Leak under the kitchen sink? How to find where it's coming from
Water under the kitchen sink comes from the supply lines, the faucet, or the drain, and each one leaks on its own schedule. Supply lines and shutoff valves leak constantly, faucet on or off. The faucet and sprayer only leak while the water runs, and drain parts only leak while water’s going down. Figuring out which one you’ve got is mostly a process of elimination.
Start by clearing everything out and drying it
Take everything out of the cabinet. Dry the bottom and every pipe and fitting with a towel, then lay a strip of paper towel or a few sheets of newspaper across the cabinet floor. Now you can test each part one at a time and see which one gets wet first, instead of guessing from a puddle.
Is it the supply lines or the shutoff valves?
These are the easiest to rule in or out, because they leak even with the faucet untouched. Feel along the two supply lines running up from the shutoff valves to the faucet, and check the valves themselves where they meet the pipe. A wet spot here shows up within a few minutes of drying everything off.
Braided supply lines wear out and start weeping at the fittings on either end. Snug up the fitting first, and if it’s still wet, replace the line. They’re cheap and quick to swap. A shutoff valve leaking around its body, not just the handle, usually needs to be replaced rather than tightened.
Is it the faucet itself?
Run the faucet and watch the base where it meets the sink deck, and the sprayer hose if you’ve got one, especially where it connects underneath. If everything’s dry with the water off but wet the moment you turn it on, the leak is in the faucet assembly: a worn cartridge, a loose connection, or a cracked sprayer hose. If the faucet’s the problem and you’d rather not wrestle with it, that’s part of our toilet and faucet repair work.
Is it the drain?
Drain leaks only show up while water’s flowing through, so the test is different. Fill the sink most of the way, then pull the plug and watch closely while it drains. Check the slip nuts on the P-trap, the curved pipe under the sink, and the basket strainer where the sink bowl meets the drain pipe.
A loose slip nut can often be hand-tightened, then snugged a bit more with pliers. Don’t crank it down hard. Plastic nuts crack under too much force. A strainer leaking around the sink itself usually means the plumber’s putty or seal underneath has dried out and needs to be redone.
A slow drain leak can look like it’s coming from the P-trap when the real problem is up at the strainer. Water runs down the outside of the pipe before it drips off somewhere lower. Wipe each joint dry, run the fill-and-drain test again, and watch the first spot that gets wet, not where the water pools.
Don’t forget the dishwasher and disposal
If you’ve got a garbage disposal or a dishwasher tied into that drain, check those connections too. A loose dishwasher drain hose or a disposal leaking at a seam shows up the same way, only when water’s running through it. If the disposal itself is acting up, here’s what to try when it’s humming but won’t spin.
A leaking soap dispenser or a sink rim that’s lost its seal can also drip down into the cabinet and get blamed on the drain. Worth ruling out before you take anything apart.
Easy fix, or time to call someone?
Snugging a slip nut or swapping a supply line is something most people can handle with basic tools. Resealing a strainer takes a little more, since the drain has to come apart. Call a plumber if a shutoff valve is corroded and won’t turn, or if you’ve traced water back into the wall itself rather than a visible fitting under the sink. Forcing a corroded valve is how a slow drip turns into a real problem fast.
A warped or soft spot on the cabinet floor means this leak’s been going on longer than you think, even if you just noticed it today.