Leak Under the Sink: How to Find and Fix It
Find where a leak under your kitchen or bathroom sink is coming from, stop it safely, make simple repairs and know when to call a plumber.
You open the cupboard under the sink and find a damp base, a puddle or a faint musty smell. Leaks here are very common, and many of them are simple, but damp can ruin a unit quickly, so it is worth acting the same day.
Stop the water making it worse
Before you search for the source, limit the damage.
- Turn off the sink's isolation valves, the small taps on the pipes beneath, or shut the main stopcock. See finding your stopcock.
- Empty the cupboard and mop up.
- Put a bowl or container under the leak.
Work out where the water is coming from
Dry everything with kitchen roll or a cloth. Then run water in stages and watch which part gets wet first.
Tap connections
If the leak appears only when the tap runs, check the flexible hoses (tap tails) and the nuts at each end. These are a frequent culprit, particularly after a tap has been replaced.
Waste pipe and trap
Run the plug-out sink and watch the waste. If water drips from the U-shaped trap or its joints, it is likely a loose compression nut or a worn washer. Put a piece of dry tissue paper around each joint, as it shows damp quickly.
Plug and overflow
A leaking plug waste or overflow shows as dripping from the bottom of the sink. Fill the basin and let it overflow slightly to check.
The sink itself
Cracks, a failed seal around the rim, or a loose tap fixing can let water run down. Water running behind the tap or around the edge points here.
Dishwasher or washing machine hoses
Sometimes the leak is not the sink at all. Check the connections of any appliance plumbed beneath it.
Simple fixes you can try
- Tighten loose nuts by hand or a quarter turn with a spanner. Do not over-tighten plastic.
- Replace a worn washer or seal. Pull the trap apart over a bowl, clean it, and fit new rubber washers.
- Replace flexible tap tails. They are inexpensive and easy to match, but make sure the water is off.
- Refit the trap correctly. Make sure threads are straight and seated before tightening.
- Silicone the sink rim. If water seeps around the edge, a fresh bead of sealant may sort it, once the old one is cleaned out.
Clean, dry and test after each change by running the water for a few minutes. Wait and recheck, because a slow weep may take time to appear.
Signs it is a bigger problem
Get a plumber if:
- the leak continues after you have tightened and replaced parts;
- you see corroded pipes or green deposits around copper;
- the wall or floor is wet or stained;
- there is a persistent damp smell or mould;
- you cannot shut off the water supply.
For typical costs, see how much a plumber costs.
Protect the cupboard
Chipboard units swell when wet and do not recover. Dry thoroughly, leave the doors open for a day or two, and use a dehumidifier or fan if the base is damp. Replace a ruined base panel rather than hoping.
Preventing repeat leaks
- Check under the sink every few months with a torch.
- Avoid storing heavy items that could knock pipes.
- Fit isolation valves if you lack them, so future repairs are quick.
- Use sink strainers to avoid food scraps blocking the trap. A blocked trap can force water out of joints, which is when you may need our guide on unblocking a kitchen sink.
Worked example: the slow drip under the bathroom basin
A tenant spots a darkened patch on the base of the vanity unit. Drying everything with kitchen roll, then running the cold tap for a minute, shows nothing. Running the hot tap, a bead of water forms at the flexible hose nut. A quarter turn with a spanner stops it for a day, but it returns, so they replace the flexible hose with a matching length. Total cost is a few pounds in parts. The unit base was swollen, so they dry it and leave doors open for two days. The key was finding the exact point rather than guessing.
The dry-tissue method
- Dry every pipe and joint.
- Wrap a strip of dry tissue paper around each joint and connection.
- Run only the cold tap for a few minutes, then check.
- Run the hot tap, then check again.
- Fill the basin or sink, then release the plug and check the waste.
- Note which tissue goes damp first.
Common culprits at a glance
| Where it drips | Typical cause |
|---|---|
| Under the tap body | Hose nut or tail washer |
| At the trap bottom | Loose or cracked trap |
| At the waste outlet under the plug | Perished seal or loose nut |
| At the wall joint | Isolation valve or pipe compression joint |
| Only during appliance cycles | Appliance hose or waste connection |
Special cases
- Rentals. Report the leak in writing to the landlord or agent, and avoid structural repairs. See plumbing in a rental.
- Flats. If the leak seems to come from above or through a wall, tell your neighbour and managing agent before assuming it is yours.
- Old lead or galvanised pipe. Do not try to fix or bend it. Call a plumber.
Insurance and records
Take dated photographs of the damage and keep receipts for any repairs or drying equipment. If you are making a claim, your insurer will usually ask for them. Check your policy before starting work, because some require approval first.
Frequently asked questions
Is a small leak under the sink urgent?
Fix it promptly. A slow leak can rot the unit and encourage mould long before you see significant water.
Can plumber's tape fix a leaking waste pipe?
PTFE tape is for threaded pipe joints, not for waste pipe compression joints. Use the correct washer or fitting instead.
Why does it only leak when I use the dishwasher?
The dishwasher drain hose may be loose or share a connection with the sink waste. Check the clip and the spigot on the trap.