Turn off the water first, using the isolation valves under the sink or your main stopcock. Then dry everything and look for where the water appears: most leaks under a sink come from the waste trap, the tap’s flexible tails or a loose compression joint. A loose plastic waste nut can often be hand-tightened; a leaking tap tail or valve is usually a job for a plumber.
Step 1: turn off the water
Isolation valves are the small screw-slot valves on the pipes under the sink. A quarter turn with a flat screwdriver, so the slot sits across the pipe, turns that supply off. The main stopcock turns off the whole house. It’s usually under the kitchen sink, but can be in a downstairs toilet, utility room, garage, cellar or under the stairs. Turn it clockwise. If it hasn’t been used for years it may be stiff; don’t force it so hard that it breaks.
It’s worth finding your stopcock today, before you ever need it.
Step 2: find where the water is coming from
Clear the cupboard, dry every pipe and joint with kitchen roll, and put a dry sheet of paper underneath. Run the cold tap, then the hot, then fill and empty the sink, watching each joint in turn.
- Only when the sink drains: the waste, trap or overflow. Often a loose nut or a perished washer.
- All the time, even with the tap off: a supply joint, tap tail or isolation valve under pressure.
- Only when the tap runs: the tap body, its base seal, or the tails where they connect to the tap.
- Around the edge of the sink: failed sealant between the sink and the worktop.
Step 3: the safe fixes
Plastic waste nuts can be gently hand-tightened; if the leak continues, the washer inside may need replacing. Don’t over-tighten plastic fittings, as they crack. Failed sealant around a sink can be cut out and redone once everything is completely dry.
Leaks from the pressurised supply side (tap tails, compression joints, valves) are best left to a plumber, because a fitting that fails completely can flood a kitchen quickly.
If water is coming through a ceiling
Turn the water off at the stopcock straight away. If water is near light fittings or sockets, switch off the electricity at the consumer unit if you can do so safely, and keep away from the area. Catch what you can, and call a plumber. If a ceiling is bulging with water, keep out of the room.
When to call a professional
Call us for any leak on the supply side, a leak you can’t find, a seized stopcock, water through a ceiling, or a cupboard base that’s already swollen. Send a photo on WhatsApp and we’ll tell you how urgent it looks.
Quick questions
Where is my stopcock?
Most often under the kitchen sink. Other common spots are a downstairs toilet, utility room, garage, cellar or under the stairs, usually where the water supply enters the house. There may also be an outside stopcock under a small cover near the boundary, which belongs to the water company.
Is a slow drip under the sink urgent?
It isn’t an emergency, but it shouldn’t wait. Slow leaks swell chipboard cupboards, rot floors and encourage mould. Put a container underneath and book it in.
Why does my sink only leak when I drain it?
That points to the waste: the trap, the waste nuts or the overflow. These are often loose or have a perished washer.
Can a leaking tap tail be tightened?
Sometimes, but flexible tap tails can fail without warning, so a weeping one is best replaced rather than tightened.
This guide is general advice for simple checks. Never attempt work on gas appliances yourself: by law, gas work must be done by a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency number on 0800 111 999.