Why Plaster Ceilings Sag (and What to Do Before One Comes Down)

A plaster ceiling that has started to droop is one of the most common calls we get, and one of the few that can turn into an emergency. The good news is that most sagging ceilings give plenty of warning, and most can be saved without tearing the whole ceiling down. This post covers why it happens, how to judge how serious it is, and what to do in the meantime. For a broader look at ceiling problems, see our guide to ceiling plaster repair.

What Holds a Plaster Ceiling Up

In most houses built before the 1940s, the ceiling is plaster over wood lath: thin wooden strips nailed across the joists with small gaps between them. When the first coat of plaster was troweled on, some of it squeezed through those gaps and curled over the back of the lath. Those curls, called keys, are what hold the ceiling up. There is no glue and no screws. The whole ceiling hangs from thousands of little plaster hooks.

Houses from the 1930s through the 1950s often have rock lath instead, gypsum panels nailed to the joists. That ceiling hangs from the nails, and it fails differently. We explain the difference in rock lath vs. wood lath.

Why the Keys Break

Age and Vibration

Keys are brittle. Decades of footsteps on the floor above, doors slamming, trucks on the street and seasonal movement of the framing snap them a few at a time. Once enough keys in one area are gone, that section of plaster is held only by its neighbors and by the paint.

Water

A leak from a bathroom, a roof or a radiator softens the plaster and the keys. Even after it dries, the plaster is weaker and often has separated from the lath. Sagging that shows up under a bathroom is very often an old leak.

Work Above the Ceiling

Someone crawling around the attic, blown-in insulation piled on the back of the lath, or a floor being replaced upstairs can all crack keys in an afternoon. We often hear that a ceiling started to drop right after other work in the house.

Old Repairs

Drywall screws driven straight through plaster, or heavy patches glued on, add weight and stress around the edges. The sag often starts at the edge of an earlier repair.

How to Tell How Bad It Is

A sag is not automatically dangerous, but some signs mean the ceiling should be dealt with soon:

• The ceiling moves when you press it gently with a flat hand or a broom head. Sound plaster does not move. If it flexes or you hear crunching, the keys are broken in that area.
• The sag is getting bigger. Mark the edge of a crack with pencil and look again in a week or two.
• Cracks are forming a closed shape, such as a rectangle or a ring around the low spot. That outlines the piece that is coming loose.
• Plaster dust or grit on the floor under the sag, or new cracks after a door slams.
• A gap opening at the wall, where the ceiling has pulled down away from the crown or the top of the wall.

A long, gentle belly with no movement and no new cracks is usually stable for now. A spot that flexes, sheds grit or has a crack drawing a circle around it is not.

What to Do Right Away

• Keep people and pets out from under it, especially beds, cribs, couches and the dining table. A section of plaster ceiling is heavy.
• Fix any active leak first. Repairing a ceiling that is still getting wet is wasted money.
• Do not push it back up or try to screw it in place with drywall screws. Pushing breaks the keys that are left, and screws without washers pull straight through old plaster.
• Take a photo and send it to a plaster contractor. A picture with a ruler or a hand in it tells us a lot about how quickly someone should look at it.

How a Sagging Ceiling Gets Saved

If most of the plaster is still in one piece, it can usually be pulled back up and held in place with plaster washers. These are thin perforated discs driven in with screws through the plaster into the lath and joists. They spread the load over a wide area so the screw cannot pull through. Sometimes an adhesive is injected behind the plaster first so it re-bonds to the lath.

Plaster that is crumbled or too far gone comes out. The openings are rebuilt in three coats, any hairline cracks are opened and filled, and the washers are buried under the finish so they disappear. If the ceiling has a lot of small repairs, a full skim coat over the top makes it one smooth surface again. That is what we did on a living room ceiling in Rogers Forge.

When most of the ceiling has let go, a new ceiling makes more sense. That can be new plaster or drywall, and there are trade-offs either way.

What We See Around Baltimore

Sagging ceilings are a regular call in the older neighborhoods: rowhouses in the city, the early 1900s houses in Catonsville, Roland Park and Towson, and older houses out toward Elkridge and Ellicott City. One we handled last year was a bedroom ceiling off Old Washington Road in Elkridge that the owner said looked like it might fall soon. It was secured and repaired before it did. Catching it at the sagging stage instead of after it falls almost always means a smaller job and much less mess.

Have a ceiling that is starting to drop? Call or text a photo to (443) 806-8077 and we will tell you how soon it needs attention.