How to Join Plaster Ceilings

Lay out dropcloths on the floor under the plaster ceiling gap. Place a ladder under the empty space between the two sides of the plaster ceiling.

Climb the ladder and measure the length and width of the gap with a tape measure.

Place a drywall panel on the floor or on a table. Using the tape measure and a pencil, draw lines equal to the gap's dimensions for a cutting template. Flip the panel over and draw the same measurements to match the other side.

Score along the lines in the drywall panel with a utility knife, then flip the panel over and score along the opposite side.

Gently break off the excess drywall from the panel to get a patch to fill the gap in the plaster ceiling.

Situate the drywall into the gap to conceal it. Hold it against the trusses and fasten it to the trusses, using a screw gun with drywall screws.

Press drywall tape over the seams between the drywall and plaster. Spread joint compound directly over the drywall tape with a putty knife, using the blade to smooth it as much as possible. Let the joint compound dry for at least 12 hours or as long as the manufacturer recommends.

Abrade the compound and the drywall patch with a sander, using medium-grit sandpaper after the compound has sufficiently dried.

Apply a coat of primer over the drywall, using a paint tray and roller. Allow the primer to dry between three and four hours. Apply the first coat of paint to the drywall with the roller and let dry two to three hours, then apply a second coat of paint to conceal the patch and make the plaster look uninterrupted.