If you live in an older home with sagging plaster on your walls and ceilings, you can fix it. Many older houses have plaster walls and ceilings with wood lath for a base. The wood lath was installed with gaps, called keys, between each piece of lath. The plaster was forced between spaced lath, and this keying action held 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. Sagging is usually obvious. 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 your hand pressure, it's a sign that the key strength has been lost. If it's not repaired, the plaster ceiling can collapse.
Whether you patch or replace the sagging plaster depends on the extent of the damage:
If the sagging is severe, meaning that it's hanging an inch or more away from the lath base, or if it covers a large portion of the ceiling, your best bet is to remove the old plaster and replaster the ceiling, or cover it with wallboard. Not an easy do-it-yourself project.
If the sagging is slight, or covers a small area, you can reattach the plaster to the wood lath by using long drywall screws fitted with plaster washers. A 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. You thread the drywall screw through a plaster washer and then drive it through the plaster and into the ceiling joists, wall studs, 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.
To reattach the sagging plaster to the lath, drive the washer with a power screwdriver or drill so that it penetrates the wood lath, wall studs, or 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.
To repair large sags, follow these steps:
Remove the loose plaster.
Install drywall screws and plaster washers around the perimeter of the loose area.
From a piece of scrap drywall, cut a patch that completely covers the hole in the wall.
Save yourself time and trouble — make the patch a square or rectangle, even though the hole may be a different shape.
Place the patch over the hole and trace around it with a pencil.
Use a straightedge to guide your knife as you cut the wallboard along these layout lines.
If the patch is large, you can make the project go much faster by using a drywall saw. Just be careful to avoid wiring and pipes that may be hidden behind the walls.
Be sure to cut away any protruding paper facing or crumbled gypsum core from the perimeter of the patch area.
Install wallboard clips on the edges of the damaged wall by using the screws supplied with the clips.
Space the clips no farther than 12 inches apart.
Insert the wallboard patch into the hole and drive screws through the wallboard patch into each wallboard repair clip.
Snap off the temporary tabs from the repair clips.
Apply wallboard tape and wallboard compound to all four sides of the patch.
When the tape and first coat are dry, apply a second, smoothing coat.
This application is intended to smooth and conceal the tape. Don't pile taping compound in a thick coat over the tape. Otherwise, the repair will be as obvious as the hole was.
Use a sanding block to smooth the repair area so that it blends with the surface of the surrounding wall.
Apply primer and a coat of paint.
Wallboard compound absorbs a lot of paint, so plan to give the patched area several coats to make it blend with the rest of the wall.