The rain test
If the stain appeared during a storm or grew the next time it rained, start with the roof. If it drips on a dry afternoon, look at plumbing above that room or at the AC condensate line before anyone pulls tile.
Bathrooms and kitchens sit under supply lines and drains. Hallways and bedrooms under an open roof are more often a valley or a flashing.
Where the stain shows up
- Next to a chimney: chimney flashing.
- Beside a skylight shaft: the curb, or a leak upslope that is using the shaft as the exit.
- In a bathroom with a vent stack above: the pipe boot.
- Along an exterior wall on the second floor: step flashing at a roof-to-wall joint.
The stain is the exit. The entry can be several feet upslope.
What to do today
Catch the water, photograph the stain, and note whether it is raining. Popcorn texture and wet drywall get heavier, so a large soft spot should not be ignored. An inspection from the roof and, when it is safe, from the attic tells you which trade you actually need.
Questions people ask next
Should I paint the stain? Not until the leak has stopped and the drywall has dried. Paint locks the mark in and hides whether it is still growing.
Can the attic tell me? Daylight or a wet trail on the underside of the deck is useful. Do not step on the drywall ceiling from above.
Who looks at it? Call (562) 555-0199. If it is not the roof, we will say so.