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What Causes Water Damage Under a Kitchen Sink?

Last updated October 26, 2026

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The short answer

Slow Leaks in Supply Lines

The flexible braided hoses that carry water to your faucet are a common culprit. Over time, the rubber inside can crack, or the metal braid can fray where it bends behind the cabinet. Even a pinhole leak sprays a fine mist against the cabinet floor, which soaks in without you noticing.

Check the connections where the hoses meet the shutoff valves and the faucet base. A loose nut or a worn out washer can drip steadily. You can feel for moisture with a dry paper towel, but do not overtighten, as that can crack the fitting and make the leak worse.

  • Look for water stains or dark spots on the cabinet floor
  • Feel the hoses for dampness or mineral buildup at the ends
  • Check the floor of the cabinet for standing water or a musty smell
  • Inspect the area behind the garbage disposal, if you have one, for drips

Worn Out Drain Seals

The seal where the sink drain meets the basin is a frequent failure point. The plumber's putty or rubber gasket can dry out and crack over time. When you run water, it seeps past the flange and runs down the outside of the tailpiece, dripping onto the cabinet floor.

A loose slip nut where the tailpiece connects to the P-trap is another spot. That joint needs to be hand tight plus a quarter turn, but it can loosen from vibration or if someone used the sink to support their weight. Water then leaks out during every use.

Clogged Drains and Backups

When food scraps and grease clog the drain line, water backs up inside the P-trap and the tailpiece. The excess pressure pushes water past the slip nuts and the flange seal. You may not see a puddle until the clog worsens, but the constant moisture rots the cabinet base.

If you have a garbage disposal, a jammed impeller can cause water to pool in the sink and overflow the overflow opening, running down the side of the cabinet. Regularly running cold water while using the disposal helps keep the line clear, but it does not prevent all blockages.

Faulty Shutoff Valves

The small valves on the hot and cold supply lines under the sink can leak from the stem packing or the bonnet nut. These valves are rarely used, so the rubber seats can dry out. A slow weep from the valve body runs down the pipe and collects in the cabinet.

Keep the location of your shutoff valves written down. That way, if a leak starts, you can turn the water off quickly. A dripping valve is a sign it needs replacement, not just a tighter packing nut, which can crack the valve body.

Condensation and Humidity

Cold water pipes running through a humid space can sweat, especially during warm, humid weather. That condensation drips off the pipes and soaks the cabinet bottom. It is not a leak, but it causes the same swelling and mold growth over time.

Foam pipe insulation helps prevent sweating, but it must be installed so no bare pipe is exposed. Also, a leaking dishwasher or ice maker line that runs under the cabinet can seep into the same area. Check the floor near the kickplate for dampness, as that points to a different source.

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Frequently asked questions

How do I know if the leak is from the supply line or the drain?

Run water through the faucet and watch the pipes while the water flows. If you see drops on the supply line or at its valve, it is a supply leak. If the drips appear on the P-trap or the tailpiece only when water drains, it is a drain leak.

Can I fix a small leak under the sink myself?

You can tighten a slip nut or replace a worn washer on a shutoff valve if you are comfortable. But if the leak is from a cracked fitting or a damaged supply line, shut off the water and call a plumber. Do not attempt a repair if you are unsure about the source.

What should I do if I find mold under the sink?

Turn off the water supply to the sink and stop using it until the leak is fixed. Wear gloves and a mask when handling any moldy materials. For small areas, clean with soap and water. For larger growth, contact a mold professional. See a doctor if you feel unwell.

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