Ceiling mold has three usual causes. AC condensation — growth in a straight line or rectangle matching a duct or diffuser, worse when the AC runs. A plumbing leak from above — an irregular stain that slowly spreads, sometimes with a bulge. Damp gypsum from a past roof or water event — a fixed area that never fully recovered. Read the pattern before you repaint.
Key takeaways
- Straight lines and rectangles point to AC ducts and diffusers.
- An irregular, slowly spreading stain points to a leak from above.
- A top-floor ceiling adds roof and slab water ingress to the list.
- Repainting a ceiling without fixing the source just delays the problem.
AC condensation
The most common cause. Ducts and diffusers in the ceiling void are cold; if they are poorly insulated, or room humidity is high, or the condensate drain backs up, moisture forms on and around them. Tell-tale signs: growth that follows a straight duct line or outlines a diffuser, damp that appears only when the system runs, and staining directly under an air handler. See AC condensation and mold.
A leak from above
A pipe in the ceiling void, a bathroom on the floor above, or a leaking AC line. The signature is an irregular stain that grows over days or weeks, often brown or yellow at the edges, sometimes with a soft bulge or a drip point. This is not a cleaning job — the water has to be found and stopped, then the cavity dried and checked. See mold after a water leak.
Damp or old gypsum
If a ceiling was soaked in the past — a roof leak, an overflow from above, construction water — the gypsum board can stay compromised even after it looks dry, and growth reappears on the same patch. Paper- faced board that was properly soaked usually needs cutting out and replacing once the source is fixed. See mold on gypsum.
What to do
- Photograph the growth and note its exact shape and whether it is spreading.
- Check whether it changes with the AC running.
- If you are on the top floor, consider roof and slab ingress, especially after weather.
- For a small, stable patch with an identified and fixed cause, clean and monitor.
- For a spreading stain, a bulge, HVAC involvement, or growth on soaked board, get an assessment before any repainting.
A ceiling stain that is still spreading is an active leak. Repainting over it traps the moisture and the paint will fail again within weeks.