The short answer
Most under-sink leaks come from the waste trap, the supply valves, the tap tails, the plughole seal or an appliance hose. Dry everything, then watch which joint wets first while you run the tap and the appliances. If the water keeps returning with no visible source, the pipe behind the units may be the cause.
Opening the cupboard to a damp patch under the kitchen sink is one of those small household moments that can quickly turn into a bigger worry. The good news is that the cupboard under a kitchen sink is one of the easier places to investigate, because almost every connection is on show once you clear out the cleaning bottles and have a proper look. The trick is knowing which fitting is most likely to be the culprit, and how to tell a simple loose joint apart from something more serious behind the units.
Below we walk through the usual sources of an under-sink leak, how to narrow down which one you are dealing with, and what it means when the damp is coming from somewhere you cannot see.
Start by drying everything and watching
Before you touch anything, take everything out of the cupboard and wipe every pipe, joint and the base of the unit completely dry. Lay a sheet of kitchen roll or newspaper across the bottom so any fresh water shows up clearly. Then run the cold tap for a minute, then the hot, and watch closely with a torch. After that, run the dishwasher or washing machine through a cycle if one drains under the sink.
The point of all this is to see which joint wets first, and whether the water only appears when you use a particular tap or appliance. A leak that shows up the moment you run the hot tap points to a different fitting than one that only appears while the dishwasher empties. That timing is often the single most useful clue you can gather.
The waste trap
The waste trap is the U-shaped or bottle-shaped fitting directly below the plughole. It holds a small amount of standing water at all times, which is what stops drain smells coming back up into the kitchen. Because it always has water sitting in it, the trap is one of the most common places for a slow drip to start.
Leaks here usually come from the threaded collars that join the sections together. The washers inside can shift out of place, perish with age, or never quite seal if the trap was overtightened when it was fitted. A trap that only weeps when the sink is full and then draining is often a joint that needs the washer reseating rather than a crack. Hand-tighten the collars first, as overtightening plastic fittings can crack them and make things worse. If the trap itself is cracked or the water keeps returning, it is an inexpensive part to replace.
The supply valves and tap tails
The two small isolation valves under the sink control the hot and cold feeds to the tap. Above them, the flexible hoses known as tap tails run up to the base of the mixer. Unlike the waste pipe, these parts are under mains pressure, so a leak here tends to be more persistent and can spray rather than simply drip.
Run your fingers carefully around each valve body and along the braided hoses while the system is at rest, then again with the tap open. A valve that is wet only at its base may need its compression nut checking. Flexible tap tails have a finite life and can corrode or split inside the braiding, sometimes leaving the outer sleeve looking fine while water seeps through. If a hose feels damp along its length or shows rust spots, it is usually replaced rather than repaired. Because these connections carry pressurised water, it is sensible to know where your stop tap is before you start, so you can shut the supply off quickly if needed.
The plughole seal
Where the strainer or waste fitting meets the bottom of the sink bowl, there is a seal and a layer of sealant holding it watertight. Over the years this can break down, and water then escapes around the top of the trap rather than from a joint lower down. This one can be deceptive, because the water runs down the outside of the waste pipe and collects at the bottom, making it look as though the trap is to blame.
To check, fill the bowl and watch the very top of the waste fitting, right where it passes through the sink, rather than the joints further down. If the leak starts up there and only when the bowl holds water, the plughole seal is the likely cause.
Appliance hoses
If you have a dishwasher or washing machine, its waste hose often connects to a spigot on the side of the trap, and its fill hose may tee off the cold supply nearby. These connections are a frequent source of under-sink water that comes and goes, because they are only wet while the machine is running.
The jubilee clip where the waste hose pushes onto the trap can loosen over time, and the hose itself can crack near the clamp. If your damp patch only appears during or just after a wash cycle, the appliance connection is the first thing to inspect. Tightening a loose clip can be enough; a split hose needs replacing.
Signs the leak may be coming from a hidden pipe
- Damp keeps returning after every joint under the sink has been dried and checked
- A musty smell in the cupboard that does not clear with the doors open
- The base of the unit or the flooring stays wet even when no tap or appliance is running
- Swollen or discoloured chipboard at the back or sides of the cupboard
- Water appearing at the front of the kickboard rather than directly under the pipes
When the damp is from a hidden pipe behind the units
If you have dried and checked every visible fitting and the water still comes back, the source may not be in the cupboard at all. Kitchen pipework often runs behind and beneath the units, buried in the wall or under the floor, and a leak there can travel a surprising distance before it shows up. Water tends to follow the path of least resistance, so the damp patch you see is frequently downhill of the actual fault, not directly below it.
This is exactly the situation where guessing becomes expensive. Pulling out base units and lifting flooring to chase a leak by eye can mean a lot of disruption for what might turn out to be a single weeping joint. It is also where non-invasive tracing earns its place, because the aim is to confirm where the water is escaping before anything is opened up.
How a hidden leak is traced
A typical trace starts by narrowing down the area with acoustic listening equipment. Water escaping a pressurised pipe makes a distinct sound, and sensitive microphones can pick it up through flooring, walls and units to focus on the likely spot. Once the area is narrowed, thermal imaging is often used to confirm it, since a hidden leak changes the surface temperature where moisture is present. Moisture meters then map how far the damp has spread.
Used together, these methods build a clear picture of where the leak is before any unit is removed or any flooring is lifted. That means repairs can be targeted at the right spot rather than working through the kitchen by trial and error. For a slow, hidden leak behind kitchen units, our water leak detection service is designed to pinpoint the source with as little disruption as possible, and you can read about how we cover homes across Devon on our area page. You will find more guides like this in our articles.
What to do right now
If the leak is steady or you cannot tell where it is coming from, turn off the isolation valves under the sink, or the main stop tap, to take the pressure off the pipework. Move anything valuable out of the cupboard, and put a towel or container down to catch drips. Photographing the wet area before you mop it up can help, as the pattern of where the water sits is a useful clue when the cause is not obvious.
A loose collar or a slack hose clip is often a simple fix you can manage yourself. But if the water keeps coming back, the cupboard smells damp, or the floor stays wet with everything turned off, it is worth having the leak traced properly before the moisture has a chance to damage the units or spread further.
Frequently Asked Questions
A waste-pipe leak usually only appears when the sink is full and draining, and the water tends to drip rather than spray. A supply leak is under mains pressure, so it is more persistent and may continue even when no tap is running. Drying everything and noting exactly when the water returns is the quickest way to tell the two apart.
That points to the appliance connection. The waste hose usually clips onto the side of the trap, and the clip can loosen or the hose can crack near the clamp. Because it is only wet during a cycle, the leak comes and goes, which is a strong clue it is the dishwasher or washing machine hose rather than the sink itself.
If every visible joint has been dried and checked and the damp keeps returning, the source may be a pipe hidden behind the units or under the floor. Water can travel before it shows, so the wet patch is not always below the fault. At that point a non-invasive trace will pinpoint it without taking the kitchen apart on guesswork.
Yes. Kitchen pipework runs behind and beneath units and through walls and floors, so a leak elsewhere can show up under the sink simply because that is where the water collects. This is why tracing the actual source matters before any repair, rather than assuming the nearest fitting is to blame.
If the leak is steady or you cannot find the source, it is sensible to close the isolation valves under the sink or the main stop tap to take the pressure off the pipes. Clear the cupboard, put something down to catch drips, and a photo of the wet area before you clean it can help when the cause is not obvious.
We carry out leak detection for homes across Devon, with same-week appointments often available. Give us a call on 07897 027775 or request a free quote and we will arrange a visit to trace the leak and confirm where it is coming from.
Worried about a leak under your kitchen sink?
If the damp keeps coming back or you cannot find where it is coming from, we can trace it accurately and tell you exactly what needs fixing. Same-week appointments often available across Devon.