Before You Begin
When plaster walls are first created the plaster is forced between the wood lath and the plaster hardens between the spaces and forms keys that hold the plaster in place. As plaster ages, these keys may break away from the lath, and the plaster coating can come loose and sag away from the lath. If you have sags in a plaster ceiling, press upward on the area with the flat of your hand. If the plaster feels spongy or gives under the pressure, it's a sign that the key strength has been lost. If it's not repaired, the plaster ceiling can collapse, and the heavy plaster can damage furnishings or hurt someone.
If the sagging is only slight, or covering a small area, it can be reattached to the lath by using long drywall screws fitted with plaster washers. The plaster washer is a thin metal disk that increases the size of the head of a drywall screw so that it doesn't pull through the plaster.
The drywall screw is threaded through a plaster washer and then driven through the plaster and into the ceiling joists or wood lath. The screw and washer pull the loose plaster tight against the framing, restoring the ceiling. By surrounding the area with plaster washers, you can stabilize the plaster so that it doesn't sag any further.
You may find plaster washers at hardware stores in neighborhoods with old houses. We found them at Charles St. Supply Co. in Boston where you can buy them online or by mail-order. Many hardware stores carry them, too.
Step 1 : Remove the loose plaster
Pull the loose plaster from the wall. Work carefully and remove only the loose plaster that has broken off from the surrounding plaster.
Step 2 : Install drywall screws and plaster washers around the perimeter of the loose area
To reattach the sagging plaster to the lath, drive the screw through the washer with a power screwdriver or drill so that it penetrates either the wood lath or the ceiling joists.
To avoid cracking the plaster and creating an even bigger repair job, don't pull the plaster tight to the lath in a single motion. Instead, start a few washers around or across the sagged area and drive them snug against the plaster face. Then tighten each of them slowly, moving from one to another, so that the plaster gradually pulls tight against the lath.
Step 3 : Apply wallboard compound and tape to patch
Apply a thin coat of wallboard compound to the patch area, then imbed the tape in the compound and apply another thin coat of compound and allow it to dry.
Step 4 : Sand the patch smooth
After the drywall compound has hardened, sand the area smooth will a sanding block. Dust the area and check for small imperfections, a second and third coat may be necessary.
Wallboard compound is very porous and will absorb paint so you will probably have to give the area two coats of latex paint to make it blend in with the rest of the surface.