Water Under the Kitchen Sink but No Leak? Find the Source by Timing

Graphic: summary of this guide, made by our team from the cited sources.

When the water shows up points to the source

When the cabinet gets wet	Most likely source	First check
All the time, even overnight	Supply side: shutoff valve (angle stop), supply lines, filter housing	Dry everything, then wrap tissue around each valve and nut
Only after the faucet has been used	Pull-down or sprayer hose connections under the deck	Run the faucet with the spray head pulled out
Only after the sink drains	Drain side: basket strainer, tailpiece, P-trap, disposer joints	Fill the basin, then release it all at once
Only after the dishwasher runs	Dishwasher drain hose clamp or connection	Watch the hose during the drain cycle
After wiping the counter or washing up	Sink rim seam, faucet base, countertop joint	Pour a cup of water along the rim
Mostly in humid summer weather	Cold pipes sweating (condensation)	Look for beads along the whole pipe

Moen's support article draws the first split. A faucet that leaks under the sink 24 hours a day points to the shutoff valves and supply lines. A leak that shows up only after the faucet runs points to the hose connections below the sink.

Run the newspaper test before you buy anything

This is InSinkErator's method for hard-to-find leaks. It works whether or not you have a disposer.

1. Empty the cabinet and dry every pipe, nut and hose with a towel.
2. Tape a sheet of newspaper flat across the cabinet floor. Any drip will leave a mark right under its source.
3. Put the stoppers in, fill the sink, and leave it full for a while. A wet spot now means a basket strainer or drain flange is seeping.
4. Pull the stoppers and let it drain. Drips now point to the tailpiece, slip nuts or P-trap.
5. Run the faucet (and the disposer, if you have one) and look again.
6. If the paper stays dry, leave it in place for a few days so a slow, intermittent source has time to show itself.

Graphic: made by our team from InSinkErator's leak-finding steps and Moen's supply-versus-hose split (linked above).

Paper beats your eyes because water travels. A drop that starts at a slip nut can run along the trap and fall well away from the joint, and that is why you "can't see the leak."

Wet all the time: the supply side

If the paper is damp even when nobody has used the sink, the leak is under pressure. Check, in this order:

• The shutoff valves (angle stops). Look for a drip at the stem behind the handle and at the nuts where the pipes enter the valve.
• The supply lines between the valves and the faucet. Feel along each line and both end nuts for moisture.
• Water filter or reverse osmosis housings under the sink, which InSinkErator also names as a hidden source.

You can confirm a pressurized leak without opening anything. EPA WaterSense suggests reading your water meter before and after a two-hour period when no water is used. If the meter moves at all, you probably have a leak somewhere in the house. It could be under this sink, a toilet or another fixture.

Wet only after the faucet runs: the hose under the deck

Pull-down and pull-out faucets have a spray hose that slides up and down through the faucet body every time you use it, with a weight and one or more quick-connect fittings below the sink. Moen says leaks that appear only after use come from that cluster of hose connections. Pull the spray head out, run water, and watch the connections with a flashlight. Moen's quick-connect instructions name two causes of a leak there: an O-ring out of its groove, or a hose pushed in at an angle instead of straight. Either one can spit water while the faucet is on and stay dry otherwise. Moen asks owners to identify the faucet model before ordering the hose parts.

Wet only after draining: the drain side

Drain joints aren't under pressure, so they only leak when water passes. A sink that drains slowly makes this worse, because water backs up against the joints. If the paper marks only during step 3 or 4, look at:

• The basket strainer nut and its gasket under each bowl.
• The slip nuts on the tailpiece and P-trap.
• The disposer's sink flange, discharge elbow and lower housing. Our guide to how to tell if your garbage disposal is bad walks through which disposer leaks are a gasket and which mean replacement.

If the sink is also slow, clear that first. See why both sides of a kitchen sink drain slowly.

Wet only after the dishwasher runs

A dishwasher pushes a lot of water through one rubber hose in a short burst, so a loose clamp may only leak during the pump-out. InSinkErator's dishwasher inlet checklist asks you to confirm the knockout plug was removed, the clamps are tight, any hose adapter is intact, and the elbow-to-trap joint is tight. The Badger 5XL manual warns that if the hose is connected without knocking out the plug, the dishwasher may overflow.

Check the height too. Whirlpool's installation instructions say to connect the drain hose above the drain trap and at least 20 inches above the floor. They recommend looping the hose up and fastening it under the counter, or using an air gap. Whirlpool's drain loop page explains that this height prevents backflow. A hose lying low in the cabinet is a sign the install was rushed, so check its clamp closely.

Wet after washing up: water coming from above

Some water never touches a pipe. If the caulk or seal around a drop-in sink has failed, water you splash on the counter seeps under the rim and runs down the inside of the cabinet. A loose faucet base does the same. InSinkErator names the sink/countertop seam and countertop joints in its list of hidden sources. To test it, dry the cabinet, then slowly pour a cup of water along the rim and around the faucet base while someone watches from below. Water that shows up on the cabinet wall, not on the pipes, confirms it.

Wet in summer: sweating pipes

Cold water pipes in a warm, humid kitchen can collect condensation, and drops run down to the cabinet floor. The EPA's mold and moisture guide treats condensation on pipes as a sign of high humidity. It suggests insulating cold water pipes, using exhaust fans while cooking and running the dishwasher, and keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Sweating looks different from a leak. The whole cold pipe is beaded, the hot pipe is dry, and the fittings aren't wetter than the straight runs. Toilets do the same thing in humid weather, as our guide to a sweating toilet tank explains.

Dry it out, whatever the cause

Many sink cabinet floors are particleboard or MDF, which swell and crumble once they soak up water. The EPA says to dry water-damaged areas and items within 24 to 48 hours to prevent mold, and to fix the water problem or the mold will come back. Leave the doors open, run a fan into the cabinet, and keep stored items out until the panel feels dry. If the floor has swelled or smells musty, replace it rather than painting over it. The EPA warns that paint applied over moldy surfaces is likely to peel.

When to call a plumber

• The water meter moves during the two-hour test and you can't find the leak at any fixture.
• An angle stop leaks from the valve body, or the valve won't turn, so you can't shut the water off safely.
• The cabinet floor or the wall behind the sink is soft or stained, which suggests the leak has been running longer than it looks.
• The disposer leaks through its body or at the motor joint, which InSinkErator says needs a call to them.

Want to know how much a slow drip is costing? Our water leak calculator turns drips per minute into gallons per year.