 11.21) applied to lath should be laid on with enough pressure to form a strong clinch or key. The coat should cover the lath to a thickness of 1â„4 in. For two-coat systems, the double-up brown coat is applied immediately. For three-coat systems, after the surface has been trued, the scratch coat should be scratched horizontally and vertically with a toothed tool to form a good bonding surface, then left to dry partly. When the surface is so hard that the edges of the scoring do not yield easily under the pressure of a thumbnail, the brown coat may be applied. Hardening may take at least 1 day, and sometimes as long as 1 week, depending on drying conditions.
The brown coat not only forms the base for the finish coat, but is also the straightening coat. The plaster should be laid on with a steel float, trued with rod or darby, and left rough in preparation for the finish coat.
