Lath and plaster is a building process used mainly for interior walls in Canada and the United States until the late 1950s. After the 1950s, drywall began to replace the lath and plaster process in the these countries. In the United Kingdom, lath and plaster was used for some interior partition walls, but was mostly used in the construction of ceilings. In the UK, plasterboard became a more common ceiling construction from 1945 onwards.
The process begins with wood laths. These are narrow strips of wood nailed horizontally across the wall studs. Each wall frame is covered in lath, tacked at the studs. The lath is typically about two inches wide by four feet long by 1/4 inch thick. Each horizontal course of lath is spaced about 1/4 inch 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..." 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 the past, horsehair 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 or plasterboard (also a type of gypsum wall board, although a bit thicker), since it can be installed without application of plaster to finish the surface. As plastering is a skilled trade, and hence expensive, reducing the amount of plastering required in a new development also reduces the building costs.
An advantage of using lath is for ornamental or unusual shapes. For instance, building a rounded wall would be difficult if drywall was used exclusively, and drywall is not flexible enough to allow tight radii.