Leak Under the Kitchen Sink? Trace It in Five Minutes
A puddle in the cabinet has only six possible sources, and a dry paper towel finds the right one in five minutes. Dry every fitting, then run water and watch where the first new drop appears. Supply-side leaks drip with the water off; drain leaks only show when you run water. A trap swap runs $100 – $250, a supply line $75 – $200, and knowing which it is saves you a guess.
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Safety first: if you smell gas, see water near electrical outlets or your panel, or sewage is contacting living areas, get people clear first. For a gas smell, leave and call 911 or your gas utility's emergency line before anything on this page.
- !Water is reaching an outlet, a power strip, or the disposal cord under the sink: unplug at the breaker, not by hand on a wet plug
- !A supply line is spraying or streaming under pressure rather than dripping: close the shutoff now
- !The cabinet floor is saturated and water is spreading to the subfloor or adjacent cabinets
- !You smell sewage or see dark, foul water, pointing at a drain or vent problem rather than clean supply
- !A braided supply line has burst and you cannot stop the flow at the angle stop: close the main
- ✓Dry everything with paper towels: the two shutoff valves, both supply lines, the faucet base from above, the trap, and the disposal
- ✓With the water off, watch the supply connections and shutoffs: any drop here is a pressurized supply leak
- ✓Run the faucet and fill the basin, then release it: a drip that only appears now is a drain or trap leak
- ✓Feel the faucet base and the sink rim from above while water runs: a leak there tracks down the supply lines and fools you into blaming them
- ✓Run the dishwasher a minute and check its hose connection at the disposal or drain: appliance leaks only show during a cycle
- →The trap or drain leaks at a joint you cannot stop by hand-tightening the slip nuts
- →A shutoff valve weeps from its stem or body and will not seal: angle stops are a plumber swap
- →The faucet leaks from its base or body, meaning a worn cartridge or a failed base seal
- →The garbage disposal leaks from its body or the bottom rather than a hose connection
- →You traced the source but the fix needs parts or access you are not set up for
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The five-minute trace
The whole diagnosis is one trick: dry everything, then watch. Pull everything out of the cabinet, wipe every fitting bone dry with paper towels, and lay a fresh dry towel across the cabinet floor. Now you can see exactly where water reappears instead of staring at a general puddle.
The single most useful split is water-on versus water-off. Supply-side parts, the two angle stop valves and the braided supply lines feeding the faucet, are under constant pressure, so they drip even when no one is using the sink. Drain-side parts, the trap and the disposal outlet, only leak while water is actually running through them. So watch the dry fittings with the water off for a minute, then run and fill the sink and watch again. Which phase produces the first drop tells you which half of the system to blame.
The six suspects
Supply connections and shutoff valves come first because they leak constantly and are easy to confirm. A drop forming at a braided line nut or weeping from an angle stop stem is your answer. The faucet base is the sneaky one: a worn faucet that leaks around its base or under the sink rim sends water straight down the supply lines, so it looks like a supply leak until you feel the rim from above while the water runs.
On the drain side, the trap and its slip-joint connections are the classic culprit, loosening or drying out and weeping only when you run water. The garbage disposal is two separate things: a hose connection leak (the dishwasher drain hose or the discharge tube) versus a leak from the disposal body itself, which is a different fix. And the dishwasher line, which connects at the disposal or the drain, only leaks during a wash cycle, so run one to rule it in or out.
What each fix costs
The trace pays off here because the prices diverge. A leaking trap or drain assembly is the most common find and runs $100 – $250 to replace, often less if it just needs new slip washers and a snug reassembly. A braided supply line or a faucet supply connection runs $75 – $200, and the lines themselves are cheap, so this is mostly labor and access.
The other two depend on what you found. A faucet that leaks from its base or body usually means a worn cartridge or a failed seal, and at that point a repair-versus-replace call applies; our faucet installation cost page covers the swap. A disposal that leaks from its body rather than a hose is rarely worth repairing, which is why a garbage disposal leaking from the bottom usually points to replacement. A weeping angle stop valve is a quick plumber swap on the same visit.
Contain it and prevent the next one
Until the fix, close the two angle stop valves under the sink to kill supply pressure, and keep the cabinet contents out and a towel down so you catch any drips and protect the cabinet floor. Particle-board cabinet bottoms swell and delaminate fast once they stay wet, so a $10 plastic under-sink mat or tray is cheap insurance that also makes the next leak obvious the moment it starts.
Most repeat leaks here come from connections that vibrate loose or supply lines that age out. Braided stainless supply lines are inexpensive and worth replacing proactively if yours are old rubber or original to the house. Hand-snug the trap slip nuts after any disposal work, since that joint gets bumped whenever someone reaches under the sink.
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