Leak under sink: isolate, identify, and stop it fast
Quick answer
A leak under the sink is usually either a waste leak (from the trap or drain) or a supply leak (from the pipework feeding the tap). The quickest way to tell them apart is whether the leak happens only when the tap runs, or continuously. Isolate the water if it’s active, then identify the source. If you can’t stop it, or it’s near electrics, treat it as urgent and call a plumber.
Safety first
- If water is near electrics (dishwasher sockets, under-cabinet electrics), don’t touch wet fittings; isolate power only if safe.
- If you can’t stop the leak, use the stopcock and call for help.
- Avoid over-tightening plastic fittings — they can crack.
Full checklist: Safety guidance
What you may notice
- The leak only happens when the tap is running — suggests a waste-side issue, such as the trap or an overflow connection.
- The leak continues even when the tap is off — points to the supply side — a valve, flexible tail, or pipe joint.
- Water is dripping from a plastic nut or fitting — may mean a seal isn’t seated correctly, which sometimes improves with a gentle re-seat.
- Water is pooling near a plug socket or under-cabinet electrics — stop immediately and follow the electrical safety guidance below rather than continuing to investigate.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, in order
- Put a towel/bowl under the leak and clear the cupboard for access.
- Check if the leak happens only when the tap runs (waste) or even when off (supply).
- Find isolation valves on the hot/cold supply (small valves on the pipes). Turn off if needed.
- Check trap nuts and seals: is it dripping from a joint on the U-bend/trap?
- Check compression joints on copper pipework and the tap tails for drips.
What you can do safely
- Isolate the water if the leak is active or worsening.
- Dry the area and identify exactly which joint or component is leaking.
- Place a container or towel underneath before testing anything.
- Tighten only obvious hand-tight items such as a loose trap nut, and stop if the leak worsens.
When to call a professional
- You can’t isolate the water supply locally and the leak continues.
- Leak is from the tap body or inaccessible pipework.
- Any signs of water reaching electrics or flooring damage.
What to tell the engineer
- Which of the checks above you carried out, and what you found at each step.
- Roughly when the problem started and whether it’s constant, intermittent, or linked to specific conditions (e.g. only when heating runs, only in cold weather).
- Any fault code, unusual noise, or visible sign you noticed.
- Whether the issue affects one fixture/radiator or several.
FAQs
Can I tighten a leaking joint myself?
Gently hand-tightening an obviously loose nut, such as a trap connection, is generally safe. Avoid over-tightening plastic fittings though, as this can crack them rather than fix the leak.
How do I find the isolation valves under my sink?
They’re usually small valves on the flexible pipes feeding the tap, often turned with a flat-blade screwdriver or by hand. If you can’t find any, use your main stopcock instead.
What if the leak is coming from the tap itself rather than the pipework?
This is a different issue — see our guide on dripping taps for cartridge versus washer causes.
Is a leak under the sink always urgent?
Not always, but it’s worth acting on fairly quickly to avoid water damage to cabinets or flooring. If it’s worsening or near electrics, treat it as more urgent.
Leave a Reply