Quick answer: the usual suspects
When water collects under a kitchen sink, the cause is usually one of these:
- A loose or worn drain connection, such as slip nuts and washers at the P-trap
- A failing seal around the drain basket (the strainer in the sink bottom)
- A worn or damaged supply line or shut-off valve
- A leak at the base of the faucet or in its connections
- A leaking garbage disposal or dishwasher hose
- A cracked basin or failed sealant, less commonly
Water travels, so where you see it is not always where it starts. For a full tracing process, see how to find and fix a leak under the sink.
Find the source
- Empty the cabinet and look for stains or soft spots.
- Dry every pipe, hose and connection with a towel or paper towels.
- Put a dry paper towel under each connection or lay a sheet of newspaper on the cabinet floor.
- Run the faucet only, with the drain stopped, and watch with a flashlight. Then fill the basin and release the water, and then run the dishwasher or disposal if you have them.
- Note when the drip appears: when the tap runs, when the sink drains, or all the time.
The timing is a strong clue. A leak only when the sink drains usually points to the drain side. A leak when the tap is on, or constantly, usually points to a supply connection.
Drain basket and tailpiece
The basket is sealed to the sink with plumber’s putty or a gasket. If the seal ages or the nut loosens, water can seep out where the basket meets the sink and run down the tailpiece. The fix usually involves removing and resealing the basket, following the manufacturer’s directions for the sealing material. Your sink material (stainless, porcelain, composite) may influence which sealant is appropriate.
Slip nuts and washers at the trap
Joints in the P-trap use slip nuts and rubber or plastic washers. They can loosen, or the washers can crack or flatten. Try hand-tightening first. If it still drips, take the joint apart, check that the washer is present and the right way around, and replace it if worn. Avoid heavy force on plastic pipe, as overtightening can crack nuts.
Faucet base and supply lines
Water can seep from the base of the faucet, around the hoses, or at the connection to the shut-off valves. A braided supply line that is corroded, kinked or has a damp connection should generally be replaced rather than patched. Close the shut-off valves first, usually by turning them clockwise. If a valve will not shut, you may need to close the home’s main water valve.
If the faucet itself seems to leak from underneath, the cause may be worn internal seals or loose mounting hardware, which vary by faucet model. Check the manufacturer’s instructions or parts diagram.
Disposal and dishwasher connections
If you have a disposal, leaks may come from its flange, discharge pipe, dishwasher hose connection or body. Turn the disposal off and keep hands away. See garbage disposal leaking from the bottom for specifics.
Cracked basin or failed sealant
A crack in the sink basin, or gaps in the sealant where the sink meets the counter, can let water through to the cabinet below. This is less common but easy to miss. A cracked basin generally means replacement rather than repair.
What does it cost to fix?
Costs vary widely by region, parts and whether you hire a plumber. Simple fixes such as replacing washers or tightening a joint may cost very little in parts if you do the work yourself. Plumber charges typically include a service call fee plus labor and parts, and replacing a sink, faucet or disposal costs more. Get a written quote before work starts.
Protect the cabinet
Dry the cabinet and check for swelling, soft spots, mold, or a musty smell. Particleboard cabinet bottoms can swell and deteriorate when wet. Put a shallow tray under the plumbing while you resolve the leak, and avoid leaving soaked towels pressed against the cabinet.
When to call a plumber
- You cannot find the source, or the drip returns after you tighten or replace parts
- Shut-off valves do not close, or they leak
- The leak comes from a pressurized pipe, not a drain
- A disposal or electrical item is wet
- The cabinet or floor is soft, or you see mold
- The sink basin is cracked
For other common problems, see kitchen sink plumbing problems and how to fix them.