Drywall Over Cracked Plaster Ceiling
How to replace missing plaster with drywall
I'm restoring a house that was built in 1840. One bedroom has a ceiling with two large areas of plaster that are missing, although the wood slats are still in place. I'd guess the missing plaster amounts to about 20 percent of the ceiling area. When I ran into this problem in another room, I wound up replastering the whole ceiling. This time I was thinking of covering the ceiling with drywall. The joists are 24 inches on center. Can the ceiling take the extra weight? — H.T., Massies Mill, VA
Tom Silva replies: The framing in the ceiling will hold it, no problem. But before you start this project, first remove any loose plaster or use plaster washers and screws to secure it to the lath, those wood slats you've referred to. Then find out how thick the plaster is: Place a straightedge on the plaster ceiling so it spans one of the areas missing plaster, and measure from the edge of the level nearest the ceiling to the exposed surface of the lath. If the plaster is 3/8 inch thick — that's pretty common — then cover the areas where the plaster is missing by screwing 3/8-inch drywall to the framing, not the lath alone. You don't have to do any fussy fitting; this drywall is there just to provide support for the finish layer of drywall.
I'd use ½-inch drywall for the finish layer because it levels out better than the 3/8-inch stuff. You're going to need help at this point from at least one person to hoist the panels up, and a "deadman" to hold them against the ceiling. A deadman is a T-shaped support made out of 2x4s that you wedge between the ceiling and the floor to hold up the drywall long enough to screw it in.
Arrange the drywall sheets so that their short edges can be screwed to the joists or strapping, if the ceiling has it. Two-inch, coarse-thread drywall screws should be long enough to do the job. Space them 8 to 12 inches apart and make sure you don't drive their heads so deep that they break through the paper. Then fill the joints and screw dimples with joint compound and paper tape. After a couple more coats, each applied after the previous one is dry, and some primer and paint, your ceiling will look as good as new.