Leaking Pipe Under the Sink: Find the Source and Stop It

First, stop the water

If water is running rather than dripping, turn it off before you do anything else. Most kitchen sinks have two small isolating valves on the pipes under the sink, one on the hot and one on the cold. A quarter turn with a flat screwdriver, so the slot sits across the pipe, closes them. If there are no isolating valves, or they will not turn, use the main stopcock, which in most London homes is under the kitchen sink anyway. If it is not there, our where is my stopcock guide covers the other usual places.

If the leak is on the hot side and you have a hot water cylinder, turning off the cold mains also stops the flow to the cylinder, so the hot pipe will soon stop too. Put a bowl or tray under the leak and take everything out of the cupboard so you can see.

If water has reached sockets, a plug-in water softener or the kitchen spotlights below, switch that circuit off at the consumer unit before you touch anything.

Supply or waste? The two-minute test

Dry every pipe, joint and the cupboard floor. Lay a sheet of dry kitchen roll under each joint. Then leave the taps off for ten minutes. A wet sheet means a supply leak, because only the supply pipes are under pressure when nothing is running.

If everything stays dry, fill the sink with water, pull the plug and watch the waste. A drip that appears only as the sink drains is a waste leak: a loose trap nut, a perished washer on the plug fitting or a split in the waste pipe. If both stay dry, run the dishwasher or washing machine, because their hoses and the valves they connect to are the next suspects.

• Wet with the taps off: supply side, under pressure, and it will not stop on its own.
• Wet only while the sink drains: waste side, low pressure, and it stops when the sink is not in use.
• Wet only when an appliance runs: its fill hose, its valve, or its drain hose where it joins the waste.
• Wet around the base of the tap on the worktop and running down: the tap itself, not the pipes under it.

Leaking compression joints and olives

Most copper pipework under a sink is joined with compression fittings: a nut that squeezes a brass ring, the olive, onto the pipe. When the olive was not seated square, the nut was never tightened fully, or the pipe has been knocked by things stored in the cupboard, the joint starts to weep. You will usually see a bead of water forming on the bottom of the nut, sometimes with a green or white crust around it.

A weeping compression joint can sometimes be stopped by nipping the nut up by about a quarter of a turn, holding the fitting body still with a second spanner so you do not twist the pipe. Do not keep tightening: an over-tightened olive distorts and leaks worse. If a quarter turn has not stopped it, the joint needs remaking with a new olive, which means isolating the water and draining that pipe.

Push-fit joints on plastic pipe leak for different reasons. The pipe was not pushed fully home, the insert was left out, or the pipe end was cut at an angle and the seal is not sitting on it properly. These do not respond to tightening. The pipe has to come out, be cut square and go back in properly. Our comparison of copper vs plastic pipes explains why each one fails the way it does.

Isolating valves and flexible tap connectors

The small isolating valves under the sink are a common leak point. They usually weep around the slotted spindle after someone has turned them for the first time in years, or at their compression nuts. Our guide to an isolation valve leaking covers that in detail, and if it is the main valve itself that is dripping, see our guide to a stopcock leaking.

Modern mixer taps are fed by braided flexible connectors screwed into the underside of the tap. These are cheap and handy, but they do not last forever. A connector that is corroded, kinked or has rust coloured staining on the braid can fail without warning and let go at full mains pressure, which is how a slow drip turns into a flooded kitchen. If a connector is weeping, replace it rather than tightening it, and replace the other one while the water is off.

Waste pipe and trap leaks

Waste leaks are low pressure and usually easy. The plastic nuts on the trap work loose as the sink is used and cupboards are packed. Hand-tighten them, and check the rubber washers inside are sitting flat and not split. If the leak is where the plug fitting passes through the sink, the seal under the sink bowl has failed and the waste fitting needs taking out and resealing.

A waste that only leaks when the sink is full and drained quickly may also be partly blocked, backing water up to a joint that is not meant to hold it. Clear the trap before replacing parts.

When it is not the pipe at all

Water running down the outside of the pipes can come from above. A mixer tap leaking from its base on the worktop, a split in the sealant around the sink, or water getting under a worktop edge all end up under the sink looking like a pipe leak. Dry the worktop around the tap, run it, and wipe underneath with a finger to see where it first appears.

On the hot pipe only, check whether the drip gets worse when the hot water has just heated up. Expansion and contraction loosen joints on the hot side more than the cold, so a hot pipe joint that weeps only in the morning is still a joint that needs remaking.

If the cupboard floor is wet but every visible joint stays dry, the leak may be in the pipe as it runs through the wall or under the floor. That needs finding properly rather than guessing, and our leak detection and repair service is the right starting point.

How urgent is it?

A drip under a sink is rarely an emergency on the day, as long as you can turn it off. It becomes urgent when you cannot isolate it, when the cupboard base or kitchen floor is already soft, or when you live above someone and the water is going through. Chipboard cupboard bases and kitchen plinths swell and crumble once they stay wet, and the damage to the floor under the units is often worse than it looks. If you have a flat below you, our guide to what to do if water is coming through the ceiling explains the next steps, and who pays is covered in who is responsible for a leak in a leasehold flat. For a leak that is spreading faster than you can mop, our guide to water escaping in the house covers the first five minutes.

Getting it fixed

Call us on 020 4652 5111 or send an enquiry with a photo of the pipes under the sink and where the water is coming from. Most under-sink leaks are fixed in a single visit: joints remade, isolating valves or tap connectors replaced, and the waste resealed if that was part of it. We check the other joints in the cupboard while we are there, because one that has started to weep is often not the only one. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.