Leaking Under the Sink
Updated February 20, 2026
Leak can come from supply lines, shutoff valves, faucet connections, drain, or P-trap. The cabinet hides it, so it often goes unnoticed until damage is underway. Finding the exact source is step one.
Overview
One of the most damaging problems precisely because it is hidden. By the time you notice warped cabinet floors, mold, or a musty smell, the leak has been going for weeks. Multiple potential sources under there: supply lines, shutoff valves, faucet connections, drain tailpiece, basket strainer, P-trap, disposal connections, dishwasher hose. Dry everything, check each component systematically. Most fix in under an hour.
Symptoms
- Water pooling on the cabinet floor or moisture on the bottom of the cabinet
- Warped, swollen, or discolored cabinet floor -- slow leak that has been going for a while
- Musty or moldy smell when you open the cabinet
- Visible dripping on supply lines, valves, or drain connections with a flashlight
- Water stains or drip marks on the wall behind the sink
- Items stored under the sink are wet or water-marked
Common Causes
- Loose or corroded supply lines -- rubber washers inside the fittings wear out, connections loosen from vibration. Older chromed copper tubes are especially prone to corrosion at the ferrule.
- Worn shutoff valve packing -- leaks from the packing nut around the stem, especially after being operated. Gate valves are more prone than ball valves. May only leak when partially turned.
- Failed faucet mounting gasket -- gasket between faucet base and sink deck deteriorates. Water seeps through mounting holes when the deck gets wet. Loose mounting nuts let the faucet shift and break the seal.
- Cracked P-trap or failed slip-joint washers -- PVC cracks from age or impact, washers dry out and shrink. Only leaks when water runs down the drain.
- Drain basket strainer leak -- plumber's putty dries out, gasket compresses. Water seeps around the strainer and drips from the underside of the basin.
- Disposal or dishwasher connection (kitchen) -- mounting ring loosens, gasket fails, or dishwasher hose clamp loosens over time.
What You'll Need
How to Fix It
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Identify the Exact Leak Source
Clear out the cabinet. Paper towels on the floor. Flashlight. Dry every pipe, fitting, and valve. Run the faucet and watch for the first sign of water. Supply lines and shutoff valves first (these leak even with the drain idle). Then run water down the drain -- check strainer, tailpiece, P-trap, wall connection. Faucet base wet on top? Mounting gasket. Find the exact source before fixing anything.
Tip: Dry paper towel under each suspect connection, check after 10 minutes. The wet one tells you which connection is leaking. -
Fix Supply Line Leaks
Tighten the compression nut a quarter turn. Often all it needs. Still leaking? Water off, disconnect, check the rubber washer inside. Replace if cracked, compressed, or missing. Lines over 8-10 years old? Replace with new braided stainless steel. Hand-tighten first, snug with a wrench. Overtightening cracks fittings.
Warning: Old chromed copper tubes with compression fittings: do not overtighten. Once the ferrule deforms, you cannot reseal it by tightening more. The ferrule (and sometimes the valve) needs replacement. -
Fix Shutoff Valve Leaks
Dripping from the valve stem? Tighten the packing nut (first nut behind the handle) a quarter turn at a time. Still leaking? Packing is worn, valve needs replacing. Gate valves are notorious for this -- replace with a quarter-turn ball valve. Valve replacement requires the main shutoff.
Tip: Not ready to replace yet? Self-fusing silicone tape around the packing nut provides a temporary seal. -
Fix P-Trap and Slip-Joint Leaks
Hand-tighten the slip-joint nut. PVC: hand-tight plus a quarter turn with pliers. Do not overtighten -- cracks the nut. Still leaking? Disassemble and inspect the slip-joint washer (beveled nylon or rubber ring). Replace if compressed, cracked, or hardened. Bevel faces up. P-trap cracked? Replace the whole assembly ($5-10).
Warning: Bucket under the P-trap before loosening anything. The trap holds water and spills when disconnected. -
Fix Drain Basket Strainer Leaks (Kitchen Sinks)
Dripping from under the basin around the drain? Strainer seal failed. Loosen the locknut from below (strainer wrench or large pliers). Remove the strainer from above. Scrape all old putty from flange and sink surface. Fresh ring of plumber's putty, press strainer in, retighten locknut. Reconnect tailpiece, test.
Tip: Granite or stone sink? Silicone sealant, not putty. Putty stains natural stone. Bead of clear silicone, set the strainer, 24 hours cure. -
Fix Faucet Base and Mounting Leaks
Water seeping from under the faucet base when the deck gets wet? Base gasket or putty seal failed. Tighten mounting nuts from below. Still leaking? Remove the faucet, clean old sealant, fresh putty or silicone, reinstall. While it is off, inspect O-rings on the faucet body -- worn ones let water leak through the mounting holes.
When to Call a Pro
Call a plumber if shutoff valves need replacement (main shutoff, possibly soldering), if the leak is inside the wall (wet drywall, dripping from behind), if the cabinet floor or subfloor is severely damaged, if corroded metal drain connections will not seal with new washers, or if you cannot work in the tight space with a basin wrench.
Prevention Tips
- Flashlight check every 6 months. Takes 30 seconds and catches leaks before they cause damage.
- Replace supply lines every 8-10 years. $5-10 each. Failure floods the cabinet.
- PVC slip-joint nuts: hand-tight plus a quarter turn. Overtightening cracks the nut.
- After any work under the sink, run water a full minute and check every connection before putting items back.
- No heavy items that can bump and loosen drain connections.
- Leak detector on the cabinet floor ($15). Alarms at the first sign of moisture.