If you find a leak under the sink, clear the cupboard and isolate the water at the isolation valves or stop tap, then dry everything and watch which fitting wets first. Most under-sink leaks come from the trap or U-bend, a loose compression nut, a worn washer or the waste seal, and many stop when you tighten or replace the part. A leak that keeps returning, or wets the floor and units, points to a hidden leak that needs tracing.
Opening the cupboard to a pool of water under the sink is never a good start to the day, but it is one of the more fixable plumbing problems. The space under a kitchen or bathroom sink is crowded with joints, and any one of them can weep. Once you have isolated the water and dried the area, you can usually narrow the source down in minutes. This guide covers what to do first, the common culprits, how to tell which one is leaking, the simple fixes, and when a hidden leak needs a professional.
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What to do right now
Before you hunt for the source, make the area safe and stop the water.
- Clear the cupboard and dry it out. Take everything out from under the sink and wipe the base and pipes dry. A dry surface makes the leak far easier to spot, and it protects whatever was stored under there.
- Isolate the water. Many taps have small isolation valves (service valves) on the supply pipes: turn the slot a quarter turn with a screwdriver so it sits across the pipe to shut that supply off. If there are none, turn off your internal stop tap, which is often under the kitchen sink itself. Turn it clockwise, slowly, and do not force it, as both WaterSafe and Thames Water advise. Not sure where yours is? Our guide on how to find the shut-off valve for water walks you through it.
- Mind any electrics. Waste disposal units, dishwashers and sockets sit close to the pipework. If water has reached anything electrical, do not touch it. Switch off the electricity at the consumer unit (the fuse box) if it is safe to do so. Electrical Safety First advises keeping the power off until a qualified electrician has confirmed it is safe.
Common sources of an under-sink leak
The pipework under a sink falls into two camps: the clean water coming in under pressure, and the waste water draining out. Knowing which group a fitting belongs to is the key to finding the leak. Here are the usual suspects.
| Source | What is going on |
|---|---|
| Trap or U-bend seal | The curved trap below the plughole holds a little water to block drain smells. Its slip-nut seals can loosen or the washer inside can perish, so it weeps when water drains through. A very common cause of a leaking pipe under the sink. |
| Compression or supply nut | The nuts joining the supply pipes to the tap or a valve can work loose, or their washer hardens. Because the supply is pressurised, this kind of leak tends to drip steadily, not only when you use the tap. |
| Tap tail connection | The flexible tap tails, the braided hoses that feed the tap, connect up under the basin. A loose connection or a worn seal at either end lets water seep down the pipe and pool below. |
| Waste connector | Where the waste pipe meets the underside of the sink, a rubber or fibre seal keeps it watertight. If that seal fails or the locking nut loosens, water escapes here when the sink drains. |
| Sealant around the basin | The silicone bead around the rim keeps splashes from running behind the sink. When it cracks or lifts, water can creep underneath and show up in the cupboard, mimicking a pipe leak. |
| Appliance hose | A dishwasher or washing machine plumbed in near the sink shares the waste and sometimes the supply. A perished hose or a loose jubilee clip can leak into the same cupboard and look like a sink problem. |
Worn or hardened rubber washers and loose coupling nuts sit behind a large share of these, which is why trades guidance from sources such as Checkatrade and Wickes often starts with tightening the nut or replacing the washer.
How to tell where it is coming from
Dry everything first, then make the water move and watch. With the cupboard empty and every pipe wiped dry, work through this:
- Run the cold tap, then the hot. Watch the supply pipes and tap connections. A drip that appears while the tap runs, or sits there constantly, points to the pressurised supply side: a compression nut, a tap tail or a valve.
- Fill the sink and let it drain. Now watch the trap, U-bend and waste connector. A leak that only shows when the water drains away points to the waste side rather than the supply.
- Feel along each joint. Run a dry tissue around each nut and seal. The fitting that comes away wet, or the highest wet point on a pipe, is usually the source, since water runs downwards from where it escapes.
- Check the basin edge. If the pipes stay dry but water still appears, press around the sealant at the rim. Splashes getting behind a failed seal look exactly like a pipe leak.
If you dry everything and no fitting wets, yet water keeps returning, the source may not be under the sink at all. A leak behind the units or under the floor can track along pipework and surface in the nearest cupboard. That is where guesswork stops being useful and tracing the leak pays off.
DIY fixes that often work
Plenty of under-sink leaks are within reach once you know which fitting is at fault.
- Tighten the nut. A weeping trap or compression joint often stops when you hand-tighten the slip nut, then nip it gently with a wrench. Do not overtighten plastic nuts, as they crack.
- Replace a worn washer. If tightening does not work, undo the joint and check the rubber washer inside. Swap a hard, flattened or split washer for a new one of the same size.
- Reseat the trap. A trap that has been knocked or cross-threaded can leak. Unscrew it, check the threads and the seal, then refit it square and snug.
- Renew the waste seal. Where the waste meets the sink, a fresh rubber seal and a properly tightened nut usually cures a drip on the waste side.
- Re-seal the basin. If the sealant is at fault rather than a pipe, strip the old silicone, let it dry, and run a clean new bead around the rim.
- Wrap threads with PTFE tape. On threaded metal joints, a few turns of PTFE tape can improve the seal when you remake the joint.
Turn the water back on slowly afterwards, watch the joint for a few minutes, then check again the next day. A leak that returns, or one you cannot trace to a visible fitting, needs more than a spanner.
When to call a professional
Some under-sink leaks are beyond a quick fix, and chasing them can do more harm than good. Call a professional when:
- the leak continues after you have tightened and resealed the visible fittings;
- water keeps appearing but no joint is obviously wet, suggesting the source is hidden;
- the cupboard base, kickboard or the floor around the units stays damp;
- there is a musty smell, swollen chipboard or staining that points to long-term, hidden water;
- water is anywhere near sockets, an under-sink appliance or wiring.
A persistent leak with no visible source usually means water is escaping behind the units or under the floor and travelling to the sink cupboard. This is what non-invasive leak detection is for: a specialist combines thermal imaging, acoustic listening, tracer gas and moisture meters to pinpoint a hidden leak, so any opening up is aimed at one small spot rather than a torn-out kitchen. Our plumbing leak detection service is built for hidden pipe leaks like these, and our wider water leak detection page explains how the methods work together.
On cost, many home policies include trace and access cover, which pays to find a hidden leak and make good the damage caused by reaching it. Limits typically range from five thousand to ten thousand pounds, though it does not usually pay to repair the pipe itself, and cover varies by policy, so check yours and keep photographs as evidence. Catching an under-sink leak early keeps the damage to your units and floor small.
Frequently asked questions
Why is there water under my kitchen sink?
Water under the sink usually comes from one of a few places: the trap or U-bend seal, a loose compression nut on the supply or waste pipe, a perished washer, the tap tail connections, the waste where it joins the sink, or an appliance hose. Dry everything, then watch to see which fitting wets first.
How do I find where the leak under my sink is coming from?
Dry the whole area, then run the tap and use the plughole while you watch each joint. A leak that only appears when the water drains points to the trap or waste. A constant drip points to the supply pipes or tap connections. Feel along each fitting to find the wet spot.
Can I fix a leaking pipe under the sink myself?
Often, yes. Many under-sink drips stop when you hand-tighten the compression nut on the trap or replace a hardened washer. Reseating a cross-threaded trap or renewing the waste seal is usually within reach too. If the leak continues, or it is not coming from a visible fitting, call a professional.
Why does the leak under my sink only appear sometimes?
An intermittent leak usually escapes only under certain conditions. A waste or trap leak shows only when water drains through it. A leak behind the units or in the floor may track along pipework and surface near the sink, even though the source is elsewhere. A leak that comes and goes is worth tracing properly.
When should I call a leak detection specialist for an under-sink leak?
Call a specialist when the leak continues after you have tightened and resealed the visible fittings, when water keeps appearing but no joint is obviously wet, or when the cupboard base, kickboard or floor stays damp. These point to a hidden leak that non-invasive detection can pinpoint without ripping the kitchen out.
Leak under the sink you cannot pin down? We will find it
If the drip keeps coming back or the cupboard stays damp with no clear source, we trace hidden leaks across Cornwall and Devon with thermal imaging, acoustic detection and tracer gas. Fast response, minimal damage, and the insurer-ready report if you need to claim.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)