Plasterboard, also known as sheetrock or drywall, is a pressed-chalk material used to give walls a straighter, smoother surface than plaster alone can achieve. Plasterboard can be installed over an old plaster ceiling directly to ceiling joists. Standard ceiling joists are spaced at either 16 or 24 inches, so 8-foot-long sheets of plasterboard span them perfectly once the first seam is correctly positioned.
Locate the ceiling joists with the electronic stud finder. Mark the location of the joists on the plaster ceiling with a pencil. Draw lines across the ceiling where the joists are with the level as a guide.
Measure from one corner of the ceiling out perpendicular to the joists (so the tape measure is crossing them). Measure to the joist that's closest to 8 feet without going over.
Mark a piece of pasterboard at the measurement. Score the board with the razor knife along the measurement line and snap. Spread carpenter's glue around the back of the plasterboard, covering the whole surface in squiggly lines.
Lift the plasterboard to the ceiling. Assistance with this is recommended. Position the cut side against the wall in the corner where you started the measurement, and the factory edge on the joist that you measured out to. Sink drywall screws every 12 inches across the surface board along the joist with the screw gun.
Hang the remaining sheets of plasterboard in the same matter, lining them up in staggered courses so there are no four-way joints.
Press mesh drywall tape along all seams. Cover with joint compound and smooth flat with a drywall knife. Allow the compound dry for five hours, then apply another layer, making it flatter and thinner than the first.
Repeat the process for a third layer of joint compound. Let dry for 24 hours. Sand lightly by hand, just enough to get it completely smooth and flat. The ceiling is now ready for painting.