Leak Under the Sink? Here's How to Find and Fix It

A kitchen sink leaking underneath is almost always a trap, waste pipe, or isolation valve fault, not the sink bowl or tap itself, because those parts are what carry every drop of water that goes down the plughole or into the tap. Dry cabinets with a wet base are the classic sign — the leak is coming from below, not from above.

The most common culprits, in order of how often we see them on London call-outs, are a loose or worn trap joint, a perished washer in a compression fitting, a dripping isolation valve, and a flexi-hose that's failed at one of its crimped ends. Each one leaves a slightly different clue, which is why drying everything and watching where the water reappears first is faster than guessing.

Yes, a leaking vanity unit under a bathroom basin has the same handful of causes as a kitchen sink — trap, waste pipe joint, isolation valve or flexi-hose — but with one extra possibility: the basin's own waste outlet seal, which sits directly under the plughole and can perish or work loose over time. Bathroom vanity units also trap moisture more than open kitchen cabinets, so a small drip does more damage faster.

Check the basin waste outlet seal first if the cabinet base is wet directly under the plughole rather than further back near the pipework — that narrows the fault before you even open the cupboard doors.

A washing machine waste pipe leaking under the kitchen sink is a separate fault from the sink's own trap, because the machine's outlet hose usually connects into the sink's waste pipe via a standpipe or a push-fit connector just below the trap. Check that connector first — it's a common weak point that gets knocked or works loose whenever the machine is moved for cleaning or a service.

If the leak only appears during a wash cycle rather than constantly, it's very likely this connector or the standpipe itself rather than the sink's own plumbing, since the extra pressure from the machine's pump is what forces water out of a loose joint.

An isolation valve dripping under the sink is usually leaking from the compression nut where the valve meets the pipe, or from the spindle at the top of the valve when it's been turned — both are fixable without replacing the whole valve in most cases. A compression nut just needs re-tightening with a wrench, while a spindle leak usually means the small O-ring inside has perished.

If the valve won't fully shut off the water even when closed, or the body of the valve itself is leaking rather than just a fitting, the valve has failed internally and needs replacing rather than tightening — that's a job worth having a plumber do, since the main supply has to be isolated first.

Finding a leak under the sink takes about ten minutes if you work through it systematically, and you don't need to disconnect anything to trace the source.

1. Empty the cupboard and wipe the trap, waste pipe, isolation valves and flexi-hoses completely dry with a cloth.
2. Run hot and cold water for a minute and watch each dried joint — the first one to bead with fresh water is your source.
3. Check the trap and waste pipe slip nuts — a loose nut just needs hand-tightening, a worn washer inside needs replacing.
4. Check the isolation valves for drips at the compression nut or from the spindle when turned.
5. Feel along each flexi-hose for dampness or a bulge, especially where the metal fitting crimps onto the hose at either end.
6. Tighten or replace whichever part is actually dripping, then run the tap again and check the joint stays dry for a full 20 minutes before you pack the cabinet back up.

Most trap and waste pipe leaks stop with nothing more than hand-tightening the slip nut — don't over-tighten with a wrench, since plastic waste fittings crack under too much force.

Call a plumber if tightening the trap or waste pipe doesn't stop the drip, if an isolation valve won't fully shut off or is leaking from the valve body itself, or if you've traced the leak to a rigid pipe joint rather than a simple slip nut or flexi-hose. RenoPlumb carries common traps, washers, flexi-hoses and isolation valves on every van and covers Tower Hamlets, Hackney, Islington, Newham, Southwark and the rest of East and Central London.

For a full diagnosis, including tracing leaks that seem to come from nowhere, see our leak detection & repair service or call 07460 824073 for a free quote.

The cost depends on whether the fix is a simple slip nut tightening or a washer swap, or a full trap, waste pipe, or isolation valve replacement, so we always confirm the price with a free, no-obligation quote before any work starts. For a general sense of how plumbing jobs in London are priced, see our pricing page.

Need this fixed by someone local? See our page for a plumber in Southwark for borough-specific coverage and response times.