Short answer
A ceiling that sags with no leak above it has lost its grip on the framing, not its strength to water. On drywall, the screws or the seam tape have let go, and a finisher refastens the board, retapes the seam and blends the texture. On plaster and lath in a pre-1950 house, the keys that hold plaster to the lath have crumbled, and loose sections get screwed back to the joists or cut out and reboarded. Stay out from under any section that bows, cracks along a line or sounds hollow, because that is the part that comes down first.
Tell loose fasteners from failed plaster in five minutes
- Look at the shape of the sag. A dip that follows a straight seam or a line of screws is drywall letting go. A rough oval with cracks running off it is plaster.
- Date the house. Built before 1950 usually means plaster over wood lath. Built after about 1960 means drywall.
- Press up gently with a flat palm, from a step stool you are steady on. Firm board that does not move is sound. Board that flexes, crunches or lifts has lost its fasteners.
- Tap across the ceiling with your knuckles. A solid thud means it is attached. A hollow drum sound means the plaster has separated from the lath behind it.
- Mark the low point of the sag with a pencil line on the wall. Check it again in 7 days.
- Look at what is above. Go into the attic or the room overhead and check for stored weight, a new floor or anything heavy sitting on the joists.
If the ceiling is firm everywhere and only a seam has cracked, that is a tape repair. If it flexes, crunches, sounds hollow or is still moving, the ceiling is coming loose from the framing and has to be refastened or replaced. Do not stand, sit or sleep under a section that is still moving, and do not pull on it to test it.
Repair or replace
| Situation | What to do |
|---|---|
| Sag follows one taped seam, board firm on both sides | Repair. Cut the tape out, refasten the seam, retape and blend the texture. |
| Drywall edges flex or feel soft along the seam | Replace that section. The screws have pulled through, and new tape over loose board sags again. |
| Plaster bulges or sounds hollow over a small area, rest of the ceiling solid | Repair. A plasterer screws the loose field back to the joists with plaster washers and skims over them. |
| An old patch has broken loose and taken plaster with it, with new cracks beside it | Replace that section. Fresh cracking means more loose plaster next to the hole. |
| Sag runs across most of the room, or the pencil mark moved in a week | Replace the ceiling. Stop using the room and get a carpenter or plasterer in this week. |
| Ceiling under a bathroom or kitchen, sagging but not stained | Repair, after a plumber checks above. A slow supply leak loads a ceiling before it ever stains it. |
What it costs
Runhome contractor estimates for this job, including the trip and assessment.
| Option | Range |
|---|---|
| Drywall ceiling, refasten, retape, retexture and paint one area | $343 to $715 |
| Plaster-and-lath ceiling, stabilize and patch, working over a stairwell | $460 to $1,040 |
Can I do it myself?
Only the smallest version of this job. A hairline crack along a seam in a firm drywall ceiling can be taped and retextured from a step stool in an afternoon. Everything past that is overhead work with weight above your head, and a loose ceiling lets go all at once rather than slowly.
Hire for this when the board flexes, when the plaster sounds hollow, when the sag runs the full length of a room, or when the ceiling is over a stairwell. Old plaster ceilings in pre-1950 houses can also hide lead paint and asbestos in the skim coat, so have the ceiling tested before anyone breaks it open.
What a contractor will ask you
Have these ready and you skip the back-and-forth.
- The age of the house, or the decade it was built. Pre-1950 usually means plaster and lath.
- Whether this ceiling has been patched or skim-coated before, and roughly when.
- When the sag or the cracking first appeared, and whether it has moved since.
- Whether there has ever been a roof or plumbing leak above this ceiling, even years ago.
- What is directly above: an unheated attic, or a finished room people walk through.
- What you feel when you press gently near the damage: firm, soft or flexing. Say so if you cannot safely reach it.
- Whether the sag runs the full length of the room or sits in one short section.
- A wide photo taken far enough back to show the whole ceiling section and where it meets the walls.
Last reviewed .