It’s not a good idea to plaster over broken plaster (unless by broken you’re referring to cracked, in which case that’s perfectly fine). However, broken plaster can be a sign of a larger underlying issue like foundation damage or severe damage to the lath holding up the plaster, so it’s worth investigating before you simply plaster on top of the damage.

Costs depend on damage size, labor rates, and whether additional services like painting are required.

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.

Yes, walls on the top floor can be load-bearing if stacked on the floor’s foundation.