Lath and plaster is a somewhat outdated building process used mainly for interior walls. Its use began to decline in the late 1950s as drywall began to replace it.
The process begins with wood laths. These are narrow strips of wood nailed horizontally across the wall studs. Over this goes two coats of plaster: a rough, sandy coat (called a brown coat) and a smooth, finish coat. After the plaster is completely dry, the walls are ready to be painted.
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).
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.
