If your plaster or drywall ceiling has been exposed to moisture for very long, it's probably gotten soft and cracked. You should deal with this immediately, since the material will eventually start crumbling and falling. After the source of the water leak is fixed, you'll want to cut out the damaged area and install a drywall patch.
Instructions
1. Mark out a square around the perimeter of the damaged area with your level and pencil. Mark it so that two of the sides of the square run along the middle edge of the ceiling joists at either side of the damaged area. If you can't see the joists to assess their positions, pull down parts of the damaged area of the ceiling until you can.
2. Cut out the square using your jig saw. Half the joist edges should be exposed on of the square.
3. Measure each side of the cutout hole and mark out the dimensions on a sheet of drywall. Use a razor knife to score and snap the piece.
4. Set the square patch into the hole. Secure it there with drywall screws along the edges that have joist behind them, putting screws about every six inches.
5. Lay mesh drywall tape over sides of the piece of drywall, so it covers the seam between the patch and the surrounding ceiling. Spread joint compound over the drywall tape with a drywall knife, completely covering each line and making it smooth. Let it dry about six hours.
6. Sand the joint compound with a drywall sander, just enough to get it flat. Apply a second layer of joint compound. Let it dry, then sand it.
7. Spread on a third layer of joint compound, let it dry, and sand it. Paint the ceiling.
Tags: damaged area, joint compound, with drywall, drywall tape, either side, layer joint