Why Is There Water Leaking Under My Sink?
For drips, dampness or a puddle inside the cupboard below a kitchen or bathroom sink, especially when you cannot immediately see which joint is leaking.
What this usually means
First work out whether the leak happens while the tap is running, when the sink drains, or even when nobody is using the sink. Water that appears mainly as the bowl empties usually points to the waste pipe, trap or plug connection. A leak that continues with the taps off is more likely to be on the pressurised supply side and should be isolated quickly. Stop using the sink, dry the cupboard, catch the water and trace the first wet joint rather than tightening everything at random.
What may be causing this
A leaking waste trap or waste-pipe joint
Clues: The cupboard stays dry while the tap is running into a plugged bowl, then starts dripping when you release the plug. You may see water around a plastic compression joint, trap or the underside of the plug fitting.
What to do: Stop using the sink until the joint is identified. A plumber can reseat or replace a failed seal, washer, trap or badly aligned waste fitting rather than simply overtightening the plastic parts.
A supply hose, isolation valve or pressurised pipe joint
Clues: Water appears even when the sink is not draining, or the joint becomes wet while the tap is running. A supply-side leak can keep dripping continuously because the pipe is under pressure.
What to do: Turn off the nearest working isolation valve if you know which one it is, or use the internal stop tap for an active leak. Arrange a plumber rather than leaving a bowl under a pressurised leak.
Water leaking from the tap base or sink fitting
Clues: The pipework below looks dry until water is splashed around the tap or sink edge. Moisture then tracks down the underside of the sink and appears to come from a pipe.
What to do: Dry the top and underside of the sink and test in small stages. The fault may be a tap seal, sink seal or plug/waste connection rather than the visible pipe directly below the drip.
A cracked, knocked or badly aligned waste pipe
Clues: Plastic pipework looks twisted, a joint has partly slipped apart, or stored items have been pushing against the trap. Water may run down the outside of the pipe when the sink empties quickly.
What to do: Stop using the sink and have the damaged or misaligned section corrected. A joint that repeatedly moves or leaks usually needs proper realignment, not more force on the nut.
How to check what is happening
That first wet point is more useful than the puddle on the cupboard floor. Water often runs along pipework before it finally drips.
Look higher up: the tap, flexible supply hoses, isolation valves or sink/tap seals are more likely than the waste pipe below.
The waste fitting, trap or waste-pipe joint is the stronger suspect because those parts only carry water when the sink drains.
Treat it as a possible pressurised supply leak or water arriving from somewhere else. Isolate the water if the leak is active.
Can you fix this yourself?
Finding the source and containing the water are sensible homeowner jobs. WaterSafe recommends using an approved plumber for the repair itself, especially for pressurised supply pipes, damaged fittings or anything you are not certain about. The useful DIY part is narrowing the fault down so the repair is quicker and you avoid tightening the wrong joint.
- Torch
- Dry kitchen roll or tissue
- Bowl or bucket
- Old towel
What you can safely try
- 1
Stop using the sink and contain the water
Empty the cupboard, put a bowl or towel below the drip and stop running water through the sink. If the leak is flowing rather than slowly dripping, turn off the relevant isolation valve or the internal stop tap.
- 2
Dry everything before you test it
Wipe the trap, waste pipe, flexible hoses, valves and underside of the sink completely dry. Wrap or hold dry tissue around individual joints. The first tissue to become wet helps identify where the leak actually starts.
- 3
Check whether the leak is on the supply side
With the waste plug closed, run only a small amount of water. Watch the tap tails, flexible hoses and valves. If they wet up before the bowl drains, the leak is probably above the waste pipe.
- 4
Then test the waste side separately
If the supply side stayed dry, release the plug and watch the trap and waste joints as the bowl empties. A leak that appears only now strongly points to the waste route.
- 5
Do not keep tightening every plastic nut
Plastic traps can distort or crack if overtightened, and a badly aligned joint may still leak. Once you know the source, photograph it and arrange the correct repair rather than turning every fitting harder.
Call a plumber if…
- Water is spraying, flowing continuously or the cupboard is flooding.
- Water has reached electrical sockets, appliances, extension leads or wiring.
- You cannot stop an active supply-side leak with a known isolation valve or stop tap.
- The pipe, trap or fitting is visibly cracked, badly misaligned or will not stay sealed.
- The leak is hidden behind the cabinet, wall or floor rather than at an accessible joint.
Get the right plumber.
We already know this problem maps primarily to a plumber. Choose the closest type of work, add your postcode and continue in the form.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- What to do with a leak under the sinkWaterSafe
- What should I do if I find a leak in or around my home or business?WaterSafe
- How do I find a water leak in or around my home or business?WaterSafe
Last reviewed: 30 August 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.