Plaster applied to wood lath is held in place by the keys that form when it squishes through the lath. Over time, these keys can disintegrate; this causes plaster to crack, crumble, and fall away from the lath. Settling of a house or the occasional earthquake can speed this process.

Some plaster is of poor quality, and this may cause the plaster to crack and crumble. In addition, water damage from roof or plumbing leaks discolor plaster and cause peeling or efflorescence, the leaching of salts and minerals to the surface.

Plaster can be damaged from the normal stresses and strains of people living in a house: holes from artwork hung on the wall, scratches from furniture, spilled liquids.

Fortunately, most of these simple problems can be easily fixed. Cracks leading from windows and doors or along seams in the ceiling can signal more significant structural damage or deterioration of the keys in the plaster throughout, so you may want to call a contractor to take a look.

If you must repair a hole in a plaster wall where there isn't a backing that will hold the plaster from behind, you'll have to install backing first.

To do this, push wire mesh into the hole and hold it in place with wire ties wound around one or two dowels or sticks. After applying the plaster, let it dry, then snip off the wire ties.