Leak under kitchen or bathroom sink
Under-sink leaks are one of the most common household plumbing problems. Most are easy to fix once you identify the source. The three main culprits are the plastic waste trap, the supply pipe connections, or the flexi hoses connecting the tap to the supply.
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Most likely cause & what to check
Clear everything out from under the sink and put a bucket or towels down. Use a torch to get a good look at all the pipework.
Run the tap and watch carefully to see where the leak is coming from. Run the plug in and fill the basin, then pull it out to check the waste trap specifically.
If leaking from the plastic waste trap (the U-bend): hand-tighten all the plastic connections — they often just need tightening. If cracked, buy a replacement trap (£5–10) and swap it out.
If leaking from the flexi hoses (the short braided pipes connecting the tap to the hot and cold supply): check both ends. PTFE tape on the thread can often cure a weeping connection. If a hose is split, replace it — they are cheap and available at any DIY store.
If leaking from compression fittings on copper pipe: try tightening the compression nut by a quarter turn. If still leaking, it may need re-making with new olive and PTFE tape.
Once you've made a repair, test thoroughly — fill the basin and drain it, then leave a piece of dry paper towel under the pipework for 30 minutes to check for any further drips.
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Frequently asked questions
How do I turn off the water under the sink?
There should be a small isolation valve on each supply pipe (hot and cold) under the sink — a screwdriver slot that you turn 90° to shut off. If there isn't one, turn off the main stopcock.
The waste pipe is leaking where it goes into the wall — what do I do?
This is where the 32mm or 40mm plastic waste pipe connects to the building drain. Check the push-fit connection is fully inserted. If it's a cemented joint that's cracked, a plumber will need to re-join it.