What a blister tells you
It tells you water has already passed through the full thickness of the board and reached the decorated face. That is more than a surface stain — the board itself is saturated at that point and the paper facing is likely delaminating from the gypsum core.
Several blisters along a line usually trace the path of a ceiling joist, because water runs along the top of the board until it meets the obstruction and backs up.
Pop it, do not wait
A blister left alone either dries and leaves a flaking patch, or grows until the weight of the water tears the paint and it comes down in one go, usually taking wet gypsum with it. Piercing it with a small screwdriver or a craft knife at the lowest point, with a bucket beneath, drains it in a controlled way.
Make the hole at the bottom edge of the bubble, not the middle, so it drains rather than pooling.
Whether the ceiling survives
Drywall that got damp and dried out usually stays sound and takes a stain-blocking primer and a repaint. Board that was saturated long enough for the paper face to separate or the core to crumble has lost its strength and gets cut out. The test is simple: press it. Firm is fine, spongy is not.
How to tell which one you have
It is probably an ice dam
Top floor, blisters near the outside wall or along a joist line, appearing in a thaw with snow on the roof.
It is probably plumbing
Directly under a wet room, following fixture use rather than weather.
It is probably paint failure
No water at all when pierced, flaking over a wide area, usually on a previously poorly prepared or repeatedly overpainted surface.
What to do in the next ten minutes
- Bucket underneath, then pierce the bottom edge of the blister and let it drain.
- Photograph before and after you drain it.
- Press the board around it — note anything spongy and keep people out from under it.
- Get air moving in the room, but do not aim a heater at a wet ceiling.
- Clear the ice before the next thaw refills it.
Questions people ask
Once the leak has stopped and the board is genuinely dry — measured, not guessed — a stain-blocking primer and a repaint is the right repair for sound board. Painting a damp ceiling traps moisture and the stain bleeds back through.
Days to a couple of weeks depending on how much water went through, the room temperature and the air movement. A cavity above a ceiling dries far slower than the visible surface, which is why a meter reading beats looking at it.
Making good after a covered water loss, including replastering and repainting the affected area, is typically within what a homeowners policy addresses. Matching the rest of the room is where policies differ, so ask your adjuster specifically about that.