Most Likely Causes (Ranked)
The P-trap (curved pipe under the sink) connects with slip-joint nuts and nylon washers. These washers compress over time and lose their seal. Metal P-traps corrode from the inside out, developing pinhole leaks after 10-20 years.
Flexible supply lines connecting shutoff valves to the faucet are the #1 cause of catastrophic under-sink flooding. Braided stainless steel lines last 8-12 years; rubber/vinyl lines only 5 years. Failure is often sudden — a burst that sprays continuously.
The gasket or plumber's putty between the faucet base and sink deck deteriorates, allowing splash water to seep underneath. The leak only appears when the faucet is running.
Kitchen sinks with disposals have an additional leak point at the disposal flange. Dried-out putty, loose mounting ring, or cracked housing cause leaks.
Porcelain, ceramic, and composite sinks can develop hairline cracks from impacts or thermal shock. Stainless steel sinks may corrode at weld seams.
How to Fix It
Tighten or Replace P-Trap
$3-15- 1Place bucket under P-trap. Unscrew slip-joint nuts by hand or with pliers.
- 2Remove P-trap and inspect washers — replace if flat, cracked, or hardened.
- 3If metal P-trap is corroded, replace entire assembly with PVC ($5-12).
- 4Reassemble hand-tight plus quarter turn. Run water 2 minutes to test.
Replace Supply Lines
$8-20- 1Turn off shutoff valves. Place towel under connections.
- 2Disconnect old lines. Install braided stainless steel lines.
- 3Hand-tighten plus quarter turn with wrench.
- 4Turn on valves slowly and check for drips. Replace every 8-10 years proactively.
Reseal Faucet Base
$3-8- 1Disconnect supply lines. Remove faucet mounting nut from below.
- 2Lift faucet, scrape old putty/gasket from sink deck.
- 3Apply fresh plumber's putty or new gasket. Reinstall and wipe excess.
When to Call a Professional
Supply line bursts can flood a kitchen in minutes — know where your shutoff valves are.
Wet cabinet floors = potential mold. Remove all items, dry completely, and check for mold.
Particle board cabinet bottoms swell and disintegrate when wet.
A slow leak unnoticed for weeks can cause subfloor rot ($500-2,000 to repair).
How to Prevent It
Inspect under sinks monthly — a 30-second flashlight check catches leaks early.
Replace flexible supply lines every 8-10 years, even if they look fine.
Install a $15-20 water leak detector under each sink.
Frequently Asked Questions
Dry everything. Lay paper towels under all connections. Run water 5 minutes, then check each towel.
Sudden supply line bursts: usually yes. Gradual leaks: usually no. Document damage with photos immediately.
Simple gasket/tighten: $75-150. P-trap replacement: $100-200. Supply line replacement: $100-250.
Putty for drain flanges (stays soft). Silicone for permanent seals on non-porous surfaces.