Most Likely Causes (Ranked)
The P-trap (the curved pipe under the drain) uses slip-joint nuts and nylon washers to seal. These loosen from vibration — especially if anything stored under the sink bumps the pipes. A common scenario: you pulled out a container, nudged the P-trap, and now it drips every time you run water. The nylon washer may also be compressed flat after years of use.
Flexible braided supply lines connecting the shut-off valves to the faucet have a 5-8 year lifespan. The internal rubber degrades from constant water pressure (even when the faucet is off, the line is pressurized). When they fail, they fail catastrophically — a burst supply line releases water at full pressure 24/7 until someone turns off the valve. Insurance data shows supply line failures are a top-5 cause of home water damage claims.
The O-ring or gasket sealing the faucet body to the sink deck compresses and hardens over time. Water pools on the counter around the faucet base and runs through the mounting hole into the cabinet below. This drip is slow and intermittent — it only occurs when water splashes around the base during use.
The tailpiece (straight pipe from drain to P-trap) and the sink strainer basket are sealed with plumber's putty or a rubber gasket. The putty dries and cracks over 10-20 years. PVC tailpieces can crack from hot water thermal cycling or from overtightened slip nuts. The strainer's lock nut under the sink can loosen from vibration.
The dishwasher drain hose connection, the discharge pipe connection, and the mounting flange are the three most common disposal leak points. The mounting ring that holds the disposal to the sink loosens over time from the disposal's weight and vibration when running. The dishwasher nipple's internal rubber diaphragm can fail.
How to Fix It
Tighten P-Trap Connections
Free- 1Place a bucket under the P-trap.
- 2Hand-tighten each slip-joint nut. If it's already hand-tight, add a quarter turn with slip-joint pliers.
- 3If it still drips, disassemble, inspect the nylon washer. If it's flat or cracked, replace it ($0.50-1 at any hardware store).
- 4Reassemble: washer first (beveled side facing the nut), then hand-tighten, then a quarter turn with pliers.
- 5Run water for 60 seconds and check every joint with a dry paper towel.
Replace Supply Lines
$8-15 per line- 1Turn off the shut-off valve under the sink. Open the faucet to relieve pressure.
- 2Unscrew the old line from both the shut-off valve and the faucet tailpiece.
- 3Install new braided stainless steel line (measure length first — too short stresses connections, too long kinks).
- 4Hand-tighten plus a quarter turn with pliers. No Teflon tape needed — these use rubber washers.
- 5Turn on the valve slowly and check both connections with a dry paper towel.
Reseal Faucet Base or Strainer
$3-8- 1Faucet base: remove the faucet, clean old sealant, apply fresh plumber's putty or silicone, reinstall and tighten evenly.
- 2Strainer: from below, loosen the lock nut, push the strainer up, clean the old putty, apply new putty around the flange, reinstall.
- 3Tighten the strainer lock nut firmly — use a basket strainer wrench ($8) or channel-locks and a screwdriver.
When to Call a Professional
A burst supply line releases water at full municipal pressure. Know where your under-sink shut-off valves are.
Wet or warped cabinet floor means the leak has been going for a while — check for mold behind and under the cabinet.
Never use a pipe wrench on plastic (PVC) fittings — it cracks them. Use slip-joint pliers or hand-tighten.
If you see active mold, wear a respirator (N95 minimum) during cleanup.
How to Prevent It
Check under every sink monthly — a 30-second visual inspection prevents thousands in damage.
Replace braided supply lines every 5-7 years proactively ($8-15 each).
Don't store heavy items against drain pipes — they loosen connections.
Exercise shut-off valves quarterly (turn off and back on) so they don't seize.
Frequently Asked Questions
Wipe everything dry. Lay paper towels under all connections. Run water at full blast for 60 seconds. The first wet paper towel marks the source.
P-trap, supply lines, and strainer reseals are beginner-level DIY ($0-15). Faucet replacement is intermediate. Only call a plumber for in-wall pipes or if the shut-off valve itself is leaking ($150-300 service call).
Minor warping: dry thoroughly with fans for 48 hours, then seal with polyurethane. Severe damage: replace the cabinet floor panel ($50-150 DIY). Check subfloor beneath the cabinet too.
Intermittent leaks usually mean the seal only fails under specific conditions: full pressure (both handles open), hot water expansion, or when items stored underneath shift against the pipes.