cling—the plaster base. A continuous concrete ormasonry surface may serve as a base without furthertreatment.BASESFor plaster bases, such as those defined by the inneredges of the studs or the lower edges of the joists, a basematerial, called lath, must be installed to form acontinuous surface spanning the spaces between thestructural members.Wood LathWood lath is made of white pine, spruce, fir,redwood, and other soft, straight-grained woods. Thestandard size of wood lath is 5/16 inch by 1 1/2 inchesby 4 feet. Each lath is nailed to the studs or joists with3-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 crackswill not occur at the joinings. Lath ends should be spaced1/4 inch apart to allow movement and prevent buckling.Figure 7-1 shows the proper layout of wood lath. Toobtain a good key (space for mortar), space the laths notless than 3/8 inch apart. Figure 7-2 shows good spacingwith strong keys.Wood laths come 50 to 100 to the bundle and aresold by the thousand. The wood should be straight-grained, and free of knots and excessive pitch. Don’t useold lath; dry or dirty lath offers a poor bonding surface.Lath must be damp when the mortar is applied Dry lathpulls the moisture out of the mortar, preventing propersetting. The best method to prevent dry lath is to wet itthoroughly the day before plastering. This lets the woodswell and reach a stable condition ideal for plasterapplication.Figure 7-1.—Wood lath with joints staggered every sixthcourse.Figure 7-2.—Wood lath, showing proper keys.Board LathOf the many kinds of lathing materials available,board lath is the most widely used today. Board lath ismanufactured from mineral and vegetable products. Itis produced in board form, and in sizes generallystandardized for each application to studs, joists, andvarious types of wood and metal timing.Board lath has a number of advantages. It is rigid,strong, stable, and reduces the possibility of dirt filteringthrough the mortar to stain the surface. It is insulatingand strengthens the framework structure. Gypsum boardlath is fire resistant. Board lath also requires the leastamount of mortar to cover the surface.Board laths are divided into two main groups:gypsum board and insulation board. Gypsum lath ismade in a number of sizes, thicknesses, and types. Eachtype 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 otherbinding agent to gypsum lath.The most commonly used size gypsum board lath isthe 3/8 inch by 16 inches by 48 inches, either solid orperforated. This lath will not burn or transmittemperatures much in excess of 212°F until the gypsumis completely calcined. The strength of the bond ofplaster to gypsum lath is great. It requires a pull of864 pounds per square foot to separate gypsum plasterfrom gypsum lath (based on a 2:1 mix of sand and plastermortar).7-4
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