How do I Repair a Water Damaged Plaster Ceiling?

Draw a square around the damaged area, using a carpenter's square and pencil. Make two parallel sides of the square run along the middle of the joists that run along either side of the area. You should be able to see or feel the joists through the damaged plaster.

Cut out the square area from the ceiling using your reciprocating saw. You should be left with a square hole that has half-exposed joist edges on either parallel side.

Measure the square. Mark it out on a piece of drywall. Cut out the square with a utility knife, just inside the lines, so the piece will be slightly smaller than the hole. Cut it by scoring, then snapping the piece.

Hold the square patch to the ceiling. Shoot drywall screws in along the edges where there are joists behind the board, putting the screws about every six inches. Sink the heads completely.

Press mesh drywall tape over the seams all around the perimeter where the drywall meets the surrounding plaster. Cover the tape with joint compound, spreading it flat with a 6-inch drywall knife. Let it set for 8 hours.

Spread a second layer of compound over the first, making it a little wider than it was and keeping it very thin. Let it dry for 8 hours. Apply a third layer, and let it dry for 8 hours.

Sand the joint compound flat and smooth with a drywall sander. Repaint the ceiling.