Lath and plaster
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Lath and plaster is a building process used mainly for interior walls in the United States until the late 1950s. After the 1950s, drywall began to replace the lath and plaster process.
The process begins with wood laths. These are narrow strips of wood nailed horizontally across the wall studs. Each wall frame is coverd in lath, tacked at the studs. The lath is typically about two inches wide, by four feet long, by a quarter inch thick. Each horizontal course of lath is spaced about a quarter away from its neighboring courses. Next, temporary lath guides are sparsely vertically to the wall, usually vertically at the studs. Plaster is then applied, typically using a wooden board as the application tool. The applier drags the board upward over the wall, forcing the plaster into the gaps between the lath, and leaving a layer on the front the depth of the temporary guides, typically about 1/4 inch. A helper feeds new plaster onto the board, as the plaster is applied in quantity. When the wall is fully covered, the vertical lath "guides" are removed, and their "slots" are filled in, leaving a fairly uniform undercoat.
It is standard to apply a second layer in the same fashion, leaving about a half inch of rough, sandy plaster (called a brown coat). A smooth white finish coat goes on last. After the plaster is completely dry, the walls are ready to be painted. In the photo, "lath seen from the back, brown coat oozing through," those curls of plaster are called "keys" and are important to keep the plaster on the lath. Insufficient "keying" and the plaster will fall off the lath. In Scotland horse-hair was used to help bind the plaster to the laths.
Eventually the wood laths were replaced with rock lath, which is a type of gypsum wall board available in sheets size 2 by 4 feet. The purpose of the four-foot length is so that the sheet of lath reaches exactly across three wall studs, which are spaced 16 inches apart on center (American building code standard measurements).
In addition to rock lath, there were various types of diamond mesh metal lath which is categorized according to weight, type of ribbing, and whether the lath is galvanized or not. Metal lathing was spaced across a 13.5 inch center, attached by tie wires using lathers' nippers. Sometimes the mesh was dimpled to be self-furring.
Lath and plaster has been replaced altogether with drywall (also a type of gypsum wall board, although a bit thicker), since no plaster is applied afterwards.
Although plastering as a building process has all but disappeared, there remain still a few skilled professionals (plasterers) who are employed mainly to do small jobs and patchwork in older buildings where drywall may not as easily match the existing finish.
A possible advantage of using lath is for ornamental or unusual shapes. For instance, building a room with rounded corners would be difficult if drywall was used exclusively.
[edit] See also
[edit] External links
- Preservation Brief No. 21 by the National Park Service. Contains detailed descriptions of Lath & Plaster construction and advisories on repairing and restoring these walls (especially in historic buildings).
