Leak Under the Sink? How to Find It and Stop It

A leak under the sink shows up as a wet cabinet floor, a swollen shelf or a bowl that keeps filling. It usually means one joint is letting water out. Most of these joints are held by nuts you can turn by hand and rubber washers you can replace, so you can usually fix it without a plumber.

The first question splits the problem: does it leak only while water runs down the drain, or all the time? A drip only when the drain is in use points to the trap, the strainer or a drain hose. A drip even when nothing runs points to the supply lines or the shutoff valves, which are always under pressure.

Start With These Quick Checks

1. 1If a garbage disposal sits under the sink, switch it off at the wall and also unplug it or turn off its breaker. Then empty the cabinet, dry every pipe and joint with a towel, and lay a dry paper towel on the cabinet floor.
2. 2Wait ten minutes without running any water. Drops on the paper now come from the supply side: the hoses to the faucet or the shutoff valves.
3. 3Then fill the sink a few inches with the stopper in and let it all drain at once. Wipe each drain joint with a dry paper towel: the trap nuts, the strainer under the sink and any dishwasher hose.
4. 4Run the faucet and look at its base on top of the sink. Water running down the outside of the hoses from above means the faucet itself leaks.

1. A Loose or Worn P-Trap

The P-trap is the U-shaped bend under the drain. Plastic traps are held together by slip nuts with a beveled washer inside each one. A nut that has worked loose, or a washer that has gone hard, lets water out every time the sink drains.

1. 1If a disposal is under this sink, switch it off at the wall switch and unplug it or turn off its breaker. Put a bucket under the trap.
2. 2Tighten each slip nut by hand, then a quarter turn more. Plastic nuts crack if you force them with pliers.
3. 3Fill the sink, let it drain and wipe the joints again. If a joint still weeps, unscrew that nut, empty the trap into the bucket and look at the washer.
4. 4Fit a new washer with its beveled side facing the joint, line the pipes up straight and screw the nut back on by hand.
5. 5Replace a cracked trap, or a metal one eaten through by rust. This Old House puts a new PVC trap at $15 to $20 in material; a universal kit costs around $26.

2. A Leaking Sink Strainer

The strainer is the metal basket in the sink's drain hole, sealed with plumber's putty and held from below by a big locknut. When the putty dries out, water seeps around it and runs down the outside of the drain pipe. Sinks with a disposal have a flange instead, see garbage disposal leaking.

1. 1If a disposal sits under the other bowl, switch it off and unplug it or turn off its breaker. Put a bucket under the drain and unscrew the slip nut that joins the trap's tailpiece to the strainer.
2. 2Hold the strainer still from above while a helper loosens the big locknut underneath with large slip joint pliers.
3. 3Lift the strainer out, scrape off all the old putty and clean both surfaces.
4. 4Roll a rope of plumber's putty about as thick as a pencil, press it under the strainer's lip and set it in the hole. On stone sinks use silicone instead, because putty can stain stone.
5. 5From below, fit the rubber gasket and the paper washer in the order your strainer's diagram shows, then tighten the locknut until the putty squeezes out evenly. Wipe the excess away, reconnect the tailpiece and test with a full sink.

3. A Dripping Supply Line or Shutoff Valve

The two hoses from the shutoff valves to the faucet are under full water pressure all the time. A drip here does not wait for you to use the sink.

1. 1If a disposal sits under the sink, switch it off and unplug it or turn off its breaker. Close both shutoff valves by turning them clockwise, then open the faucet to let the pressure out. Keep a bucket ready.
2. 2If the drip is at a hose nut, tighten it a quarter turn with a wrench while you hold the valve still with a second wrench.
3. 3If the hose is wet along its length, bulging or rusty, replace it with a new braided supply line of the same length and fittings.
4. 4If the drip comes from behind the valve's handle, tighten the small packing nut on the valve by an eighth of a turn.
5. 5Open the valves slowly and watch for a minute. A valve that will not close fully, or drips from its body, should be replaced by a plumber.

4. The Faucet Leaking From Above

If the hoses are dry at the valves but wet higher up, the water comes from the faucet. It runs down the outside of the hoses or the faucet body and drips into the cabinet.

1. 1Run the faucet and watch its base on top of the sink. Water pooling there means a worn seal inside the faucet.
2. 2If you have a pull-out sprayer, pull the hose out and look for a split. Before you reach under the sink to check its connector, switch off any disposal and unplug it or turn off its breaker.
3. 3Look at the caulk along the sink's rim and around the faucet: splashes that get under a failed bead can drip down inside the cabinet.
4. 4Worn faucet seals are the same repair as a dripping faucet, see how to fix a leaking faucet.

5. The Dishwasher Drain Hose

In a kitchen, the dishwasher's drain hose joins the disposal or a branch on the drain under the sink. A loose clamp leaks only while the dishwasher pumps out.

1. 1Start a short dishwasher cycle and watch the hose connection under the sink with a flashlight while it drains. Keep your hands out.
2. 2Before you touch the clamp, unplug the dishwasher or turn off its breaker, and switch the disposal off at its wall switch and at its plug or breaker.
3. 3Tighten the clamp, or cut off a cracked hose end and refit it with a new clamp.
4. 4Turn the power back on with your hands clear and run another cycle to check.

When to Call a Plumber

HomeAdvisor puts a plumber's drain pipe repair under the sink at $100 to $500. It is worth it in these cases:

1. 1Water comes from a pipe inside the wall, or from a pinhole in a copper pipe.
2. 2A shutoff valve will not close, or leaks from its body.
3. 3The leak comes back within days after you replaced the washers and tightened the nuts.
4. 4The cabinet floor is soft or moldy: the leak may have run for months, and the floor below may be wet too.