Do this first
- Empty the cupboard. Everything out, including whatever is already wet. You cannot diagnose a leak around bottles of cleaner.
- Dry every pipe and fitting with a cloth, then lay white paper across the base. The pattern on the paper is the diagnosis.
- Do nothing with the water for twenty minutes, then look. Wet paper with the taps untouched means a pressurised supply leak. Dry paper means the fault is on the waste side.
- Then run the tap for a minute and look again. New water that arrives while the sink drains and stops afterwards is the trap or the waste connection.
- Isolate what you can. The small valves on the tap tails close with a quarter turn of a screwdriver. If the supply side is the culprit and there are no valves, use the stopcock.
- Check the base of the unit and the floor under it. Chipboard carcasses swell and go soft, and the floor below is often wetter than the cupboard.
- Do not put a bucket under it and leave it. A supply-side leak in a kitchen at mains pressure gets worse, not better.
Supply side or waste side
This is the only fork that matters, and the paper test settles it in twenty minutes. Everything else follows from it.
| When it wets | What it is | Urgency |
|---|---|---|
| Constantly, nothing running | Supply: a flexible tail, an isolation valve, a compression joint or the tap back-nut | Isolate now. It runs while the flat is empty. |
| Only as the sink drains, then dries out | Waste: the trap connections, the sink outlet or the waste branch to the stack | Usable with care until repaired |
| Only when the dishwasher or washing machine pumps out | The appliance waste spigot on the trap, or the standpipe behind it | Stop using the appliance |
| Around the rim of the sink itself, on the worktop side | Failed sealant at the sink or tap, water getting in from above | Reseal |
| Rust-coloured staining on a braided tail, no drip yet | A flexible tail near the end of its life | Replace before it fails |
The braided flexible tail deserves a paragraph on its own. It is the single most common cause of serious kitchen and bathroom floods in London flats, because it is at full mains pressure around the clock, it has a finite service life, and when it goes it does not drip, it sprays. If the braiding is rusting anywhere along its length, replace it whether or not it is leaking today. The same applies to the tails behind a basin: see a dripping tap for the tap-side faults that look similar from underneath.
What the repair involves
Supply-side work is isolate, replace the failed tail or valve or remake the joint, then test under full pressure and leave the paper down for an hour before signing it off. Waste-side work means stripping the trap, cleaning the seals and faces, renewing washers, and correcting the geometry, because a large share of under-sink waste leaks are caused by a trap that has been forced to line up with a badly sited waste branch and is being held together by strain. Fixing the alignment is what stops it recurring.
Then the cupboard and the floor. Swollen carcass bases, delaminated plinths and a wet floor under the unit all get dealt with here rather than handed on, along with the pipework itself, under pipework repair and replacement. If water has already gone through to a room or a flat below, the stain downstairs is the next thing to look at.
When it is not us
- The trap nut was finger-loose. Hand-tighten it and dry the area. Plastic trap nuts are meant to be hand-tight, not spannered.
- The washing machine hose clip had worked loose. Tighten the clip. That is the whole repair.
- Water is coming from the appliance itself rather than the plumbing. See a leaking washing machine or a leaking dishwasher: an appliance engineer handles the machine, we handle everything it connects to.
- The sink seal has failed and water is getting in from the worktop. That is a reseal, and it is a job a competent homeowner can do with a tube of sanitary silicone.
- A blocked waste that only leaks because it is backing up. Clear the blockage first; the joint may be fine.
- You rent. Report it in writing. Installations for the supply of water and for sanitation are the landlord's to keep in repair.