Backing plaster is a type of plaster used in construction to provide a base layer for the application of other plasters. It is usually applied to LATH, which is a thin, narrow strip of wood or metal used as a backing material for plaster. LATH is typically nailed to the wall studs and then covered with backing plaster. The backing plaster provides a strong, durable base for the application of other plasters, such as finish plaster.