Leak Under the Bathroom Sink: How to Find It and Fix It

Updated July 2026 · Emergency Plumber Chelsea

Get to the pipework first. On a vanity unit, empty the cupboard. On a pedestal basin, the pedestal hides the trap and the pipes, and it can usually be eased away from the wall once any fixing screw at the base is removed; take care, as it can be heavy and the basin may rely on it for support. On a wall-hung basin with the pipes boxed in or concealed, look for an access panel or a removable shroud under the basin. Dry every pipe, valve and fitting with kitchen roll and lay a dry sheet on the floor or cupboard base so you can see exactly where drops land.

Test the supply side. Leave the taps off and do not use the basin for half an hour. Fresh drops mean the leak is on something under constant pressure. Most modern basin mixers are fed by two flexible braided connectors that screw into the base of the tap and connect to the pipes below, usually through small isolation valves. Traditional pillar taps have a separate tap connector on each tail, with a fibre or rubber washer inside the nut. Check each nut and valve with a dry finger. A drip from a compression nut or a valve can often be stopped with a gentle quarter turn, holding the valve body with a second spanner so the pipe does not twist. Our guide on how to fix a leaking pipe joint explains how far to go.

Look closely at the braided connectors. If a flexible connector has a bulge, rust-coloured marks on the braid, broken strands or a kink, turn the water off at the isolation valves or the stopcock straight away. A failing braided hose can burst without warning at full mains pressure, and in a bathroom above another room that means a ceiling coming down. Replacing a pair of connectors is a small job, but it means working in a cramped space under the basin with the right size of fitting, which is why many people call a plumber for it. If the isolation valves are seized, turn off the main stopcock instead; our guide on how to turn off a stopcock shows how.

Test the waste side. Put the plug in or close the pop-up, fill the basin to about two thirds, then release it and watch underneath as it drains. Waste leaks show in a few places. The slip nuts on the trap can work loose, and the washer inside can split or slip. On a bottle trap, the base unscrews for cleaning and needs its seal seated properly when it goes back. Where the waste fitting passes through the basin, the seal or washer between the fitting and the ceramic can fail, and water then runs down the outside of the waste. A leak only while draining that sits right at the top of the waste, just under the basin, usually means that seal needs remaking.

Check a lever pop-up waste. Older lever-operated pop-up wastes have a pivot rod that goes into the side of the waste pipe behind the basin, sealed by a small nut and washer. As those seals wear, water drips from the pivot whenever the basin drains. Tightening the pivot nut slightly sometimes stops it; if not, the seal or the whole waste needs replacing. Push-button click wastes do not have the rod, but their seals can fail in the same way at the fitting under the basin.

Then check the overflow. Most basins have an overflow slot near the top of the bowl that drains through a channel inside the ceramic into the waste. Fill the basin until water runs into the overflow and watch underneath. If the leak appears only then, the problem is the seal where the overflow channel meets the waste fitting. Our guide to a sink overflow leaking or smelling covers this in more detail, including why a basin overflow can start to smell.

Make sure the water is not coming from above. Water that gets past a worn O-ring at the base of the tap, seeps under the tap where it sits on the basin, or runs over the edge of a countertop basin and down the side of the vanity can drip into the cupboard and look exactly like a pipe leak. Splash water around the base of the tap and along the back edge of the basin with the taps off, and see whether it appears underneath. A tap that leaks from its base or drips from the spout needs its own repair, which our guide on how to stop a dripping tap explains. Check the basin itself for a hairline crack too, especially around the waste hole and the tap hole.

Look for longer-term damage. A vanity unit base made of chipboard swells and crumbles when it has been wet for a while, and a swollen base means the leak has been going for weeks, not days. Check the floor around a pedestal for lifting vinyl, loose tiles or dark grout. On an upper floor, check the ceiling below for staining. A slow basin leak in a Chelsea mansion flat can go unnoticed for months if it drips into a boxed-in space, and the first sign may be a stain in the flat below. Our guides to a leak in the ceiling below a bathroom and signs of a hidden water leak explain what to look for.

Know what is urgent and what can wait. A supply leak is urgent because it never stops, and a swollen braided connector can burst. Turn it off at the isolation valves and use another basin until it is fixed. A drip from the trap only while the basin drains is not an emergency, but it should be fixed within days, because it will ruin a vanity base or the floor underneath. A leak you cannot find, or one that comes back after tightening, needs a plumber to look at it properly.

If water is leaking under your bathroom sink and you cannot stop it, or the connectors or valves need replacing, call Emergency Plumber Chelsea on 020 3002 6667. Our bathroom plumbing service replaces tap connectors, isolation valves, traps and basin wastes, and our leaking tap repair service deals with dripping and leaking basin taps, across Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham. If the basin is draining slowly as well as leaking, the trap may be partly blocked, which our blocked sinks service clears. There is no call-out fee on jobs over 3 hours.

Quick tips

• Dry everything and lay kitchen roll underneath so you can see exactly where drops land.
• Wet with the taps off means a supply leak; wet only after draining means a waste leak.
• A bulging, rusty or frayed braided tap connector can burst: isolate it and have it replaced.
• Fill the basin to the overflow to check the overflow seal, which only leaks when filled high.
• Splash round the tap base and basin edge to rule out water coming from above.

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