In a traditional lath and plaster system, what backs the plaster?

• A. Gypsum board backing with veneer plaster
• B. Metal lath backing only
• C. Lath backing
• D. No backing

Answer: (C)

Back the plaster is the lath. In a traditional lath and plaster system, a lattice of wood strips or metal lath is fastened to the studs, and the plaster is built up over this backing. The lath provides a rough, keyable surface for the first coat to grip and also creates a small air space that helps the plaster dry and move with temperature and humidity changes. This backing is essential for the plaster to hold and resist cracking; without it, or with a different modern substrate such as gypsum board, you’d be using a different system. So the plaster is backed by the lath itself, which can be wood or metal.