Things You'll Need
Drywall screws and washers
A cracked or sagging plaster ceiling is not only unsightly but also poses a structural hazard unless immediately repaired. Ceiling cracks and droops occur for a variety of reasons, including improper installation, moisture damage and a slipped foundation. Left untreated, ceiling cracks will enlarge, or the weight of a sagging plaster ceiling can pull the surrounding ceiling down with it. However, the ceiling will need to be replaced if the entire section is sagging. Spread drop cloths on the floor, and set a sturdy stepladder under the section of the ceiling you want to repair.
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Score the ceiling plaster around the crack using a putty knife, forming a rectangle. Space the knife's blade several inches from the crack. Repeat this process to mark the locations of the remaining cracks on the ceiling.
Insert a tube of silicone caulk into the caulking gun. Break the tube's seal with a nail. Position the tip of the gun at a corner of the crack and press the trigger to fill the void.
Maintain steady pressure as you spread a thin bead of caulk into the crack. Press the caulk with a wet thumb until smooth. Wipe away excess caulk from around the edges using a rag.
Bore 3/16-inch holes every 6 to 8 inches along the section of the sagging plaster ceiling and into the wood lath underneath.
Use a caulking gun to inject acrylic adhesive through the holes, until you feel a bulge on the plaster.
Insert drywall screws with washers in the holes to draw the sagging plaster ceiling upward on the lath. The adhesive will seep out through the drywall screw holes. Allow the glue to cure for the time specified on the label directions.
Cover the repaired section of the ceiling with joint compound, using a trowel. Ensure you cover the washers as well.
Measure and cut mesh screen to fit over the repaired ceiling section. Press the screen onto the compound and smooth it with a trowel. Allow the compound to cure for the time specified on the label directions.
Smooth high spots on the repaired section of the ceiling using 100-grit sandpaper. Prime the section before painting it the desired color, so it blends with the surrounding ceiling. Allow the primer to dry thoroughly before covering it with paint.