Leaky Faucet or Old Sink? How to Find the Real Culprit

Leaky Faucet or Old Sink? How to Find the Real Culprit

A water leak under your kitchen sink can be catastrophic, leading to cabinet rot, mold, and damaged flooring. But before you replace your entire faucet or your entire sink, you need to determine the precise source. Water travels in mysterious ways, and a leak that shows up on the floor might have originated higher up.

Here is your guide to playing plumbing detective.

Step 1: Test the Faucet (Supply Line Leaks)

The easiest leaks to find are those related to the water supply lines or the faucet base.

• Turn Off the Water: Shut off the angle stops (valves) under the sink that feed water to the faucet. Dry the faucet base, supply lines, and the underside of the counter with a towel.
• Wait and Watch: Keep the valves off for several hours. If the leak stops, the culprit is a failure in the faucet cartridge (inside the handle) or the supply lines. If the leak continues, the source is related to drainage.

Step 2: Test the Drain Assembly (Usage Leaks)

Drain-related leaks only happen when water is actively moving through the system.

• Fill the Sink: Plug the sink drain and fill the basin halfway with water.
• Watch the Flange: Look under the sink and observe the drain flange (where the strainer/disposal meets the sink). If water begins to drip immediately, the leak is from a failed seal (plumber’s putty or silicone) where the drain flange meets the sink basin. This is a common old sink failure.
• Pull the Plug: Now, unplug the sink and let the water rush down. If the leak starts only when the P-trap or connecting pipes fill up, the culprit is a loose slip joint nut or a degraded washer in the P-trap assembly. This is an easier fix involving simple plumbing parts.

Step 3: Test the Sink-to-Counter Seal (Top Mount/Drop-in Leaks)

For drop-in sinks, water often seeps through the failed silicone seal between the sink rim and the countertop, especially during splashing.

• Clean and Dry: Dry the countertop around the sink rim and the underside of the counter lip.
• Splash Test: Splash water heavily onto the countertop near the sink rim. If water appears beneath the counter lip, the silicone seal has failed. This is a leak related to the sink itself, and it requires scraping out the old caulk and applying a new, fresh bead of 100% silicone.

By isolating the water source, you can pinpoint the exact failure point and avoid wasting money on unnecessary repairs.

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