Quick Answer:

If you see dampness or stains below a drain after using the fixture, stop using it, dry the area, then run a controlled water test while watching the trap, joints and overflow. Most leaks show up only when the fixture is draining; inspecting the visible trap and watching for dripping during drain vs during fill will usually find the source. If you can’t confirm the exact spot, call a pro.

Why This Happens

Water under the floor after use usually comes from one of three places: a loose or damaged trap/joint, an overflow or secondary path (common with tubs and some sinks), or a crack in a drain line further downstream. Small hairline leaks often only appear when water flows, so the area is dry between uses. Sometimes air and gurgles relate to partial blockages or venting problems—see Drain Burps Air After Water Shutoff—and other times a trap seals fine until a clog causes unexpected flow changes noted in Drain Drains Fine Then Suddenly Stops.

Step-by-Step What to Do

1. Stop using the fixture

Don’t run the sink, tub, or shower until you’ve done basic checks. Continued use can make the damage worse and spread water.

2. Dry the visible area and surrounding surfaces

Use towels and a fan or dehumidifier to dry the ceiling or floor under the drain area. This gives you a clean baseline so new moisture is easy to spot during testing.

3. Inspect visible trap and joints

• Look at the P-trap and any horizontal joints you can see. Check for loose slip nuts, corrosion, or hairline cracks on plastic traps.
• Hand-tighten slip nuts moderately — not more than finger-tight + a quarter turn — and re-check for leaks during the water test.

4. Run a controlled water test

• Fill the fixture with a small amount of water, then let it drain while someone watches under the floor or in the access area.
• For larger fixtures, run a steady flow for a minute while observing. The goal is to reproduce the leak without flooding.

5. Watch for dripping during drain vs during fill

Note whether wetting appears only when water is draining (indicates drain-line or trap issue) or while the fixture is filling (may indicate supply-related drips or an overflow bypass). This distinction narrows the problem quickly.

6. Check the overflow path (tub/sink)

For tubs and many sinks, the overflow assembly can leak into the wall or floor cavity. Run water into the overflow inlet or fill the fixture above the normal drain level and watch the area below for moisture.

7. Dry and re-check

After any minor adjustments or tightening, dry the area again and repeat the controlled test. If the leak stops, monitor the location over the next day for recurrence.

What Not to Do

• Do not keep using the fixture while you’re seeing active leakage. That can increase damage and hide the leak’s origin.
• Do not open walls or floors before you confirm the leak location. Cutting into structural elements without a confirmed source can be unnecessary and costly.

When to Call a Professional

Call a plumber if you cannot reproduce the leak in a controlled test, if tightening visible fittings doesn’t stop it, or if the leak appears to come from inside the wall or under a finished floor. Also call if there’s significant water damage, mold growth, persistent odors, or any structural sagging. A professional can use a camera, dye tests, or access panels to pinpoint the problem with minimal damage.

Safety Notes

• Turn off electrical breakers for any lights or devices under the wet area before you work there. Water and electricity are a dangerous combination.
• If you find mold, avoid disturbing large areas yourself; wear a mask and consider professionals for extensive mold remediation.
• Be cautious when tightening metal plumbing parts — over-tightening can crack plastic fittings or strip threads.

Common Homeowner Questions

• How long can I wait to fix this? Fix it as soon as you can; even small leaks can cause mold and structural damage over weeks to months.
• Can I use sealant on a leaking joint? Temporary plumber’s tape or joint compound can help short-term, but a proper repair or part replacement is usually needed for a lasting fix.
• Will homeowner’s insurance cover this? It depends on cause and policy; sudden accidental leaks are more likely covered than long-term neglect.