Mixing Plaster
A mechanical mixer disperses the ingredients of a mix more evenly and therefore is to be preferred over box mixing. Recommended practice is as follows: (1) Place the anticipated water requirements in the mixer; (2) add about half the required sand (or all required perlite or vermiculite); (3) add all the plaster; (4) add the rest of the sand; (5) mix at least 2 mm, but not more than 5 mm, adding water, if necessary, to obtain proper workability; and (6) dump the entire batch at once.
The mixer should be thoroughly cleaned when it is not in use. If partly set material is left in it, the set of the plaster might be accelerated. For this reason also, tools should be kept clean.
For hand mixing, first sand and plaster should be mixed dry to a uniform color in a mixing box, water added, and the plaster hoed into the water immediately and thoroughly mixed. Undermixed plaster is difficult to apply and will produce soft and hard spots in the plastered surface.
Plaster should not be mixed more than 1 hr in advance. Nor should a new mix, or gaging, be mixed in with a previously prepared one. And once plaster has started to set, it should not be remixed or retempered.
Plaster Drying
A minimum temperature of 55F should be maintained in the building where walls are to be plastered when outdoor temperatures are less than 55F, and held for at least 1 week before plaster is applied and 1 week after the plaster is dry.
In hot, dry weather, precautions should be taken to prevent water from evaporating before the plaster has set. Plastered surfaces should not be exposed to drafts, and openings to the outside should be closed off temporarily. After the plaster sets, the excess moisture it contains evaporates. Hence, the room should be adequately ventilated to allow this moisture to escape.
(Architect Data Book Construction Products and Systems, Gold Bond Building Products, a National Gypsum Division, 2001 Rexford Road, Charlotte, NC 28211; Gypsum Products Design Data, Gypsum Association, 810 First Street, NE, #510, Washington, D.C., 20002; Gypsum Construction Handbook, United States Gypsum, 125 South Franklin Street, Chicago, IL 60606.)
Gypsum Bases for Plaster
One commonly used base for plaster is gypsum lath. This is a noncombustible sheet generally 16 x 48 in by 3â„8 or 1â„2 in thick, or 16 x 96 in by 3â„8 in thick. It is composed principally of calcined gypsum that has been mixed with water, hardened, dried, and then sandwiched between two paper sheets (ASTM C37). Insulating
gypsum lath is made by cementing shiny aluminum foil to the back of plain lath.
It is used for vapor control, and for insulation against heat loss or gain. Also available is gypsum lath with a core specially formulated with minerals for high resistance to fire.
Gypsum lath has the advantage over metal lath that less plaster is required, because the first, or scratch, coat is applied only over the lath surface. Also, when used for suspended ceilings or hollow partitions, plaster on gypsum lath is less susceptible to cracking than on metal lath.
Installation of gypsum lath should meet the requirements of ASTM C841, Installation of Interior Lathing and Furring. The lath should be applied to studs with long dimension horizontal, and vertical joints should be staggered (Fig. 11.20). In ceilings, the long sides should span supports. Ends should rest on or be nailed to framing, headers, or nailing blocks. Each lath should be in contact with adjoining sheets, but if spaces more than 1â„2 in wide are necessary, the plaster should be reinforced with self-furring metal lath that is stapled or tied with wire to the gypsum lath.
When nailed to wood members, 3â„8- in-thick gypsum lath should be attached with four nails, and 1â„2-in lath with five nails, to each framing member covered.
Nails should be blued gypsum lath nails, made of 13-ga wire, 11â„8 in long, with a 19â„64-in-diam flat head. They should be driven until the head is just below the paper surface without breaking the paper.
Lath also may be attached to wood framing with four or five 16-ga staples, 7â„16 in wide, with 3â„8-in divergent legs. With metal framing, screws should be used, as recommended by the lath manufacturer.
Clips, however, are a suitable alternative for use with wood or metal framing.
Fasteners should be driven at least 3â„8 in away from ends and edges. Clips must secure the lath to framing at each intersection with the framing.
Studs or ceiling members supporting lath may be spaced up to 16 in c to c with 3â„8-in gypsum lath, and up to 24 in c to c with 1â„2-in lath.
Except at intersections that are to be unrestrained, reentrant corners should be reinforced with cornerite stapled or tied with wire to the gypsum lath. Exteriorangle corners should be finished with corner beads set to true grounds and nailed or tied with wire to the structural frame or to furring. Casing beads should be used around wall openings and at intersections of plaster with other finishes and of lath and lathless construction.
Gypsum Base for Veneer Plasters. Special gypsum lath meeting requirements of ASTM C588 is required as a base for veneer plasters. It is formulated to provide the strength and absorption necessary for proper application and performance of these thin coatings. The lath comes in thicknesses of 3â„8 (for two-coat systems), 1â„2, and 5â„8 in, the last permitting 24-in spacing of wood framing. Installation should meet the requirements of ASTM C844. In general, gypsum base should be applied first to the ceiling, then to the walls. Maximum spacing of nails is 7 in on ceilings and 8 in on walls. Screw spacing should not exceed 12 in for wood framing 24 in c to c or for steel framing or for wood ceiling framing, or 16 in for wood studs spaced 16 in c to c.
