Historic Charleston Foundation - Charleston, SC

laster was the traditional interior wall covering in Charleston until the 1920s and '30s because of its ability to be painted, whitewashed or wallpapered. Plaster is a beautiful, strong and durable wall covering that also has some practical properties such as resisting fire and reducing the transmission of sound. Whenever possible, flat wall plaster and ornamental plaster should always be retained in a historic house.

Ceiling lath

Lath is the frame that holds plaster to the wall or ceiling. Made of thin wooden strips, or later of metal mesh, lath is attached directly to the structure of the building and receives the plaster, made of lime and gypsum, in its wet form in three coats. When the plaster dries, it becomes an extremely durable material that should last almost indefinitely. When plaster does crack or separate from the lath, it can be an indication of a more serious problem.

Tension cracks, usually identified as diagonal cracks, may indicate the settling of the structure itself or exposure to severe vibrations. Building settlement can be caused by settling over time due to unstable soil, insufficient footings or deterioration of the foundation. Visualize a perpendicular line from the crack line to the floor and it will generally point to the trouble area.

Tension fracture in masonry

Map cracking occurs when the finish or top layer of plaster is pulling away from the base coats. This is generally caused by water damage or excessive moisture. Plaster that appears to be "bunching up" in a fashion similar to an alligator's skin is a sign of old plaster that is no longer adhering to the substrate, or lath, and needs replacing. Generally, only plaster that is failing should be replaced. Wholesale removal of interior plaster is often unnecessary.

Hairline cracks in plaster, caused by shrinking materials, should be left alone or painted over. Continue to watch these cracks, however, for further cracking.

Additional Resources
Preservation Brief #21, "Repairing Historic Flat Plaster/ Walls and Ceilings" by Mary Lee MacDonald

Preservation Brief #23, "Preserving Historic Ornamental Plaster" by David Flaherty