Empty the cabinet, dry everything, and test one part at a time. Water with the faucet off points to the supply lines or shutoff valves. Water only when the faucet runs points to the faucet body or sprayer. Water when the sink drains points to the drain, P-trap, disposal or dishwasher connection. If the cabinet floor is wet but every connection is dry, the leak may be in the wall behind the sink.
A wet sink cabinet is one of the most common leaks in any home, and usually one of the easiest to fix, once you know which of the half-dozen connections under there is the culprit. Here’s how to track it down in a kitchen or bathroom, and what to do when the leak is actually in the wall behind the cabinet.
Before you start
- Empty the cabinet.
- Dry everything: pipes, valves, the cabinet floor.
- Wrap connections in dry paper towel or tissue so drips show up clearly.
- Have a flashlight and a bucket ready.
Test one thing at a time
| Test | If it leaks | Likely source |
|---|---|---|
| Faucet off, wait 10 minutes | Water anyway | Supply lines or shutoff valves |
| Faucet running, stopper out | Water while running | Faucet body, sprayer hose, base seal |
| Fill the sink, then drain | Water while draining | Drain, P-trap, basket strainer, tailpiece |
| Run the disposal | Water from the disposal | Disposal seals or connections |
| Run the dishwasher | Water during the cycle | Dishwasher drain or supply connection |
Common causes and fixes
- Supply lines: replace old plastic or rubber lines with braided stainless lines. Tighten, but don’t overtighten.
- Shutoff valves: a valve that weeps at the stem can often be snugged; an old multi-turn valve is better replaced with a quarter-turn valve.
- P-trap and drain: hand-tighten slip nuts, replace worn washers, and make sure the pieces are aligned.
- Basket strainer (kitchen): a leak around the drain opening usually needs the strainer reset with fresh plumber’s putty.
- Faucet: leaks from the base or under the deck are often a worn O-ring or cartridge, or a loose sprayer hose.
- Garbage disposal: leaks at the top are the sink flange; at the side, the drain or dishwasher connection; from the bottom, often an internal seal, which usually means replacing the disposal.
- Dishwasher: check the drain hose clamp at the disposal or tailpiece, and the supply connection under the sink.
Bathroom sinks add the pop-up drain assembly, which leaks at the pivot nut.
When the leak is in the wall
If the cabinet floor or back wall is wet but every connection stays dry:
- The supply stubs or drain line inside the wall may be leaking.
- A refrigerator water line or dishwasher line running through the cabinet may be leaking. See refrigerator water line leak.
- Water from a window or exterior wall above the sink may be running down inside the wall.
Do a meter test with everything off. If the meter moves, a pressurized line is leaking somewhere. A moisture meter on the back wall of the cabinet shows whether the wall is wet. See how to find a water leak inside a wall.
Check for damage
- Swollen or soft cabinet floor (particleboard swells when wet)
- Mold on the back of the cabinet or wall
- Stains on the ceiling below, if the sink is upstairs
- Flooring damage in front of the cabinet
A long-running leak under a sink can soak the subfloor. Hidden water leak detection with moisture mapping shows how far it went, so only damaged material is replaced.
Prevent it
- Replace supply lines with braided lines and check them once a year.
- Put a leak sensor in the sink cabinet.
- Don’t store heavy items that can knock the P-trap loose.
Get help
If your sink cabinet keeps getting wet and you can’t see why, we find hidden leaks across Bargersville and Johnson County, including Greenwood, Whiteland and Center Grove.
Still can’t find it?
Our hidden water leak detection pinpoints the leak without tearing things up, so the repair starts in the right spot.
Frequently asked questions
Why is there water under my kitchen sink?
Common causes are a loose or worn supply line, a leaking shutoff valve, a loose P-trap or drain connection, a leaking garbage disposal, a dishwasher drain or supply connection, or a failed faucet seal or sprayer hose.
How do I find a leak under the sink?
Dry everything, then test one thing at a time: faucet off, faucet running, sink draining, disposal running, dishwasher running. Wrap connections in dry paper towel to see which one gets wet.
Is a leak under the sink an emergency?
A drip from a drain isn't usually an emergency, but a supply line leak is under constant pressure and can burst. Close the shutoff valves under the sink until it's fixed.
What if the cabinet is wet but the pipes are dry?
The leak may be in the wall behind the sink, from the supply stubs or drain line, or from the dishwasher or refrigerator line. A meter test and moisture readings on the back wall help narrow it down.