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The short answer is yes, a sagging plaster ceiling is dangerous. Sagging plaster on a wall is something you should fix immediately, but it isn’t a major concern. If the same type of sagging occurs on a ceiling, however, you could be in danger of your ceiling falling in large chunks. When plaster sags, it loses its strength and ability to hold itself up. The bonds that hold it together are strong, though, so if a sagging area falls, it will take large pieces down with it.

You’ll want to inspect your plaster walls for signs of damage before painting. If you notice soft spots, deep cracks, bubbling, or areas where the plaster is crumbling or pulling away from the wall, it likely needs repair. While small hairline cracks are common and likely cosmetic, you should address larger issues before painting to ensure a quality, smooth finish.

You can paint over plaster, but repair any damage to the plaster first, or the paint will crack and bubble. You should also prepare the surface of the plaster to accept paint, which gives your paint job the best chance of looking good and lasting a long time. Before applying the color you want, apply a coat or two of primer. Additionally, use latex paint, and choose a latex paint with a satin finish for areas with dampness, like bathrooms.

Plaster itself has a higher R-value than drywall. However, there are other considerations surrounding plaster and insulation. As a general rule, it's easier to obtain consistency in insulating value and quality when working with drywall rather than plaster. Plaster walls often don't contain any insulation behind the wall covering by design, and careful evaluation is necessary when choosing to add it.