Leak under the kitchen sink: finding the source

First, stop any damage

If water is spraying or running rather than dripping, turn off the supply before you do anything else. Under most kitchen sinks there are two small isolation valves, one on the hot pipe and one on the cold, with a slot you turn a quarter turn with a flat screwdriver. Slot across the pipe means off. If there are none, or the leak is before them, turn off the stop tap. In many London homes that is under the kitchen sink too. Our guide to finding and labelling water isolation valves shows where else to look.

Empty the cupboard and mop out the base. Kitchen cupboard bases are usually chipboard, which swells and crumbles once it has been wet for a while, so the sooner it dries the better. If water has gone behind the units or under the floor, tell us when you call, because it may need to be dried before anything is refitted.

The dry paper test

Once everything is dry, wrap a piece of kitchen roll around each joint in turn, or lay a sheet flat under the pipework, and leave it for ten minutes without using the sink. A damp patch that appears while nothing is running is a supply leak. Then run the tap into the sink with the plug out, and fill and drain the bowl a couple of times. A patch that appears only now is on the waste side.

Run a hand gently along the pipes from the top down. Water runs downhill and along the underside of pipes, so the place it drips from is often lower than the place it is leaking. Check the underside of the sink itself and the back wall of the cupboard as well.

Supply-side leaks

Most modern kitchen taps are connected with flexible braided hoses. They are convenient, but the stainless steel braid can corrode in a damp cupboard, especially if cleaning products are stored next to them, and the rubber tube inside can split with age. A hose that is rusty, frayed, kinked or bulging needs replacing, even if it has not leaked yet, because when these hoses fail they can let go all at once. Our guide to a leaking flexible tap hose covers what the replacement has to match.

Compression joints and isolation valves can weep from the nut or the spindle, often after someone has knocked them while reaching into the cupboard. A gentle nip with a spanner sometimes stops a joint weeping, but overtightening crushes the olive and makes it worse. Push-fit joints that leak usually need remaking rather than pushing in harder. Why does the same push-fit pipe joint keep leaking? Our guide explains what is usually wrong.

If the leak is from a pipe coming out of the wall or floor, or you cannot find a wet joint at all but the meter keeps moving with everything turned off, the leak may be behind the units. Our guide to a water meter moving when the taps are off explains how to check.

Waste-side leaks

Waste leaks are low pressure and only happen when water is draining, so they are less urgent, but they are dirty water and they soak the cupboard base a little more each time. Plastic trap nuts loosen with use, washers go missing when a trap has been taken off to clear it, and older traps crack.

Hand-tighten each nut on the trap and waste. Do not use a wrench on plastic fittings. If the leak is at the joint where the waste goes into the wall, or the trap is cracked, it needs a new part. If the sink is also slow to drain, a partial blockage can push water out of joints that would otherwise be fine; our guide on how to unblock a sink covers that.

A leak that only appears when the bowl is filled high is usually the overflow. That is covered in our guide to a kitchen sink overflow leaking into the cupboard.

The tap, the worktop and appliances

Not every leak under the sink starts there. Water that runs down the back of the cupboard after washing up is often getting past failed sealant around the sink rim or under the tap. Water that appears only when the tap is used, and runs down the tap body, points to worn seals in the spout; our guide to a kitchen tap leaking around the swivel spout explains the repair. A loose kitchen tap that moves when you use it can also let water past its base.

If the dishwasher or washing machine connects under the sink, check the waste hose where it joins the sink trap and the supply hose on the valve. A leak that appears only during a cycle is almost always one of these. A pull-out kitchen tap hose leaking or failing to retract is another source inside the cupboard.

When to call a plumber

Call straight away if water is spraying or running, if you cannot turn it off at the isolation valves, if a braided hose is visibly corroded or bulging, or if water is going under the units or through to a room below. It is also worth calling if the leak is from a pipe in the wall, or the cupboard is wet but nothing you can see is dripping.

Smaller weeping joints and waste leaks can wait for a booked visit, as long as you keep a bowl under them and the supply side is off if it is a supply leak. If the dishwasher on the same waste is also leaving water behind, see our guide to a dishwasher not draining.

If water has reached sockets, a plug-in water heater or appliance wiring under the sink, switch the circuit off at the consumer unit before touching anything.

What an engineer will do

An engineer will isolate the supply, dry the area and test each joint under pressure to find the one that is failing. Worn hoses, valves and olives are replaced, traps and wastes are remade or renewed, and failed sealant is cut out and replaced. If the leak is behind the units or in the wall, they will trace it before opening anything up, so only the section that needs to come out is disturbed. Our guide to repairing pipework behind fitted units explains how that access is planned.

Where the cupboard base has swollen or the floor is wet, they will tell you what needs drying before anything is refitted.

Get a kitchen leak fixed

Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. We fix leaking kitchen taps, hoses, valves and wastes and trace hidden leaks behind units. Call 020 4652 8585 and tell us whether the leak drips all the time or only when the sink is used.

Our call-out charge is £60, and the engineer explains what they have found and what the repair involves before starting. You can read more on our leak detection and repair page and our kitchen plumbing page.