Tap won't turn off fully
A tap that won't shut off completely is almost always a worn washer or faulty ceramic cartridge. Left unchecked it wastes thousands of litres a year and can lead to limescale build-up in the seat. This is one of the most straightforward DIY plumbing jobs you can do.
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Most likely cause & what to check
Confirm the tap is the problem — turn off the isolation valve under the sink and see if the dripping stops. If it does, the tap is at fault. If it continues, the isolation valve itself is faulty.
Shut off the water supply using the isolation valve or main stopcock. Open the tap to drain residual water.
Remove the handle by unscrewing the grub screw under the decorative cap.
For a washer-type tap: use a spanner to unscrew the headgear (the brass threaded section). At the bottom you will see a rubber washer held by a small nut. Replace the washer and refit.
For a cartridge tap: pull out the cartridge and replace it with an identical one from a merchant.
Check the tap seat inside the tap body — if it feels rough or pitted, it may need re-facing with a tap re-seating tool or the whole tap may need replacing.
Reassemble and turn the water back on slowly. Test that the tap now closes completely.
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Frequently asked questions
My isolation valve won't close fully — what do I do?
If the isolation valve is also passing water, turn off the main stopcock. Isolation valves are cheap and easy to replace — a plumber can do this in 15 minutes.