Water in the cupboard under a sink usually comes from a leak in the plumbing below, such as the waste pipe and trap, the hoses or pipes feeding the tap, or an appliance connection. It can also come from above, where water gets past the seal around the sink or the tap and drips down inside the cupboard.
Because several parts sit close together, the water is often found some distance from where it starts. A few simple checks can help narrow it down before a repair is planned.
Leaks From the Waste
The waste pipe and trap carry water away from the sink. Their joints rely on washers and seals that can wear or loosen, especially if the cupboard is used for storage and the pipes get knocked. A waste leak usually shows only when the sink is draining, not all the time.
If a dishwasher or washing machine drains into the same waste, the leak may appear only when the appliance empties.
Leaks From the Water Supply
Taps are usually fed by flexible hoses or copper pipes with isolating valves. A loose connection, a worn washer or a damaged hose can leak at any time, even when the tap is not being used, because the supply is under pressure.
Older flexible hoses can also bulge, kink or corrode at their ends, which may lead to a leak without much warning.
This kind of leak can be slow and steady, so water may collect quietly on the cupboard floor for some time before it is noticed.
Water Getting In From Above
Water splashed on the worktop can find its way past worn sealant around the sink, or past the seal under the tap. It may then run down the underside of the sink and drip into the cupboard.
An overflow on some sinks has its own connection to the waste. If this works loose, water may escape when the sink is full.
Moisture in a cupboard does not prove where the leak starts. Condensation on cold pipes can also cause drips, especially in a warm, steamy kitchen.
Narrowing It Down
Empty the cupboard, dry everything and place a dry towel or paper on the floor of the unit. Then check whether water appears when the tap runs, when the sink drains, when an appliance empties or all the time.
If the leak is steady and the isolating valves turn easily, closing them may stop it while you get help. Avoid taking apart pipes that are under pressure. If the tap itself seems to be leaking, our leaking tap repair page covers that in more detail.
Our page on sink repair in Poole explains how leaks from sinks and their plumbing are traced and repaired.
If you keep finding water under your sink, a plumber can trace the source and explain the repair options.