Plaster walls were commonly used in homes in the early 20th century until cheaper, easier-to-install gypsum board hit the market. Plaster is applied to wood lath attached to wall studs. Wet plaster is pushed through the lath, and plaster ears hang behind the lath to help plaster adhere to the wall. While plaster is more durable than drywall, it is more likely to need repairs as it ages because it can dry out and weaken.
Moderate settling of the house can cause small hairline cracks, but more commonly these occur as the plaster ages and slightly weakens. Because the cracks are small, there is no reason to remove and repair the plaster. Instead, the crack can be filled in with a small amount of drywall patching compound and sanded to a smooth finish.
If the plaster has deep, wide cracks more than 1/4 inch wide, it could be a sign of structural issues with the home, especially if the cracks are near the corners of the house, doors or windows. If you have deep cracks, remove any loose or broken pieces of plaster inside the crack and patch the crack with mesh tape and drywall compound. Deeper cracks can be a sign that the plaster ears between the lath have begun to break off and can no longer hold the plaster in place on the wall.
Bulging or Sagging Plaster
If the plaster is bulging or sagging away from the wall, it indicates a major problem with the plaster. When this problem occurs, the plaster ears have broken off from the lath. If this is a new problem, you might be able to correct it by trying to screw the plaster back into the lath, then patching over the screw holes. If the bulge is severe, tearing out the plaster and either replastering or hanging new drywall on may be your only option.
Difficulty Removing Plaster
Removing plaster walls can be a problem because of the weight of the material, plus the underlying lath means you have to remove two layers of material. Removal first requires you to scrape the heavy plaster off the wall with a shovel and dispose of it. Once that is done, you have to go back and pry all of the wood lath off the walls. From start to finish, it's a messy and strenuous job.