Why the stain's position can mislead
Water travels along joists, pipes and the top of the drywall before it soaks through, so a stain can show up a few feet from the leak above. A stain directly under the tub doesn't prove the tub is the cause, and a stain under the hallway can still come from the shower valve.
What tells you more is when the stain grows. Write down whether it gets worse after showers, after baths, after toilet flushes, or all the time, even overnight when nobody uses the bathroom.
When it grows, and what that suggests
| The stain grows | Most likely source |
|---|---|
| All the time, even with nothing running | A supply line or the shower valve under pressure |
| After showers only | Shower arm or head joint, grout, caulk, spout or drain |
| After deep baths only | Tub overflow gasket |
| After any bath or shower draining | Tub drain shoe or drain seal |
| After toilet flushes | Toilet wax ring or tank connection |
| After the sink drains | Sink drain or trap |
Tracing it one fixture at a time
- Check the meter.With every fixture off, watch the water meter. If it moves, something under pressure is leaking.
- Test one fixture per day.Use only the shower one day, only the tub the next, and so on. Let the ceiling dry between tests so changes are visible.
- Mark the stain edge.Lightly pencil the outline after each test so growth is easy to see.
- Photograph everything.Photos of the stain over time, plus the bathroom above, help a plumber decide where to look first.
After the leak is fixed
- Make sure the wet drywall and insulation dry out completely
- Open and dry the area if the drywall stays soft or smells musty
- Prime the stain with a stain-blocking primer before repainting
- Keep the photos and any written repair details for your records
- Check with your insurer about water damage if it was extensive
What to ask a plumber
- How will you find the source without opening more ceiling than needed?
- If you open the ceiling, how large an opening, and who patches it?
- Did the testing confirm the source, or is it a best guess?
- Is there an access panel upstairs that avoids cutting the ceiling?
Common questions
Should I cut a hole in the ceiling to let the water out?
If water is pooling above a bulging ceiling, a small opening with a bucket under it can relieve it. Turn off power to that area first and stay out from directly underneath.
Can I paint over the stain once it's dry?
Only after the leak is fixed and the area is fully dry. Use a stain-blocking primer first or it will bleed through.
Why does the stain only show sometimes?
Intermittent stains usually come from leaks that happen only when a fixture is used, such as a tub overflow or drain.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.