Is a sagging ceiling dangerous?
Yes, treat it as dangerous. A ceiling that bows, feels soft, or has pulled away from the framing has lost either its fastening or its strength, and saturated drywall is heavy. It does not fail gradually. It holds, and then a whole section comes down at once.
Before anyone arrives
Keep people and pets out of the room and out from under the sag. Move what you can from underneath it, working from the edges rather than standing under the middle.
Do not push on it to test it, and do not put a bucket under a bulge and leave it. A bulge full of water is holding weight that the board was never rated to carry.
If there is a light fixture or a fan in the sagging area, turn that circuit off at the panel. Water and a ceiling box in the same place is an electrical problem as well as a structural one.
Why it is sagging
Water is the common cause. Drywall loses most of its strength once it is wet, and the paper facing that holds it together softens.
The other causes are mechanical. Fasteners that missed the framing or backed out over time, board hung on joists spaced too far apart for its thickness, or a second layer of ceiling added over the first without the fasteners to carry it. Older houses sometimes have drywall screwed straight over failing plaster and lath, and when the plaster keys let go the whole assembly moves.
Why nobody should patch it
A patch relies on the board around it being sound. On a sagging ceiling it is not. Screwing the bow back up to the joists tears the softened paper and gives you a flat ceiling that fails again in a month.
The compromised section comes down, the framing and insulation above it get checked while they are visible, and new board goes up. On a ceiling the finish then usually gets carried across the whole plane, because a repaired square in the middle of a ceiling is visible from every seat in the room.
How fast this needs to happen
Same day if it is actively dripping or the bow is growing. Within a few days if it is stable and dry.
We respond to urgent property damage within 24 to 48 hours. If it is an immediate safety problem, say that when you call rather than leaving it in a form.
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