A kitchen sink leaking underneath is usually the trap seal, a loose waste connection, a worn flexi hose, or a dripping isolation valve. Check each in turn with the cabinet dry and a torch. If the cabinet floor is stained, swollen or soft rather than freshly wet, the leak has likely been running for some time and is worth a proper inspection.
In This Guide
Where to start looking
A kitchen sink leaking underneath has a shorter list of suspects than most people expect. Nearly everything under a kitchen sink is a joint, a seal or a hose, and each one can fail on its own without the others being involved. Working through them methodically saves you pulling the whole cabinet apart.
Clear the cabinet out, dry every surface with an old towel, then run the tap and watch. Fill the sink and let it drain too, since a leak that only shows on draining points to the waste side, not the supply side. Watching where the first drop appears tells you almost everything.
The trap and waste pipe
The trap, the curved or bottle-shaped section under the plughole, holds a small amount of water to stop drain smells coming back up. It is usually the first thing to check because it is held together with plastic compression nuts that loosen over time, especially if the cabinet gets knocked around during cleaning.
Check the compression joints
Hand-tighten the nuts at each end of the trap. Do not overtighten plastic fittings, since cracking the nut creates a new leak rather than fixing the old one.
Look at the washer
Inside each compression joint is a rubber or plastic washer. If it has perished or sits crooked, water will weep out slowly even with the nut tight.
Check where the waste meets the sink
The plughole fitting itself seals against the underside of the sink with a rubber gasket or plumber's putty. A failed seal here drips only when water is standing in the sink or draining through it.
Flexi hoses and tap connections
Flexible tap connectors are common in kitchens because they make fitting a new tap or mixer far easier than rigid pipe. They do wear out though, usually from the inside, where the braided outer looks fine but the inner liner has split. A hose failing this way can go from a small drip to a proper spray in a short space of time.
Check both ends of every flexi hose: where it screws to the tap tail and where it screws to the isolation valve or supply pipe. A slightly damp thread at either end, even without an obvious drip, means the seal is starting to go.
Working through the connections methodically
An engineer checking under a sink goes joint by joint, drying and re-checking each one rather than guessing at the source. That same order, trap, waste, hoses, valves, is worth following before you call anyone out.
Isolation valves
Isolation valves sit between the supply pipe and the tap or appliance, letting you shut off water to one point without turning off the whole house. They are also a common source of small drips, usually where the valve body meets the pipe rather than through the valve mechanism itself.
- Compression fitting where the valve meets the pipe: tighten gently or check the olive inside
- Spindle seal where the handle passes through the valve body: a slow weep here often means the internal seal has worn
- Outlet thread where the flexi hose screws on: reseat and check the washer
If you are not confident isolating water yourself, our guide on finding and using your stopcock covers how to shut off the whole supply safely while you work.
Still finding water where it shouldn't be?
If you have checked the trap, hoses and valves and the cabinet is still damp, the leak may be running along a pipe run out of sight. Our engineers can trace it without ripping the cabinet apart.
What the cabinet floor tells you
A wet patch you can wipe up in seconds is one story. Swollen chipboard, a dark stain that has already dried at the edges, or a musty smell when you open the doors is a different story altogether. That points to a leak that has been dripping on and off for weeks or months, not something that started today.
We see this pattern often on older kitchen fits, where a slow weep from a waste joint has been quietly soaking into the cabinet base for a long time before anyone notices the smell. By the time the floor looks damaged, the leak has usually been there longer than the homeowner realises.
Signs it's a simple fix
- Fresh water only, no staining on the cabinet base
- Drip stops as soon as you tighten or reseat a joint
- No smell, no swelling in the chipboard
Signs it needs a closer look
- Staining or swelling on the cabinet floor
- Damp smell even when everything looks dry
- Leak returns within days of tightening a joint
If water has been tracking behind the cabinet into the wall or under adjoining flooring, tightening the visible joint will not stop the damage spreading. Get it checked properly before refitting the cabinet or the kickboard.
When to call in a specialist
Most kitchen sink leaks are genuinely simple: a nut needs tightening, a washer needs replacing, a flexi hose needs swapping. Where it gets harder is when you have fixed every joint you can see and the cabinet is still damp, or the leak seems to move around rather than staying at one point.
That usually means the water is coming from somewhere you cannot see directly, behind the cabinet, under the floor, or along a pipe run hidden in a stud wall. A plumbing leak detection survey uses acoustic and thermal equipment to trace exactly where the water is travelling without pulling the kitchen apart to find out.
Honestly, most call-outs we get for kitchen leaks are straightforward once we are on site. It's the ones that have been ignored for months that turn into bigger jobs, once the water has found its way into the subfloor or a stud wall behind the units.
Fitted kitchens make this worse, since the units, worktop and often a run of tiled splashback all sit tight to the pipework. Pulling a full kitchen apart to chase a leak is expensive and disruptive, so tracing the source accurately before any units come out saves both time and money.
Frequently Asked Questions
This usually points to the dishwasher's own waste connection into the sink pipework, rather than the sink's trap or tap. Check the hose clip where the dishwasher waste joins the sink's waste pipe or an air gap fitting.
For a minor drip you can usually keep using the sink while you fix it, provided you catch the water and check the cabinet regularly. Stop using it if the leak is worsening or water is reaching electrics or a neighbouring wall.
Sealant can mask a leak temporarily but rarely fixes a compression joint or valve for good. It's better to replace the worn washer or olive, since sealant makes the joint harder to service properly next time.
Dry the cabinet, then run the tap without letting water drain, watching for drips near the tap tails and isolation valves. Then let the sink drain fully and watch the trap and waste joints separately for drips.
Not necessarily. A soft or swollen cabinet base usually means the chipboard has absorbed water over time, not that the sink unit has failed. The cabinet base may need replacing, but the sink and tap can often stay.
A leak that returns after tightening often means the actual source is elsewhere, perhaps a pipe run behind the cabinet or under the floor, and what you are seeing is water tracking to the nearest visible joint.
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Don't let a small problem become a big one
If a kitchen sink leak has already left the cabinet swollen or stained, get it traced properly before you refit anything. Our non-invasive surveys find the source without unnecessary damage.