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Yes, you can install drywall over plaster, but there are important factors to consider. While it can create a smooth, flat surface, it is often not the best approach. Installing drywall over plaster will increase the wall's thickness, which can cause issues with existing window and door frames that may no longer fit. You might also need to adjust power outlets to keep them accessible. A better, though more expensive, option is to remove the old plaster and lath entirely before installing new drywall. Drywalling directly over plaster seals in the old material, and if any problems arise later, you will need to remove both layers down to the structural beams for repairs. Given the potential complications, this job is best handled by professionals.

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.

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.

You can put certain types of plaster over other types, but doing so isn't always the best idea. For example, putting a coat of gypsum over lime plaster is possible, but doing so will limit the lime plaster's breathability and potentially affect its performance.

Since cement plaster tends to leave a textured surface, no matter how much you smooth it, some people prefer to use gypsum plaster as a topcoat over cement plaster, creating a smoother surface.