The candidates are braided tap tails, waste joints, the sink-to-worktop seal, isolation valves, appliance hoses and the tap body. Dry everything, then run each in turn and watch – it is the only reliable method.
The six candidates
| Source | How it presents |
|---|---|
| Braided flexible tap tails | Weeping at the crimped ends, or a sudden catastrophic failure. The single most common cause of serious kitchen water damage |
| Waste trap and compression joints | Slow drip, worse when the sink is drained. Seals harden, or the joint was cross-threaded at installation |
| Sink-to-worktop seal | Water around the rim, soaking the chipboard from above rather than below. The cabinet base may be dry while the worktop swells |
| Isolation valves | Weeping from the spindle, often after being operated for the first time in years |
| Appliance hoses | Washing machine or dishwasher inlet and outlet. Frequently the last thing anyone checks |
| Tap body or back-nut | Water running down the tap tails from above, which looks exactly like a tail failure |
A braided flexible connector has a rubber inner hose inside a steel braid, and the inner hose perishes with age. When it fails it is not a drip – it is full mains pressure, continuously, until somebody turns the water off. If yours are old, discoloured or you have no idea how old they are, replacing them is cheap insurance.
Finding it properly
The method is unglamorous and it is the only one that works reliably.
- Empty the cupboard completelyEverything out. You need to see the base and all four corners.
- Dry everythingThoroughly – kitchen roll on every joint, the base, and the underside of the sink. You cannot see a new drip against existing water.
- Put dry kitchen roll under each jointIndividually. Under each tap tail, under the trap, under each waste joint, under each isolation valve. Paper shows a drip long before you would see it on a wet surface.
- Wait, without using anythingFifteen minutes. If paper gets wet now, it is a supply-side leak – a tail, a valve or the tap – because nothing has been drained.
- Run the cold tapThirty seconds. Check every piece of paper.
- Run the hot tapSame. Distinguishing hot from cold narrows it usefully.
- Fill the bowl and drain itThis loads the waste properly. A trap that does not drip on a trickle will often drip on a full bowl.
- Run each applianceWashing machine and dishwasher, through a cycle if you can.
- Wet the worktop around the sink rimAnd watch the underside of the sink cutout. This catches the seal failure that everything else misses.
Between those steps, you will have identified it. If nothing produces a drip, the water is coming from somewhere else and running to the low point under the sink – which is a different investigation.
What the damage tells you
The state of the cabinet is a timeline.
- Damp but sound chipboard – recent, days to weeks.
- Swollen, dark, crumbling edges – months. The board has been wet repeatedly and has broken down.
- Mould on the back wall or under the base – long-standing, and there may be more behind the unit.
- A silicone-sealed ‘repair’ somebody has done – the leak predates you, and silicone over a failing joint does not fix it.
- Water staining on the floor beyond the cabinet – it has been substantial, and the floor may need checking.
- A swollen worktop around the sink rim – the seal, not the plumbing underneath.
A slow leak under a sink can do more cumulative damage over a year than a burst does in an hour, precisely because nobody sees it.
The fixes
- Braided tails – replace, do not attempt to repair. Use good quality ones, the correct length, not pulled taut and not kinked. Replace both while you are there.
- Compression waste joints – dismantle, clean the seal faces, fit a new washer, reassemble hand-tight plus a little. Overtightening plastic waste fittings distorts them and makes the leak worse.
- Trap – if it is old and brittle, replace it rather than reseal it. They are inexpensive.
- Sink seal – rake out the old silicone completely, clean and dry the surfaces properly, and reseal. Silicone on a damp or dirty surface will not bond and you will be doing it again.
- Isolation valves – if a valve weeps after being operated, it usually needs replacing rather than adjusting. Cheap valves are a false economy here.
- Appliance hoses – replace, and check the aqua-stop if fitted.
- Tap back-nut – tighten carefully, or if the tap body itself is leaking, the tap needs attention. See taps.
While the cupboard is empty
- Fit isolation valves if there are none, on both tap tails. It turns every future job from a drain-down into five minutes.
- Replace elderly braided hoses whether or not they are the culprit.
- Check the appliance hoses and their connections.
- Dry the cabinet out properly before putting anything back – a fan heater aimed into the cupboard for a day, or at minimum the door left open.
- Consider a leak alarm. A battery water sensor under the sink costs very little and tells you the next time, immediately.
- Replace a failed cabinet base rather than covering it. Wet chipboard does not recover.
Cabinet base swollen and dark? That has been going on for months. Worth finding it properly rather than guessing.
Questions we get asked about this
How do I find a leak under the sink?
Dry everything, put kitchen roll under every joint individually, then run each thing in turn – cold tap, hot tap, full bowl drained, appliances – checking the paper after each. It identifies the source reliably.
My flexible tap hoses look old. Should I replace them?
Yes. The rubber inner perishes with age and when it fails it releases full mains pressure continuously. They are inexpensive and it is the most cost-effective preventative job under a sink.
Water is on the cabinet base but everything looks dry.
Check the sink-to-worktop seal – water can run around the rim and down inside the cutout, bypassing everything underneath. Wet the worktop around the rim and watch from below.
Can I just put more silicone on it?
Only if the problem is genuinely the seal, and only after removing the old silicone completely and drying the surfaces. Silicone over a failing joint or onto a damp surface does not hold.
The waste keeps leaking after I tighten it.
Plastic compression waste fittings are hand-tight plus a little. Overtightening distorts the seal and the olive, and makes it leak worse. Dismantle, clean, new washer, reassemble gently.
Is a slow drip urgent?
Not an emergency, but do not leave it indefinitely. A slow leak destroys a cabinet base, gets into the floor, and can do more cumulative damage over a year than a burst does in an hour.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.