Plaster ceilings tend to crack from settling, and they don't hold up well to moisture damage. A leak in your roof or in an upstairs room above a first-floor ceiling can result in moisture damage to your ceiling. If the damaged area is relatively small, you probably can fix it yourself. If the damage is extensive, however, you might have to call in a professional. And don't neglect to fix the leak that caused the damage, or you will end up fixing the ceiling again.
Tap a finishing nail into the ceiling plaster around the damaged area to locate the lath slats. Mark each hole where you hit the lath. Pull out the nail, and continue until you have found all the lath locations where the water damage is.
Place flathead screws into plaster washers, and drill them into the holes. The washers are round disks that will hold up the plaster. Try to get the washer situated so that it covers both sides of a crack.
Use a flat-edged trowel to cover the damage and washers with joint compound.
Cut a piece of mesh to fit over the space where you spread the compound. Press it into the ceiling coating while the coating is still moist. Let this dry for a couple of hours or so, then apply another coat of compound over top. Be sure that it completely covers the mesh. Spread it thinner along the edges of the patch to bring it down smooth with the ceiling. Wait 24 hours for it to dry.
Sand the rough surface of the compound to smooth it out. If there is a thin area, add some more compound to fill it in. Let this dry another day.
Paint the patch to match the rest of the ceiling. Use a paintbrush to apply the paint.