When hairline cracks start showing up in old ceilings, most old-house owners fear the ceiling is going to cave in. Plaster ceiling cracks are most often due to building movement, water damage or just plain age. They seldom mean the ceiling is falling down. Working overhead may make repairing cracks more inconvenient, but with some basic equipment and patience, just about anyone can repair minor cracks and make the ceiling look like it was fixed by a "pro."
Spread newspaper or dust sheets on the floor--you'll make a mess and wet plaster can stain carpets. Clean out the crack with a matt knife, the sharp tip of a can opener or a screw driver, depending on the width of the crack. Tap the plaster around the crack as you clean it out to make sure that the surrounding plaster is solidly "keyed" (meaning its caught on to) to the underlying lath (the narrow strips of wood nailed across studs to hold the plaster).
Remove any loose plaster; it will just shift to reopen the crack if you ignore it. Brush the dust out of the openings and spray with a fine mist of water so the plaster does not absorb the water out of the new plaster too rapidly. Mix about a quart of joint compound to use as "mud." If necessary, add a little more water to make it easy to work with, but not so thin that it drips off the ceiling.
Fabricate backing for holes with broken lath. Tie a string through the centre of a piece of wire mesh and slip it behind the broken lath. Pull the string to hold the mesh flat against the lath as you fill the hole with mud. The openings in the mesh will serve as keys for the plaster. A piece of wallboard or even fiberboard also can be used as backing, but they will not provide the same spaces on which the plaster can key. Use a dab of glue or construction adhesive to hold flat backing to the lath. Put a string through the centre of whatever backing you choose, which will allow you to pull the backing toward you as you plaster, insuring contact with the mud coat. Let larger holes dry completely before adding tape.
Wait until any patches are completely dry; plaster shrinks as it dries and may need to be filled with a second coat of mud before taping. Fill each crack and draw the knife along it to leave a thin bed in which to place your tape. Lay the tape along the crack and gently draw the knife along the tape, easing the air and unevenness out and setting the tape into the mud. Hold the knife at a 45-degree angle as you pull it evenly toward you down the line of tape.
Finish your work with a "skim" coat. Scrape away any high spots and lay on a thin coat of plaster over the mud coat, overlapping its edges. Feather the edges of this coat with your knife at a low angle, working the patch until the new plaster is level with the surrounding ceiling. Allow the plaster to dry before repairing detail, removing imperfections and sealing the new plaster with a bit of drywall sealer to prepare it for painting.
Always fix any underlying problems (leaks, lath deterioration) before starting on the ceiling to avoid having to fix the same crack again. Professionals have lively arguments over whether paper or fibreglass tape is better. Paper is thinner and easier to skim flat, but fibreglass has great grip. Choose what's easiest to use for you. There are all sorts of labour-saving materials, from tape that does not need mud to a repair compound the consistency of whipped cream that can be used without tape. Some work quite well. Once you've mastered the traditional method, try some of these materials.
Always wear safety glasses when working with plaster, especially when working overhead. If you discover plaster "hanging" loose from the lath around the crack, you have a problem larger than a simple crack. "Delamination" will require the use of plaster "washers"--hardware that reattaches plaster to lath, a technology involving drilling holes and injecting adhesive between plaster and lath. If the problem is extensive, the old plaster will probably have to be removed and the ceiling replastered.