Refasten any loose plaster areas of your plaster bathroom ceiling by screwing these loose areas to the ceiling’s wood laths.
Look for any parts of the plaster bathroom ceiling that are missing plaster boards. In these areas, position a level against the plaster ceiling so that it covers a portion of the plastered part and a portion of the part lacking plaster. Using a measuring tape, measure the distance from the top part of the level facing the ceiling and the bottom part of the wood lath in the area lacking plaster, as this will let you know the thickness your replacement drywall board needs to be.
Measure the length and width of the area lacking plaster. Measure the same area on a fresh drywall board that features the required thickness you determined and cut the drywall board to the appropriate size. Lift the drywall piece to the area of the ceiling that needs it, with the help of another individual if necessary, and screw it to the ceiling framing. Make sure the drywall board is greenboard, which is necessary for bathroom use because it is moisture-resistant. Do this for all ceiling areas missing plaster.
Lift a drywall board that is 1/2 inch thick toward the ceiling starting in one corner and hold it up using a T-shaped brace called a deadman, which stands between the floor and ceiling to support the drywall board.
Screw the short edges of the rectangular drywall board to the ceiling joists using drywall screws that are 2 inches long and have coarse threads. Put the screws about a foot apart. Repeat this until the entire ceiling is covered with new drywall boards.
Apply a drywall joint compound to the heads of the screws as well as the drywall board seams, and put drywall tape on the seams to get rid of gaps between drywall sheets. Put another coat of the compound on the screw heads, and when it dries, put yet another coat on the heads.
Apply a primer to your new drywall bathroom ceiling and allow the primer to dry. Put paint on the ceiling drywall using a color that matches the décor of your bathroom.