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Why Is There Water Damage Under My Kitchen Sink?

Last updated October 26, 2026

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The short answer

Common Leak Sources

The most frequent cause is a loose or worn connection where the water supply lines meet the shutoff valves or the faucet. Over time, the rubber washers inside can degrade, or the nuts can loosen from temperature changes and vibration. Even a tiny drip can saturate the cabinet bottom.

Drain pipes are another usual suspect. The slip nuts that hold the P-trap and drain extensions can work loose, and the gaskets can crack. Also check the sink rim where it meets the countertop; a failed seal lets water run down the outside of the sink basin and pool underneath.

  • Loose supply line nuts
  • Cracked or missing washers
  • Deteriorated drain gaskets
  • Faulty sink rim seal
  • Corroded or pinhole leaks in pipes

Signs to Look For

Start by clearing everything out from under the sink. Look for water stains, dark streaks, or a soft, spongy feel on the cabinet floor. A musty smell often indicates prolonged dampness, which can lead to mold growth on wood and cardboard boxes.

Check the pipes themselves for moisture. Run water for a minute, then wipe the connections with a dry cloth. If you see fresh droplets or a steady drip, you have found the leak. Also inspect the cabinet back and side panels for swelling or discoloration that points to a hidden leak behind the wall.

Quick Checks You Can Do

Tighten the slip nuts and supply line connections by hand. Do not overtighten, as that can crack the fittings. Replace any visible washers that look flattened or cracked. If the leak stops, you have saved yourself a bigger repair.

If the leak persists, test the sink seal by filling the sink with a few inches of water and letting it drain while you watch underneath with a flashlight. Sometimes the leak only appears when water runs, so this test helps you pinpoint the source.

When to Call the Pros

If you cannot find the source, or if the cabinet is already warped, stained, or shows mold, it is time to bring in local water damage pros. They can perform a thorough inspection to find hidden leaks and assess the extent of the damage, including any that may have spread to the subfloor.

The pros can also help with drying, cleaning, and repairs. In St. Petersburg, where humidity is high, mold can develop quickly after a leak, so acting fast is important. For a complete evaluation, you might consider a water damage inspection service.

Preventing Future Leaks

Check under the sink every few months. Look for dampness, rust, or mineral deposits on fittings. Tighten connections gently and replace washers at the first sign of wear. Also, avoid storing harsh chemicals that can corrode pipes over time.

Know where your shutoff valve is and how to turn it off in an emergency. If you live in St. Petersburg, keeping that location written down can save you from a major mess. For ongoing peace of mind, consider periodic inspections by a professional.

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Frequently asked questions

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

Dry all pipes and connections, then run water through both the faucet and the drain separately. If the leak appears only when water runs, it is likely the drain. If it leaks all the time, check the supply lines and shutoff valves.

Can I fix a small leak myself?

Yes, if it is a loose nut or a worn washer, you can often tighten or replace it yourself. Turn off the water supply first. If the leak continues after your repair, or if you see damaged wood, call the pros to avoid bigger problems.

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

Stop using the sink and dry the area as much as you can. Wear gloves and a mask when cleaning small areas with soap and water. For larger mold growth, consult a mold professional, as they have the proper safety equipment and methods.

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