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Lath and Plaster


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 United States. 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, many lath and plaster ceilings are replaced[1] with plasterboard, which 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. In the UK, riven or split hardwood laths were used of random lengths and sizes. Splitting the timber, as opposed to sawing in straight lines, followed the grain of the timber which greatly improved strength and durability.

Next, temporary lath guides are placed 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. Traditional lime based mortar/plaster often incorporated horsehair which reinforced the plasterwork, helping to prevent the keys breaking away.


Lath and Plaster

Laths seen from the back with plaster visible between them. ©2005 Asmithmd1