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What Causes Water Damage Under a Kitchen Sink?

Last updated October 26, 2026

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The short answer

Loose Fittings and Supply Lines

The most common culprit is a loose connection where the supply line meets the shutoff valve or the faucet base. A tiny drip here often goes unnoticed because it runs down the pipe and pools under the cabinet. Over time, the constant moisture softens the particleboard base and stains the wood. You might only spot it when you pull out a pot and feel dampness.

Tightening these fittings can stop the leak, but the damage underneath may already be done. If the cabinet floor feels spongy or shows dark rings, the water has soaked in. Even a small, steady drip can saturate the wood within a few weeks. Check the fittings every few months by wiping them dry and looking for fresh moisture.

Loose compression nuts on supply lines, worn rubber washers inside the connections, corroded metal threads on older pipes, faucet base gaskets that no longer seal, and hand-tightened fittings that vibrate loose are all common failure points. Each one can produce a slow drip that goes unnoticed for weeks.

Corroded Pipes and Drain Traps

Older galvanized steel or copper pipes can corrode from the inside out, especially where water sits for long periods. The drain trap, the curved pipe under the sink, is another weak spot. Food particles and debris can settle there, and the constant moisture causes rust or pinhole leaks. These leaks often appear as a slow drip or a thin stream when you run the water.

A pinhole leak in a supply line can spray a fine mist that coats the cabinet interior without you noticing. The wood absorbs the moisture and swells, making the cabinet door stick. You might see a white mineral deposit on the pipe or a greenish crust on copper. Replacing the damaged section is usually the fix, but the surrounding wood needs drying and often replacement.

Clogged Drain and Overflow Leaks

A slow drain causes water to back up in the sink basin, and that pressure pushes water past the drain flange gasket. If the flange is old or the putty has dried out, water seeps down the outside of the pipe and into the cabinet. You might notice a musty smell or see water stains on the cabinet bottom that appear only after using the sink.

The overflow hole on the side of the sink can also leak if its internal gasket fails. Water travels through the overflow channel and drips out from under the sink rim. This is easy to miss because the leak is intermittent and depends on how full the sink gets. Cleaning the drain regularly and replacing the flange gasket can prevent this type of damage.

Detecting and Preventing Leaks

Look under the sink with a flashlight at least once a month. Wipe all pipes and fittings dry, then check again after a few hours. Feel the cabinet floor for dampness or soft spots. A dark stain or a musty odor is a clear warning sign. If you live in Bellingham, keep the shutoff valve location written down so you can act fast in an emergency.

Replace rubber washers in supply lines every few years, as they harden and crack with age. Use a pipe wrench to snug the compression nuts, but avoid overtightening, which can crack the nut. Check the caulk around the sink rim and the drain flange annually. A fresh bead of plumber's putty on the drain flange creates a watertight seal. Consider installing a leak detector under the sink that sounds an alarm when it gets wet.

When to Call the Pros

Once you spot moisture, shut off the water to the sink and remove everything from the cabinet. Dry the interior with towels and set up a fan to circulate air. Leave the cabinet doors open so the wood can dry out. If the particleboard is swollen or crumbling, it will need to be cut out and replaced. The longer the wood stays wet, the more likely mold will develop.

  • Shut off the shutoff valve under the sink
  • Remove all stored items and dry surfaces
  • Run a fan for at least 24 hours
  • Look for discoloration on the cabinet walls
  • Take photos of the leak and the soaked wood

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Frequently asked questions

How can I tell if a leak is active under my sink?

Wipe all pipes and the cabinet floor dry with a paper towel. Wait a few hours, then check again. If you see fresh moisture, a leak is active. A musty smell or a dark stain that grows is another sign. You can also place a dry paper towel under the pipes and check it the next day.

Can I fix a small leak myself?

A loose compression nut can often be tightened with a wrench, and a worn washer can be replaced. If the pipe is corroded or cracked, you may need a plumber. For damage already done to the cabinet, the pros can assess and dry it properly. Never ignore a drip, even a tiny one.

What should I do if the cabinet floor is already swollen?

The swollen particleboard has absorbed water and will not return to its original shape. You will likely need to cut out the damaged section and replace it. The pros can check for damage to the subfloor and dry it thoroughly. If mold is present, a mold professional should evaluate the area.

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