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You can usually tell if you have plaster or drywall by touch (plaster is harder, colder, and smoother than drywall) or by tapping on it (drywall sounds hollow, plaster does not). If you still cannot tell, peek under an outlet cover and check the cut edges. The rough edges show the composition of the wall. You will see either drywall (two paper sheets sandwiching gypsum material) or only plaster material (no paper sheets).

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.

A 50-pound mirror can be hung on drywall, but only if it has the support of strong hardware. Ideally, you can find the studs behind the drywall and attach the mirror to them; this will give your mirror better stability. Otherwise, confirm that you are using anchors appropriate to the weight of the mirrors. Another technique is to use French cleats—two pieces of wood that fit together, with one piece attached to the wall and the other to the mirror. This allows the weight to be distributed over a wider area.

In most cases, no, you won’t need a permit to replace drywall. If you’re changing the floorplan of your home by adding or removing walls during the drywall project, then you might need a permit, but replacement or repairs usually doesn’t require a permit. Your drywall professional should be able to confirm whether or not a permit is required for your project. If it is, it’s likely to add between $50 and $150 to your total cost. Keep in mind that you will need permits if you plan to install new electrical wiring or plumbing lines behind your new drywall.