Check for leaks under sinks

Perform a visual and tactile inspection of the drain pipes, P-trap, and supply lines under bathroom and kitchen sinks to catch leaks before they cause damage.

Why it matters#

Under-sink leaks often go unnoticed until they've caused significant damage—rotting cabinet floors, warped particle board, mold growth, and even structural issues. A slow drip might only create a small puddle, but over weeks it saturates cabinetry and creates conditions for unhealthy mold spores. Early detection saves hundreds to thousands in water damage repairs and helps prevent musty odors and health hazards.

What you'll need#

Tools#

• Flashlight
• Dry paper towels or white cloth
• Small bucket or towel (to place under P-trap)

Steps#

1. Clear the Area and Prepare#

Remove everything from under the sink—cleaning supplies, trash cans, fire extinguishers. Items bumping against pipes are a common cause of loosened connections. Place a dry towel on the cabinet floor to make spotting new moisture easier.

2. Do a Visual Inspection#

Using a flashlight, look at every connection point under the sink: where the drain meets the sink basin (the tailpiece), all P-trap connections, where pipes enter the wall, and both hot and cold supply lines running to the faucet. Look for water stains, mineral deposits (white crusty buildup), corrosion (greenish on copper, rusty on steel), or active dripping.

3. Perform the Touch Test#

Wipe all pipes, connections, and the P-trap completely dry with paper towels. Run your fingers along every joint and connection. Feel for dampness, hidden moisture in crevices, or the slick feel of a slow seep. Pay special attention to the slip nuts connecting P-trap sections—these loosen over time.

4. Run Water and Watch#

Turn on both hot and cold water and let them run for 2-3 minutes while watching closely underneath with your flashlight. Check the highest connection points first (where drain meets sink), then work down. Use a dry paper towel to wipe suspicious spots—it will show water immediately.

5. Stress Test the Drain#

Plug the sink and fill it with several inches of water. Release the stopper and watch the drain pipes while water rushes through—this high-volume flow puts maximum stress on connections. This is when marginal leaks reveal themselves.

6. Inspect Supply Lines#

Check the braided steel or plastic supply lines running from the shutoff valves to the faucet. Look for bulges, cracks, corrosion at connection points, or dampness. Feel around the shutoff valve handles—these can develop slow leaks. Supply line failures cause flooding, so replace any that look questionable.

7. Check the P-Trap Condition#

Look at the P-trap itself (the curved U-shaped pipe that holds water to block sewer gases). Check for corrosion, cracks, or white mineral deposits at joints. Metal P-traps can corrode inside and develop pinhole leaks. If your P-trap is metal and over 15-20 years old, consider replacing it with PVC.

Pro tips#

• If you detect a sewer smell but no visible leak, your P-trap may have dried out (common in rarely-used sinks). Run water for 30 seconds to refill the trap and restore the gas barrier.
• After finding and drying a damp spot, place a piece of white paper towel there and check back in 24 hours—this helps confirm whether you have an active leak.
• When slip joint nuts (the big plastic or metal nuts on P-trap connections) leak, try hand-tightening first. If that fails, the rubber washer inside may need replacement—a 50-cent fix.

Warnings#

• Don't over-tighten plastic slip nuts—they crack easily, turning a minor leak into a replacement job.
• If you see active water spraying from a supply line, shut off the valve immediately; if you can't, turn off the main water supply to your house.
• Black mold on cabinet floors or walls indicates a long-term moisture problem—address the leak AND the mold, which may require professional remediation.

When to call a pro#

If you discover leaks from connections inside the wall, corroded shutoff valves that won't turn, or if you've tightened connections and replaced washers but leaking persists, it's time for professional help. Plumbers can also inspect and replace corroded supply lines before they fail catastrophically. Plumbing repair typically costs $150-$850 depending on complexity, far less than water damage restoration.