Things You'll Need
1-foot by 2-foot Wood furring strips
Plastic sheeting or dropcloths
Chisel or stiff-bladed putty knife
T-square or straightedge
Screw gun with drywall screw attachment
Joint compound (mud)
6-inch and 10-inch drywall knives
Sanding blocks or drywall sanding tool
Fixing an old plaster ceiling that is falling down may require some extensive repairs, or may even require complete replacement of the old plaster with sheetrock. First, you have to assess the extent of the damage and determine whether there is a leak or moisture problem up in the attic.
How to Fix a Falling Plaster Ceiling
Make sure there is no leak in the roof that is causing the plaster to come down. No matter what you need to do next, any necessary roof repairs must be done to ensure everything above the ceiling is dry. Sometimes past leaks or simply old age is making the plaster fail. Hire a roofer if you need to make any repairs to the roof.
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Understand how the old lathe and plaster was originally done. Thin strips of wood called lathe were nailed in rows to the studs and ceiling joists. Over this was spread a layer of mortar, often with horse hair or even chicken feathers mixed in to give it strength and flexibility. One this was dry, the plasterer spread a thin layer of lime plaster. This wall system was much harder and more durable than modern gypsum sheetrock walls, but it was a lengthy and expensive process done by true craftsmen. Few people nowadays know how to do old-time plaster work. The overwhelming majority of modern walls and ceilings are done with sheetrock, joint compound and tape. If your home is of particular historical significance, it might be worthwhile to find someone who can do a true plaster repair. However, most repairs can be done quite well by patching in with sheetrock.
If the damage is confined to a small enough area. you may be able to simply patch that area in with sheetrock, tape and mud. Remove all the loose and damaged plaster from the ceiling. You will probably want to cover the area below with plastic sheeting or drop cloths. Cut a rectangular or square area so you can cut a piece of sheetrock to fit. You will expose the lathe, so try to remove the plaster back to a ceiling joist so you can attach the sheetrock securely to the joist instead of leaving it hanging only screwed to the lathe.
Use the utility knife to score the surface of the plaster along a straightedge as deep as you can. Then use the hammer and chisel or putty knife to carefully remove the old plaster in as straight a line as you can.
Buy the right thickness of sheetrock. The regular 1/2-inch sheetrock may be too thick and will be higher than the original surface. Sheetrock is available in 3/8-inch thickness, which may work better. Measure the height of the surrounding ceiling plaster as accurately as you can.
Cut the sheetrock to fit, and screw it into the joists. Space the screws about six inches apart along the joists for a secure patch.
Fill the mud pan with joint compound (commonly called mud). Using the six-inch drywall knife, lay a good bed of mud along the seams. Cut the tape to the correct length and embed it well into the mud by pressing it in firmly with the drywall knife. Patch over the screw dimples as well. Let this dry for 24 hours. It may shrink quite a bit.
Using the 10-inch drywall knife, go over the seams again with joint compound, this time feathering it out as smoothly as you can. Since this is a thinner coat, it will take between four and eight hours to dry. If you are very good, two coats may be sufficient and will sand smooth. If you are matching a texture over the repair, two coats will be plenty. Once dry you can sand it smooth and do the texture. If your ceiling is smooth, chances are you will need to do a third light "touch up" coat of mud to look really good.
How to hang a whole new ceiling over the old one.
Cover the floor and any remaining furniture with plastic sheeting or drop cloths. Remove the worst areas of loose plaster from the ceiling.
Use a stud finder to locate all the ceiling joists and mark them along the ceiling.
Attach wood furring strips right over the old plaster along the joists with 2-inch screws. Run the furring strips around the edge of the entire ceiling where the ceiling meets the wall. This gives you a framework to hang new sheetrock right over the bad ceiling.
Hang new sheetrock to the furring strips, following Steps 6 to 8 above.
Don't do this work around children. Keep children well away from the area when you are removing the old plaster. At least some of the old layers of paint will be lead-based, and you do not want to expose children to lead. Keep the work area clean and vacuumed. Never put unused mud back into the original bucket as you can contaminate it with dried material. Dispose of it into a trash can or bag, not down the sink.