Leak Under the Kitchen Sink? How to Find It and Fix It

Updated July 2026 · Emergency Plumber Chelsea

Clear the cupboard completely and dry every pipe, valve and fitting with kitchen roll. Lay a sheet of dry kitchen roll or newspaper on the cupboard base, because it will show exactly where drops land. Use a torch and look at the back of the cupboard too, since the pipes often run behind a false back panel. If the cupboard base is chipboard and has already swollen or gone soft, the leak has probably been going on for a while.

Test the supply side first. Leave the tap off and do not use any appliances for half an hour. If fresh drops appear, the leak is on a pipe that is under constant pressure. Then run the hot tap on its own for a minute and check again, then the cold. Supply-side leaks tend to show at the joints: where the flexible hoses screw onto the tap tails, where they connect to the pipes or isolation valves below, and at the compression nuts on the copper pipework.

Flexible tap connectors are the most common supply-side culprit. Most modern kitchen mixer taps are fed by two braided hoses. The joints can loosen, the small rubber washers inside the nuts can perish, and the hoses themselves can fail. Look closely at each braided hose along its length. If you see a bulge, rust-coloured staining on the braid or broken strands, turn the water off at once. A failing braided hose can burst without warning and flood the kitchen, and in a mansion block the flat below, at full mains pressure.

Isolation valves under the sink, the small valves with a screwdriver slot, can weep from the spindle or from their compression nuts. A quarter turn of a weeping compression nut, holding the valve body steady with a second spanner, often stops it. A valve that weeps from the slot needs replacing. These valves are also how you isolate the tap for a repair: turn the slot so it sits across the pipe. If they are stiff or seized, turn off the main stopcock instead; our guide on how to turn off a stopcock shows which way to turn it.

Next, test the waste side. Put the plug in, fill the sink to a few centimetres below the overflow, then pull the plug and watch underneath. Waste leaks show at the slip nuts on the trap, at the joint where the waste pipe goes into the wall, around the sink waste fitting where it passes through the bowl, and at the overflow connection. A slip nut that has worked loose can usually be tightened by hand. If the plastic washer inside is split or has slipped, a new washer or a new trap is a cheap part. A leak from around the waste fitting itself means the seal between the fitting and the sink has failed and it needs remaking with fresh sealant. If the drip only appears when the bowl is filled right up, the overflow hose or its connector is the likely culprit, and our guide to a sink overflow leaking or smelling explains the fix.

Then run the dishwasher and washing machine if they are plumbed in nearby. Their drain hoses usually connect to a spigot on the sink trap or to a standpipe. A hose that is not pushed fully onto the spigot, a perished hose clip, or a blocked trap that forces water back up the hose will all leak only while the appliance is emptying. The fill hoses connect to small appliance valves on the supply pipes, and those washers can fail too. Our guides to washing machine plumbing problems and a dishwasher not draining cover the appliance side.

Check that the water is not coming from above. Water that seeps past the base of the tap, around a worn tap body or under the edge of the sink where the sealant has failed runs down the underside of the worktop and drips into the cupboard, where it looks like a pipe leak. Splash water on the worktop around the tap and the rim of the sink and see whether it appears underneath. If the tap itself is leaking at its base or spout, our guide to a kitchen mixer tap leaking takes you through it. If the tap is beyond repair, see our guide to replacing a kitchen tap. For a basin rather than a kitchen sink, our guide to a leak under the bathroom sink covers pop-up wastes, bottle traps and pedestal basins.

A slow waste leak that only shows when you use the sink is not an emergency, but it should be fixed soon because it will rot the cupboard base and can soak into the floor. A supply leak is more urgent because it never stops. Until it is repaired, keep the isolation valves or the stopcock off and use a bowl under any drip. If you cannot find the stopcock, our guide on what to do if you can't find your stopcock helps you track it down.

Some leaks under a sink are not at any joint you can see. A pipe that runs through the wall or under the floor behind the cupboard can leak out of sight, and the first sign may be a damp patch on the other side of the wall, a musty smell or a water meter that keeps turning when everything is off. Our guide to the signs of a hidden water leak explains what to check.

If you cannot find the source, a braided hose looks swollen, a valve is seized or the leak comes back after tightening, call Emergency Plumber Chelsea on 020 3002 6667. Our kitchen plumbing repairs service replaces tap connectors, isolation valves, traps and waste fittings and reconnects appliances across SW3, SW10, Knightsbridge, Sloane Square, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours.

Quick tips

• Dry everything and lay kitchen roll on the cupboard base so you can see exactly where drops land.
• Wet with the tap off means a supply leak; wet only when draining means a waste leak.
• A bulging or rusty braided tap hose can burst: turn the water off and have it replaced.
• Hand-tighten trap slip nuts first; a split washer or cracked trap is a cheap part.
• Splash the worktop around the tap to check the water is not coming from above.

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