Flaking and cracking plaster are relatively minor maintenance items, but when the plaster starts to sag, or “belly” out, from a wall or ceiling, it indicates deeper problems that require your full attention.

Plaster is heavy, and it needs a solid, well-anchored base to support its weight. This base is usually strips of wood or metal wire mesh (both referred to as lath) that are nailed to the wall and ceiling framing. When the plaster is applied, it squeezes through the lath, creating “keys” that harden to form a strong integral bond with the wall.

Over the years, plaster can dry out and lose its holding strength, or weaken from vibration, and the keys begin to break away. The lath can also pull away from the framing. Gravity and the weight of the plaster exert themselves, and the first sign is often sagging, followed eventually by the collapse of the plaster surface.

At this point, you may want to call in a professional. Repairing a large wall area is difficult enough, but if the ceiling is beginning to sag, working over your head with heavy, hard-to-handle materials is not an easy job. If you still want to attempt your own repairs, here are a few ways to proceed:

1) First, protect the flooring under your work area, because once you begin, the entire affected area could give way. To prevent plaster dust from spreading into the rest of the house, hang damp sheets or tape plastic sheeting over the doorways and put an exhaust fan in a window. If you’re worried that much of the ceiling could come down at once, build T-shaped supports from 2 by 4s and use them to hold a piece of plywood flat against the ceiling while you work.

2) Use a hammer and cold chisel or wrecking bar to chip out a small area at the edge of the bulge (wear safety glasses). Once you can see behind the surface, you should be able to tell if the plaster has pulled away from the lath or the lath itself has come loose from the framing. If the lath has pulled away and the plaster is still well-adhered to it, you may be able to refasten the lath to the framing without removing the plaster.

3) Use long drywall screws that will penetrate at least half their length into the wood framing. Start near the edge of the bulge and press the ceiling upward as you drive the screws (you may have to shift your plywood support, and the tees holding it, ahead of you as you work).

Because the plaster and lath form an integral sheet, like a piece of drywall, it may go back up without a problem. However, if the lath has warped, or if the old nails in the framing prevent the lath from returning to its original position, this may prove impossible. You might have to first remove much or all of the plaster just to get the lath back up.

4) If the lath is still anchored to the ceiling joists, and the plaster has pulled away, your only option is to remove the old plaster. It’s a dirty job, but if there’s nothing holding the plaster to the framing, it will come down quickly.

There is another trick that professionals use to repair plaster walls and ceilings that have failed—leave the old plaster in place and reface the entire area with new drywall. Long screws with washer heads are used to pull the drywall and old ceiling back up to the framing, or as close to level as possible. You end up with a drywall surface, but it eliminates the problems of sagging, cracking, and flaking plaster once and for all.