PLASTER BASE INSTALLATION
For plastering, there must be a continuous surface to which the plaster can be applied and to which it will clingthe plaster base. A continuous concrete or masonry surface may serve as a base without further treatment.
For plaster bases, such as those defined by the inner edges of the studs or the lower edges of the joists, a base material, called lath, must be installed to form a continuous surface spanning the spaces between the structural members.
Wood lath is made of white pine, spruce, fir, redwood, and other soft, straight-grained woods. The standard size of wood lath is 5/16 inch by 1 1/2 inches by 4 feet. Each lath is nailed to the studs or joists with 3-penny (3d) blued lathing nails.
Laths are nailed six in a row, one above the other. The next six rows of lath are set over two stud places. The joints of the lath are staggered in this way so cracks will not occur at the joinings. Lath ends should be spaced 1/4 inch apart to allow movement and prevent buckling. Figure 7-1 shows the proper layout of wood lath. To obtain a good key (space for mortar), space the laths not less than 3/8 inch apart. Figure 7-2 shows good spacing with strong keys.
Wood laths come 50 to 100 to the bundle and are sold by the thousand. The wood should be straight-grained, and free of knots and excessive pitch. Dont use old lath; dry or dirty lath offers a poor bonding surface. Lath must be damp when the mortar is applied Dry lath pulls the moisture out of the mortar, preventing proper setting. The best method to prevent dry lath is to wet it thoroughly the day before plastering. This lets the wood swell and reach a stable condition ideal for plaster application.
Of the many kinds of lathing materials available, board lath is the most widely used today. Board lath is manufactured from mineral and vegetable products. It is produced in board form, and in sizes generally standardized for each application to studs, joists, and various types of wood and metal timing.
Board lath has a number of advantages. It is rigid, strong, stable, and reduces the possibility of dirt filtering through the mortar to stain the surface. It is insulating and strengthens the framework structure. Gypsum board lath is fire resistant. Board lath also requires the least amount of mortar to cover the surface.
Board laths are divided into two main groups: gypsum board and insulation board. Gypsum lath is made in a number of sizes, thicknesses, and types. Each type is used for a specific purpose or condition. Note: Only gypsum mortar can be used over gypsum lath. Never apply lime mortar, portland cement, or any other binding agent to gypsum lath.
The most commonly used size gypsum board lath is the 3/8 inch by 16 inches by 48 inches, either solid or perforated. This lath will not burn or transmit temperatures much in excess of 212° F until the gypsum is completely calcined. The strength of the bond of plaster to gypsum lath is great. It requires a pull of 864 pounds per square foot to separate gypsum plaster from gypsum lath (based on a 2:1 mix of sand and plaster mortar).
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 manufacturers materials for a specified job or area. This ensures compatibility. Always strictly follow the manufacturers specifications for materials and conditions of application.
Plain gypsum lath plaster base is used in several situations: for applying nails and staples to wood and nailable steel framing; for attaching clips to wood framing, steel studs, and suspended metal grillage; and for attaching screws to metal studs and furring channels. Common sizes include 16 by 48 inches, 3/8 or 1/2 inch thick, and 16 by 96 inches, 3/8 inch thick.
Perforated gypsum lath plaster base is the same as plain gypsum lath except that 3/4-inch round holes are punched through the lath 4 inches on center (OC) in each direction. This gives one 3/4-inch hole for each 16 square inches of lath area. This provides mechanical keys in addition to the natural plaster bond and obtains higher fire ratings. Figure 7-3 shows back and side views of a completed application.
Figure 7-3.Keys formed with perforated gypsum board.
Insulating gypsum lath plaster base is the same as plain gypsum lath, but with bright aluminum foil laminated to the back. This creates an effective vapor barrier at no additional labor cost. In addition, it provides positive insulation when installed with the foil facing a 3/4-inch minimum air space. When insulating gypsum lath plaster is used as a ceiling, and under winter heating conditions, its heat-resistance value is approximately the same as that for 1/2-inch insulation board.
Long lengths of gypsum lath are primarily used for furring the interior side of exterior masonry walls. It is available in sizes 24 inches wide, 3/8 inch thick, and up to 12 feet in length.
Gypsum lath is easily cut by scoring one or both sides with a utility knife. Break the lath along the scored line. Be sure to make neatly fitted cutouts for utility openings, such as plumbing pipes and electrical outlets.
Metal lath is perhaps the most versatile of all plaster bases. Essentially a metal screen, the bond is created by keys formed by plaster forced through the openings. As the plaster hardens, it becomes rigidly interlocked with the metal lath.
Three types of metal lath are commonly used: diamond mesh (expanded metal), expanded rib, and wire mesh (woven wire). These are shown in figure 7-4.
Figure 7-4.-Types of metal lath.
Lets now look at the basic installation procedures for plaster bases and accessories.
Gypsum lath is applied horizontally with staggered end joints, as shown in figure 7-5. Vertical end joints should be made over the center of studs or joists. Lath joints over openings should not occur at the jamb line. Do not force the boards tightly together; let them butt loosel y so the board is not under compression before the plaster is applied. Use small pieces only where necessary. The most common method of attaching the boards has been the lath nail. More recently, though, staples have gained wider use (due mainly to the ready availability of power guns).
Figure 7-5.-Lath joints.
The nails used are 1 1/8 inches by 13 gauge, flat headed, blued gypsum lath nails for 3/8-inch-thick boards and 1 1/4 inches for 1/2-inch boards. There are also resin-coated nails, barbed-shaft nails, and screw-type nails in use. Staples should be No. 16 U.S. gauge flattened galvanized wire formed with a 7/16-inch-wide crown and 7/8-inch legs with divergent points for 3/8-inch lath. For 1/2-inch lath, use 1-inch-long staples.
Four nails or staples are used on each support for 16-inch-wide lath and five for 2-foot-wide lath. Some special fire ratings, however, require five nails or staples per 16-inch board. Five nails or staples are also recommended when the framing members are spaced 24 inches apart.
Start nailing or stapling 1/2 inch from the edges of the board. Nail on the framing members falling on the center of the board first, then work to either end. This should prevent buckling.
Insulating lath should be installed much the same as gypsum lath except that slightly longer blued nails are used. A special waterproof facing is provided on one type of gypsum board for use as a ceramic tile base when the tile is applied with an adhesive.
All metal lath is installed with the sides and ends lapped over each other. The laps between supports should be securely tied, using 18-gauge tie wire. In general, metal lath is applied with the long length at right angles to the supports. Rib lath is placed with the ribs against the supports and the ribs nested where the lath overlaps. Generally, metal lath and wire lath are lapped at least 1 inch at the ends and 1/2 inch at the sides. Some wire lath manufacturers specify up to 4 1/2-inch end lapping and 2-inch side laps. This is done to mesh the wires and the paper backing.
Lath is either nailed, stapled, or hog-tied (heavy wire ring installed with a special gun) to the supports at 6-inch intervals. Use 1 1/2-inch barbed roofing nails with 7/16-inch heads or 1 inch 4-gauge staples for the flat lath on wood supports. For ribbed lath, heavy wire lath, and sheet lath, nails or staples must penetrate the wood 1 3/8 inches for horizontal application and at least 3/4 inch for vertical application. When common nails are used, they must be bent across at least three lath strands.
On channel iron supports, the lath is tied with No. 18-gauge tie wire at 4-inch intervals using lathers nippers. For wire lath, the hog tie gun can be used. Lath must be stretched tight as it is applied so that no sags or buckles occur. Start tying or nailing at the center of the sheet and work toward the ends. Rib lath should have ties looped around each rib at all supports, as the main supporting power for rib lath is the rib.
When you install metal laths at both inside and outside corners, bend the lath to forma comer and carry it at least 4 inches in or around the corner. This provides the proper reinforcement for the angle or comer.
A wide variety of metal accessories is produced for use with gypsum and metal lathing. Lathing accessories are usually installed before plastering to form true corners, act as screeds for the plasterer, reinforce possible weak points, provide control joints, and provide structural support.
Lathing accessories consist of structural components and miscellaneous accessories. The principal use of structural components is in the construction of hollow partitions. A hollow partition is one containing no building framing members, such as studs and plates. Structural components are lathing accessories that take the place of the missing framing members supporting the lath. These include prefabricated metal studs and floor and ceiling runner tracks. The runner tracks take the place of missing stud top and bottom plates. They usually consist of metal channels. Channels are also used for furring and bracing.
Miscellaneous accessories consist of components attached to the lath at various locations. They serve to define and reinforce comers, provide dividing strips between plaster and the edges of baseboard or other trim, and define plaster edges at unframed openings.
Comer beads fit over gypsum lath outside corners to provide a true, reinforced comer. They are available in either small-nose or bullnose types, with flanges of either solid or perforated (fig. 7-6) metal. They are available with expanded metal flanges.
Figure 7-6.Perforated flanged corner bead.
Casing beads are similar to comer beads and are used both as finish casings around openings in plaster walls and as screeds to obtain true surfaces around doors and windows. They are also used as stops between a plaster surface and another material, such as masonry or wood paneling. Casing beads are available as square sections, modified-square sections, and quarter-rounds.
Base or parting screeds are used to separate plaster from other flush surfaces, such as concrete. Ventilating expansion screed is used on the underside of closed soffits and in protected vertical surfaces for ventilation of enclosed attic spaces. Drip screeds act as terminators of exterior portland cement plaster at concrete foundation walls. They are also used on external horizontal comers of plaster soffits to prevent drip stains on the underside of the soffit. A metal base acts as a flush base at the bottom of a plaster wall. It also serves as a plaster screed.
Because some drying usually takes place in the wood framing members after a structure is completed, some shrinkage is expected. This, in turn, may cause plaster cracks to develop around openings and in the comers. To minimize, if not eliminate, these cracks, use expanded metal lath in key positions over the plaster-base material as reinforcements. Strip reinforcement (strips of expanded metal lath) can be used over door and window openings (fig. 7-7, view A). A 10- to 20-inch strip is placed diagonally across each upper comer of the opening and tacked in place.
Figure 7-7.-Metal lath used to minimize cracking.
Strip reinforcement should also be used under flush ceiling beams (fig. 7-7, view B) to prevent plaster cracks. On wood drop beams extending below the ceiling line, the metal lath is applied with furring nails to provide space for keying the plaster.
Corner beads of expanded metal lath or of perforated metal (fig. 7-8) should be installed on all outside comers. They should be applied plumb and level. Each bead acts as a leveling edge when walls are plastered and reinforces the comer against mechanical damage. To minimize plaster cracks, reinforce the inside comers at the juncture of walls and ceilings. Metal lath, or wire fabric, is tacked lightly in place in these corners.
Figure 7-8.-Plaster reinforcing at corners.
Control joints (an example of which is shown in fig. 7-9) are formed metal strips used to relieve stresses and strains in large plaster areas or at junctures of dissimilar materials on walls and ceilings. Cracks can develop in plaster or stucco from a single cause or a combination of causes, such as foundation settlement, material shrinkage, building movement, and so forth. The control joint minimizes plaster cracking and assures proper plaster thickness. The use of control joints is extremely important when Portland cement plaster is used.
Figure 7-9.-Control joint.
Plastering grounds are strips of wood used as plastering guides or strike-off edges and are located around window and door openings and at the base of the walls. Grounds around interior door openings (such as fig. 7-10, view A) are full-width pieces nailed to the sides over the studs and to the underside of the header. They are 5 1/4 inches wide, which coincides with the standard jamb width for interior walls with a plaster finish. They are removed after the plaster has dried. Narrow strip grounds (fig. 7-10, view B) can also be used around interior openings.
Figure 7-10.Plaster grounds.
In window and exterior door openings, the frames are normally in place before the plaster is applied. Thus, the inside edges of the side and head jamb can, and often do, serve as grounds. The edge of the window might also be used as a ground, or you can use a narrow 7/8-inch-thick ground strip nailed to the edge of the 2-by 4-inch sill (fig. 7-10, view C). These are normally left in place and covered by the casing.
A similar narrow ground or screed is used at the bottom of the wall to control the thickness of the gypsum plaster and to provide an even surface for the baseboard and molding. This screed is also left in place after the plaster has been applied.
Some plaster comes ready-mixed, requiring only the addition of enough water to attain minimum required
workability. For job mixing, tables are available giving recommended ingredient proportions for gypsum, lime, lime-portland cement, and portland cement plaster for base coats on lath or on various types of concrete or masonry surfaces, and for finish coats of various types. In this chapter, well cover recommended proportions for only the more common types of plastering situations.
In the following discussion, one part of cementitious material means 100 pounds (one sack) of gypsum, 100 pounds (two sacks) of hydrated lime, 1 cubic foot of lime putty, or 94 pounds (one sack) of port land cement. One part of aggregate means 100 pounds of sand or 1 cubic foot of vermiculite or perlite. Note: Vermiculite and perlite are not used with lime plaster. While aggregate parts given for gypsum or portland cement plaster may be presumed to refer to either sand or vermiculite/perlite, the aggregate part given for lime plaster means sand only.
Table 7-1.Base Coat Proportions for Different Types of Work
Table 7-2.Recommended Base Coat Proportions for Gypsum Plaster
The total volume of plaster required for a job is the product of the thickness of the plaster times the net area to be covered. Plaster specifications state a minimum thickness, which you must not go under. Also, you should exceed the specs as little as possible due to the increased tendency of plaster to crack with increased thickness.
The two basic operations in mixing plaster are determining the correct proportions and the actual mixing methods used.
Figure 7-11.Plaster mixing machine.
Personnel handling cement or lime bags should wear recommended personnel protective gear. Always practice personal cleanliness. Never wear clothing that is hard and stiff with cement. Such clothing irritates the skin and may cause serious infection. Any susceptibility of skin to cement and lime bums should be immediately reported to your supervisor.
Dont pile bags of cement or lime more than 10 bags high on a pallet except when stored in bins or enclosures built for such purposes. Place the bags around the outside of the pallet with the tops of the bags facing the center. To prevent piled bags from falling outward, crosspile the first five tiers of bags, each way from any comer, and make a setback starting with the sixth tier. If you have to pile above the 10th tier, make another setback. The back tier, when not resting against a wall of sufficient strength to withstand the pressure, should be set back one bag every five tiers, the same as the end tiers.
During unpiling, the entire top of the pile should be kept level and the setbacks maintained for every five tiers.
Lime and cement must be stored in a dry place to help prevent the lime from crumbling and the cement from hydrating before it is used.
|David L. Heiserman, Editor||
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Revised: May 18, 2013