Gypsum (Rock) Lath

Gypsum or rock lath is a pre-manufactured plaster board, generally 16 inches by 48 inches in size, and 3/8 inch thick.  Rock lath became popular in the 1930s as a less expensive alternative to wood lath.  It is nailed directly to the wall studs and receives two coats of plaster over it.  The rock lath is called the first coat and replaces the wood lath and the brown coat of the previous wet plaster system.  The second coat is a cement plaster about 1/4 inch to 3/8 inch thick.  The finish coat is then applied, which is comprised of hard finish plaster, and is approximately 1/8 inch thick.

Rock laths can be identified by a crack pattern that may be visible in ceilings and walls or may be visible on the backside of walls in areas such as the attic.