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If the holes in the plaster are small, you can fill them with layers of patching plaster until they’re filled. After it dries, you can sand the plaster and paint over it. If the hole is much larger, consider using bonding plaster first and then a layer of patching plaster, completing the process with sanding and a layer of paint.

The two most common drywall thicknesses for ceilings are 1/2-inch and 5/8-inch. For most standard residential applications, 1/2-inch drywall is sufficient and widely used. It provides adequate support and is lighter and easier to handle during installation compared to thicker sheets. However, 5/8-inch drywall is often used for specific cases where additional benefits are needed. It is heavier but provides superior sound dampening and fire resistance. You should consider using 5/8-inch drywall for ceilings in commercial buildings, between a garage and a living space, or above a finished basement to meet building codes or for enhanced soundproofing.

This depends on how much plaster is applied and in how many layers, but a decent estimate is around 150 square feet. If you are applying only two coats on a small wall, there is a chance you may only need one or two gallons for the entire project. If you are plastering a large living room wall with high ceilings, using one of the more involved techniques–like Scagliola–that requires several coats, expect the project materials costs to add up quickly. 

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.