Plaster on wood lath. This is one of the oldest types of plaster. Wood lath are narrow strips of wood which are nailed onto the wall studs and ceiling joists. They are spaced approximately 1/4" (6 mm).
The plaster is applied in two or three coats. When the first coat is applied it squeezes between the wood lath and curls behind them. The hardened curls of plaster are called "keys" and hold the plaster in place. The plaster does not adhere to the wood lath. When the keys break off, the plaster loosens its grip, which will cause irregular cracks and, in severe cases, falling plaster.
Minor plaster repairs can be made with drywall patches. In most cases it is easier and faster to install new drywall directly over the old plaster using long drywall screws. This method, however, adds a considerable amount of weight to a structure and should be limited to a few walls or ceilings. Plaster on gypsum lath. Lath came in small 16"x48" sheets and took considerable less time to install than the wood lath it replaced. Hairline cracks are typical for this kind of application. Parallel running cracks are 16" (400 mm) apart. Wire lath was installed around door and window lintels and inside/outside corners.
