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PLASTER BASE INSTALLATION

LEARNING OBJECTIVE:

Upon completing this section, you should be able to associate the names and purposes of each type of lath used as a plaster base. You should also be able to describe the procedures used in plastering, including estimating materials and the procedures for mixing and applying plaster bases.

For plastering, there must be a continuous surface to which the plaster can be applied and to which it will cling—the plaster base. A continuous concrete or masonry surface may serve as a base without further treatment.

BASES

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

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.

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Figure 7-1.—Wood lath with joints staggered every sixth course.

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Figure 7-2.—Wood lath, showing proper 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. Don’t 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.

Board Lath

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 manufacturer’s materials for a specified job or area. This ensures compatibility. Always strictly follow the manufacturer’s 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.

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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

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.

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Figure 7-4.-Types of metal lath.

DIAMOND MESH.— The terms "diamond mesh" and "expanded metal" refer to the same type of lath (fig. 7-4). It is manufactured by first cutting staggered slits in a sheet and then expanding or stretching the sheet to form the screen openings. The standard diamond mesh lath has a mesh size of 5/16 by 9/16 inch. Lath is made in sheets of 27 by 96 inches and is packed 10 sheets to a bundle (20 square yards).

Diamond mesh lath is also made in a large mesh. This is used for stucco work, concrete reinforcement, and support for rock wool and similar insulating materials. Sheet sizes are the same as for the small mesh. The small diamond mesh lath is also made into a self-furring lath by forming dimples into the surface that hold the lath approximately 1/4 inch away from the wall surface. This lath may be nailed to smooth concrete or masonry surfaces. It is widely used when replastering old walls and ceilings when the removal of the old plaster is not desired. Another lath form is paper-backed where the lath has a waterproof or kraft paper glued to the back of the sheet. The paper acts as a moisture barrier and plaster saver.

EXPANDED RIB.— Expanded rib lath (fig. 7-4) is like diamond mesh lath except that various size ribs are formed in the lath to stiffen it. Ribs run lengthwise of the lath and are made for plastering use in 1/8-, 3/8-, and 3/4-inch rib height. The sheet sizes are 27 to 96 inches in width, and 5-,10-, and 12-foot lengths for the 3/4-inch rib lath.

WIRE MESH.— Woven wire lath (fig. 7-4) is made of galvanized wire of various gauges woven or twisted together to form either squares or hexagons. It is commonly used as a stucco mesh where it is placed over tar paper on open-stud construction or over various sheathing.

INSTALLATION

Let’s now look at the basic installation procedures for plaster bases and accessories.

Gypsum Lath

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).

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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.

Metal Lath

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.

Lath Accessories

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.

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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.

Joint Reinforcing

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.

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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.

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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.

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Figure 7-9.-Control joint.

Plastering Grounds

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.

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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.

Mixing

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, we’ll 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.

BASE COAT PROPORTIONS.— Two-coat plasterwork consists of a single base coat and a finish coat. Three-coat plasterwork consists of two base coats (the scratch coat and the brown coat) and a finish coat.

Portland cement plaster cannot be applied to a gypsum base. Lime plaster can, but, in practice, only gypsum plaster is applied to gypsum lath as a base coat. For two-coat work on gypsum lath, the recommended base coat proportions for gypsum plaster are 1:2.5. For two-coat work on a masonry (either monolithic concrete or masonry) base, the recommended base coat proportions are shown in table 7-1. Also shown in table 7-1 are proportions for three-coat work on a masonry base and proportions for work on metal lath.

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Table 7-1.—Base Coat Proportions for Different Types of Work

For three-coat work on gypsum lath, the recommended base coat proportions for gypsum plaster are shown in table 7-2.

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Table 7-2.—Recommended Base Coat Proportions for Gypsum Plaster

FINISH COAT PROPORTIONS.— A lime finish can be applied over a lime, gypsum, or portland cement base coat. Other finishes should be applied only to base coats containing the same cementitious material. A gypsum-vermiculite finish should be applied only to a gypsum-vermiculite base coat.

Finish coat proportions vary according to whether the surface is to be finished with a trowel or with a float. (These tools are described later.) The trowel attains a smooth finish; the float produces a textured finish.

For a trowel-finish coat using gypsum plaster, the recommended proportions are 200 pounds of hydrated lime or 5 cubic feet of lime putty to 100 pounds of gypsum gauging plaster.

For a trowel-finish coat using lime-Keene’s cement plaster, the recommended proportions are, for a medium-hard finish, 50 pounds of hydrated lime or 100 pounds of lime putty to 100 pounds of Keene’s cement. For a hard finish, the recommended proportions are 25 pounds of hydrated lime or 50 pounds of lime putty to 100 pounds of Keene’s cement.

For a trowel-finish coat using lime-portland cement plaster, the recommended proportions are 200 prods of hydrated lime or 5 cubic feet of lime putty to 94 pounds of Portland cement.

For a finish coat using portland cement-sand plaster, the recommended proportions are 300 pounds of sand to 94 pounds of Portland cement. This plaster may be either troweled or floated. Hydrated lime up to 10 percent by weight of the portland cement, or lime putty up to 24 percent of the volume of the portland cement, may be added as a plasticizer.

For a trowel-finish coat using gypsum gauging or gypsum neat plaster and vermiculite aggregate, the recommended proportions are 1 cubic foot of vermiculite to 100 pounds of plaster.

Estimates

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.

Mixing Plaster

The two basic operations in mixing plaster are determining the correct proportions and the actual mixing methods used.

PROPORTIONS.— The proper proportions of the raw ingredients required for any plastering job are found in the job specifications. The specs also list the types of materials to use and the type of finish required for each area. Hardness and durability of the plaster surface depend upon how accurately you follow the correct proportions. Too much water gives you a fluid plaster that is hard to apply. It also causes small holes to develop in the finish mortar coat. Too much aggregate in the mix, without sufficient binder to unite the mixture, causes aggregate particles to crumble off. Without exception, consult the specifications prior to the commencement of any plaster job.

MIXING METHODS.— As a Builder, you will be mixing plaster either by hand or using a machine.

Hand Mixing.— To hand-mix plaster, you will need a flat, shallow mixing box and a hoe. The hoe usually has one or more holes in the blade. Mixed plaster is transferred from the mixing box to a mortar board, similar to that used in bricklaying. Personnel applying the plaster pick it up from the mortarboard.

In hand mixing, first place the dry ingredients in a mixing box and thoroughly mix until a uniform color is obtained. After thoroughly blending the dry ingredients, you then cone the pile and add water to the mix. Begin mixing by pulling the dry material into the water with short strokes. Mixing is continued until the materials have been thoroughly blended and proper consistency has been attained. With experience, a person squires a feel for proper consistency. Mixing should not be continued for more than 10 to 15 minutes after the materials have been thoroughly blended. Excessive agitation may hasten the rate of solution of the cementitious material and reduce initial set time.

Finish-coat lime plaster is usually hand-mixed on a 5- by 5-foot mortar board called a finishing board. Hydrated lime is first converted to lime putty by soaking in an equal amount of water for 16 hours. In mixing the plaster, you first form the lime putty into a ring on the finishing board. Next, pour water into the ring and sift the gypsum or Keene’s cement into the water to avoid lumping. Last, allow the mix to stand for 1 minute, then thoroughly blend the materials. Sand, if used, is then added and mixed in,

Machine Mixing.— For a quicker, more thorough mix, use a plaster mixing machine. A typical plaster mixing machine (shown in fig. 7-11) consists primarily of a metal drum containing mixing blades, mounted on a chassis equipped with wheels for road towing. Sizes range from 3 1/2 to 10 cubic feet and can be powered by an electric or a gasoline motor. Mixing takes place either by rotation of the drum or by rotation of the blades inside the drum. Tilt the drum to discharge plaster into a wheelbarrow or other receptacle.

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Figure 7-11.—Plaster mixing machine.

When using a plaster mixer, add the water first, then add about half the sand. Next, add the cement and any admixture desired. Last, add the rest of the sand. Mix until the batch is uniform and has the proper consistency—3 to 4 minutes is usually sufficient. Note that excessive agitation of mortar speeds up the setting time. Most mixers operate at top capacity when the mortar is about 2 inches, at most, above the blades. When the mixer is charged higher than this, proper mixing fails to take place. In stead of blending the materials, the mixer simply folds the material over and over, resulting in excessively dry mix on top and too wet mix underneath-a bad mix. Eliminate this situation by not overloading the machine.

Handling Materials

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.

Don’t 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

Copyright   SweetHaven Publishing Services
All Rights Reserved

Revised: December 03, 2014