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Leak Under The Kitchen Sink
Water is pooling in the base of the sink cabinet, or the chipboard floor of the unit has gone soft.
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What's happening?
Under a kitchen sink there are two separate systems and identifying which one is leaking halves the diagnosis. The supply side is pressurised — flexible tails, isolating valves, the tap body — and leaks continuously whether or not you are using the sink. The waste side is not pressurised: the trap, the waste pipe and the washing machine or dishwasher hose only leak while water is running through them. A puddle that appears overnight with nothing running is a supply leak. A puddle that appears only when the sink is emptied or the dishwasher discharges is a waste leak.
What usually causes it?
- Failed flexible tap tail — the braided hose splits inside its sleeve and weeps at the crimp
- Isolating valve gland weeping, often after it has been operated for the first time in years
- Loose or cross-threaded trap connection after someone cleared a blockage and did not seat the washer
- Perished sink waste seal or an under-tightened waste flange letting water through the sink cut-out
- Dishwasher or washing machine hose clipped badly, so it discharges partly outside the standpipe
- Blocked trap causing water to back up and escape at the weakest joint rather than draining away
- Split rigid waste pipe or a solvent joint that was never fully made
Is it dangerous?
Not to people, but it is quietly expensive. Kitchen carcasses are chipboard, and chipboard that stays damp swells, delaminates and eventually collapses under the sink. A slow supply leak inside a plinth can run for months, rotting the floor and reaching adjacent units before anyone notices. Where the leak is above an electrical socket, an appliance spur or a boiler, treat it as urgent and isolate the electrics to that area first.
Can I fix it myself?
Empty the cabinet, dry it out and put kitchen roll down so you can see exactly where the water reappears and when. Run the tap, then fill and empty the bowl, then run the dishwasher, checking each time — that sequence usually identifies the culprit in ten minutes. Hand-tighten a loose plastic trap nut, never with grips. Close the isolating valve under a weeping tap tail to stop the flow. Anything past that — replacing tails, re-bedding a waste flange, or a leak you cannot locate — is worth a plumber.
When should you call a plumber?
Call a plumber when you cannot see where the water is coming from, when a joint still weeps after being remade, when the flexible tails need changing in a tight cabinet, or when the cabinet base and floor have already been damaged and need the pipework repositioned. Call urgently if the leak is a pressurised one you cannot isolate, or if water is reaching sockets or an appliance spur.
What will the plumber actually do?
- Dry the cabinet and pressure-test the supply side separately from the waste to prove which system is leaking
- Replace failed flexible tails, isolating valves and the tap back-nut where the seal has gone
- Remake the trap and waste connections with new washers, and re-bed the sink waste flange
- Re-plumb appliance hoses and standpipes so they discharge correctly and are properly clipped
- Clear a blocked trap that is causing water to back up past a joint
- Check the cabinet base and floor for rot and advise what needs replacing before the unit fails
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