Leak under sink: isolate, identify, and stop it fast
Quick summary
Isolate first, then identify whether it’s a waste leak (trap) or a supply leak (pipework/valves).
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 to check (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 the result means
- Leaks only when tap runs: waste/trap or sink overflow connection.
- Leaks even when tap is off: supply side (valve, flexi tail, joint) — isolate.
- Drip from a plastic nut: seal may be mis-seated; gentle re-seat often helps.
What you can safely do
- 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.
Engineer notes
Classify leak: waste vs supply. For waste, inspect trap washer seating and overflow connections; avoid cross-threading plastic. For supply, check valve gland, flexi tail integrity, compression olives, and pipe support. Pressure-test after any adjustment. Record whether leak is pressure-dependent or flow-dependent.
Leave a Reply