Leak Under the Kitchen Sink? How to Find the Source

A leak under the kitchen sink comes from the supply pipes and tap hoses, the waste trap and its connections, the sink's plug hole seal, water getting in from above, or an appliance hose in the same cupboard. The fastest way to narrow it down is timing. If the cupboard is wet even when nobody has used the sink, the leak is on the supply side, which is under mains pressure and runs all the time. If it only gets wet when the sink drains, it's the waste, the trap or the plug hole seal. Supply leaks can be stopped at the isolating valve or the stopcock, and should be dealt with the same day.

Stop the damage first

1. Empty the cupboard and put a bowl or tray under the drip.
2. If water is spraying or dripping steadily from a pipe or hose, find the isolating valve on that pipe. It's a small fitting with a screwdriver slot. Turn the slot a quarter turn so it sits across the pipe. If there's no valve or it won't close, turn off the stopcock. Our guide to a stopcock that won't turn helps if yours is stuck.
3. If the leak is on the waste side, stop using the sink and the dishwasher or washing machine if they drain into it.
4. If water has reached electrics, sockets or an appliance behind the units, switch that circuit off at the consumer unit before touching anything.

In a flat, check the ceiling of the flat below if you can, or let your neighbour know. Water sitting in the base of a kitchen unit can find its way down long before you see it.

Find where it's coming from

Dry every pipe and joint with a towel, then wrap kitchen roll around each joint or lay it on the cupboard base. Dry paper shows a fresh drip clearly, and you can see which way the water ran. Then test one thing at a time:

Test	If it gets wet
Leave everything off for half an hour	Supply side: tap hoses, isolating valves, compression joints or the pipe itself
Run the hot and then the cold tap into the sink with the plug out	The tap body or its base seal, or the waste
Fill the sink, then pull the plug	The trap, waste pipe or plug hole seal. This puts the most water through the waste at once.
Wipe water along the worktop edge and around the tap	The sink's sealant line or the tap's base seal letting water in from above
Run the dishwasher or washing machine	The appliance hose, spigot connection or appliance valve

Supply side leaks

These are the ones not to leave, because they're under pressure all the time.

• Flexible tap hoses. Most modern kitchen taps are fed by braided flexible hoses. A drip at the nut can be a washer that isn't seated. A hose that's weeping through the braid, kinked, bulging or rusty should be replaced, not tightened. A hose that fails under mains pressure lets out a lot of water quickly.
• Isolating valves. These can weep from the screwdriver slot as they age, especially after being turned for the first time in years. They're cheap to replace but need the water off.
• Compression joints. The brass nuts and olives on copper pipes. A small drip can sometimes be stopped by nipping the nut a little tighter, while holding the fitting's body steady with a second spanner. Over-tightening can distort the olive and make it worse. If a gentle nip doesn't stop it, the joint needs remaking.
• The pipe itself. Look for green or white crusting on copper pipes, which is a sign of a slow weep that has dried out. A pinhole in the pipe needs a section replacing.

New or altered pipework and fittings on the water supply have to meet the Water Supply (Water Fittings) Regulations 1999, so if the supply needs remaking, get it done properly rather than patched. If the tap itself is dripping into the sink rather than leaking below, see how to fix a dripping tap. If the tap is past repair, our guide on how to replace a kitchen tap covers the hoses and valves too.

Waste and trap leaks

Waste leaks only show when water is draining, which is why they can go unnoticed for weeks and slowly rot the cupboard base.

• Trap nuts. Plastic nuts on the trap are tightened by hand. If a joint drips, undo it, check the rubber washer is sitting flat and isn't split, and refit. Don't use a wrench on plastic, which can crack the nut.
• A cracked trap. Plastic traps can crack if they've been overtightened or knocked. Replace it.
• Appliance spigot. The washing machine or dishwasher hose connects to a spigot on the trap with a clip. A loose clip or perished hose end drips when the appliance drains. If the washing machine is the source, see our guide to a washing machine leaking.
• A partial blockage. If the sink drains slowly, water backs up and can push out of joints that are fine under normal flow. Clear the blockage first. See our guide to a blocked kitchen sink.

Sink seal and water from above

Plug hole seal. The waste fitting in the bottom of the sink is sealed to the sink with a washer or sealant. When that fails, water leaks around the outside of the fitting under the sink, but only when there's water in the bowl. Filling the sink with the plug in and waiting a few minutes before draining it shows it. Remaking the seal means taking the waste fitting out.

Overflow. If your sink has an overflow slot, a thin pipe runs from it to the waste. A loose connection drips only when the sink is very full.

Sealant and the tap base. Water splashed along the back of the sink can run under a worn sealant line and drip into the cupboard, which looks exactly like a pipe leak. So can water running down the outside of a tap with a worn base seal. If the wet patch is at the back or directly under the tap, and the pipes are dry, this is the likely cause. Resealing the sink is a simple fix once the old sealant is cleaned off. If the water is coming out of the tap itself, round the spout or under the handle, see our guide to a tap leaking from the base.

When to call a plumber

Call a plumber when a supply leak won't stop after a gentle tighten, when a braided hose or isolating valve needs replacing and you'd rather not turn off the water yourself, when the leak is at a soldered joint or in the pipe itself, or when you can't find the source. If the cupboard is damp but none of the tests above produce a drip, the water may be coming from a pipe in the wall or under the floor. Our leak detection service can trace hidden leaks.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our leak repair and kitchen tap repair services cover London, and the pricing page explains how jobs are charged.

If the drip is coming from a nut on a valve or pipe joint, our guide to a leaking compression fitting explains how to tighten or remake it properly.

Frequently Asked Questions