Even though old plaster ceilings were built very differently than modern ceilings, they can be repaired much the same way: with sheetrock, mud, and tape. If you want to restore an old home to its original plaster condition, contact a professional. Otherwise, follow these instructions for patching a plaster ceiling.
Step 1 - Remove Damaged Material
Put a drop cloth down on the floor and remove any furniture you don't want stained or damaged during this DIY task. Then remove the damaged plaster and mortar in the shape of a square or rectangle, so that you can easily cut a replacement piece to fill in the hole of sheetrock. Use a utility knife to mark the section to be removed. Next, use a hammer and chisel to knock out the damaged area.
Depending upon the size and location of the damage, you may want to remove more than the damaged area so that you can attach your patch to a ceiling joist. Plaster ceilings were commonly made using wooden strips or lathes as braces that were attached to the studs and joists. Although you can attach a patch to a lathe, your patch will be better supported by screwing it to the joists rather than the lathes.
Step 2 - Cut Replacement Patch from Sheetrock
From sheetrock, cut a patch the same measurements as the hole you knocked from the ceiling. Be aware of the thickness of your plaster ceiling when choosing sheetrock. Sheetrock comes in various thicknesses. The average sheet of sheetrock is 1/2 inch thick. If your ceiling is thinner, you can purchase sheetrock 3/8 inches thick instead. You may need to knock out your hole before buying the sheetrock if you are unsure what size you will need.
Step 3 - Attach Patch to Ceiling
Screw your ceiling patch to the ceiling joists with drywall screws. Space screws several inches apart, trying to keep the distance between screws uniform.
Step 4 - Apply Mud and Tape
With a smaller drywall knife (a 6-inch knife works well), spread a layer of mud over the joints between your old plaster and the new patch. Fill in screw holes as well. Measure out pieces of drywall tape to go over the joints, being careful not to overlap pieces of tape. At the same time, cover all the joint space with tape. Go over the tape with the knife, holding it as parallel to the wall as possible. Make firm strokes, pushing out some mud as you stroke lengthwise with the tape and joints. Allow it to dry for 24 hours.
Step 5 - Apply More Coats of Mud
Add more mud with a 10-inch knife to cover the tape and smooth together the old and new. Feather outward as you apply mud so that the mud reaches out farther than the original damage. Allow it to dry in between additional coats.
If you plan to add texture over your patch, fewer coats will be necessary as the texture will cover up imperfections. Sand lightly after your final coat. Cover the patch and any newly mudded areas with a primer. Finally, paint over the entire plaster ceiling to smooth out and bring together the old and the new.