If the sink and trap are dry but water keeps appearing under the kitchen units, it is usually from a dishwasher or washing machine connection, a pipe in the wall behind the units or a pipe under the floor. A swelling kickboard is often the first clue. Take the plinth off, dry the void and watch where the water arrives first.
You have emptied the cupboard under the sink, run a hand around the trap and the tap tails, and everything is dry. Yet the floor in front of the units is damp again, or the kickboard has started to swell along its bottom edge. The obvious suspect has an alibi. The good news is that a leak under kitchen units with a dry sink comes from a short list of places, and most of them can be narrowed down in an afternoon with a torch, some kitchen roll and a little patience.
Rule the sink out properly first
A dry cupboard is not quite the same as a dry sink. A trap that only weeps while the bowl drains can dry out between uses and look fine when you check it.
- Fill and drain. Fill the bowl, pull the plug and watch the trap and waste connection with a torch while the water rushes through.
- Wrap the joints. Dry kitchen roll around each compression nut shows a single drop within minutes.
- Check the rim. A failed silicone bead where the sink meets the worktop lets splashes run down the back of the cabinet and appear at the floor rather than in the cupboard, which is exactly the pattern people find confusing.
If all of that stays dry, the sink is cleared. Everything from here on is hidden behind something, which is where our water leak detection service starts to become relevant.
The three places the water usually comes from
Kitchens are crowded with pipework. Hot and cold supplies, a heating circuit on its way to a radiator, appliance feeds, waste runs and sometimes the incoming mains all share the same run of units. When the sink is dry, the water is almost always from one of these.
1. Dishwasher and washing machine connections
The most common culprit by a distance. Both appliances are fed through a flexible hose from a small valve on the cold supply, and both drain through a hose pushed into a standpipe or a spigot on the sink trap. Every one of those connections can weep. The inlet washer hardens, the jubilee clip on the drain hose loosens, the standpipe overflows if the drain is slow, and a perished dishwasher door seal leaks a little onto the floor on every cycle without you ever seeing it. Because the appliance sits on top of the leak, the water spreads sideways under the plinth and shows up a metre away.
2. The pipe in the wall behind the units
Kitchen supplies are often chased into the wall or run behind the back panels of the base units. A compression joint tucked behind a cabinet, or a pipe rubbing on a fixing where it passes through the wall, can drip steadily into the plinth void. The clue is damp on the wall behind the units, a soft or bowed back panel, or a leak that carries on whether or not the appliances have been used.
3. A pipe under the floor
In many Cornish homes the kitchen sits on a solid floor, and in older granite and cob cottages that floor has often been screeded over more than once. Heating pipes and sometimes the supply pipe run through that screed. A pinhole or a failed joint there produces no drip anywhere. The water rises through the screed and appears at the lowest point, which is frequently under the plinth because that is where the floor dries slowest. A warm patch of floor in front of the units points strongly at the heating circuit.
Not every wet floor is a leak, though. A fridge freezer with a blocked defrost drain, a boiler condensate pipe that has come adrift, a cold mains pipe sweating in a warm kitchen or rain getting in around an outside tap can all put water under the units. These come and go with the weather or the appliance cycle, which is a useful pattern to look for before you assume the worst.
What the kickboard is telling you
The plinth, or kickboard, is usually a strip of chipboard with a thin facing. It is the cheapest part of the kitchen and the most sensitive moisture meter you own. Chipboard swells within days of getting wet and never fully goes back down, so the shape of the swelling records where the water has been.
- Swelling in one short section. The water is arriving there. Look directly behind it.
- Swelling along the whole length. Water has spread across the floor under the units, so it has either been running for a while or it is coming up from below.
- A tide mark on the back of the plinth. The line shows how deep the water has stood.
Most kickboards clip onto the cabinet legs and pull away with a firm tug at the bottom edge. Once it is off, get a torch on the floor and work along the run slowly. Dry everything with a towel, leave it an hour and look again. Water that comes back in one particular spot has just told you where to look.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Wet under the dishwasher or washing machine, dry elsewhere | Inlet hose, drain hose, door seal or a cracked sump | Pull the appliance forward and run a cycle while you watch |
| Damp along the back wall, back panels soft or stained | Pipe or joint in the wall behind the units | Isolate the supply and see whether it stops |
| Damp patch in the floor with no drip above it, floor feels warm | Heating pipe under the screed | Watch the boiler pressure gauge for a few days |
| Damp patch on the floor, cold, there whether appliances run or not | Cold supply pipe under the floor or in the wall | Do the meter test below |
| Only wet after heavy rain, worst by the outside wall | Water getting in from outside | Check and seal the pipe exit or outside tap |
Three tests that cost nothing
- The appliance test. Turn off the isolation valves feeding the dishwasher and washing machine, dry the void, and leave them off for two days. If the floor stays dry, you have your answer. If it gets wet anyway, the appliances are cleared.
- The meter test. With every tap and appliance off, check whether the small dial or the last digits on the water meter move. Our guide to a water meter moving when nothing is on walks through it. Movement points to a leak on the cold supply, which includes any pipe under the kitchen floor.
- The pressure test. On a combi or sealed heating system, note the boiler pressure when cold (normally about 1 to 1.5 bar) and check it again each morning. A slow fall with no visible drip makes a heating pipe under the floor the prime suspect. Do not keep topping it up and living with it: a wet screed under laminate or vinyl soon ruins the flooring, as our article on laminate floors lifting with water underneath explains.
When it becomes a leak detection job
If the appliances are cleared and the meter or the boiler gauge says water is escaping, the leak is inside a wall or under the floor. This is the point to stop guessing. Taking a kitchen apart or breaking up a screed on a hunch is expensive, and it is easy to open the wrong spot because water travels along pipes and through screed before it surfaces.
A survey works from the outside in. A thermal imaging camera picks up the temperature difference a heating leak leaves in a floor, and often shows the route of the pipe as well as the wet patch. For a cold supply leak, acoustic listening equipment picks up the sound of water escaping under pressure and narrows it to a short section of pipe. Where a pipe can be isolated and drained, tracer gas surfaces at the fault, even through a screed. Moisture readings then confirm how far the water has spread so that any opening up is small and in the right place. If you would like to know what that looks like on the day, read what to expect from a leak detection survey.
Where the leak has damaged the units or the floor, most buildings policies include trace and access cover, which pays for finding the leak and making good afterwards, though not for repairing the pipe itself. Our report is written with that claim in mind. We provide leak detection across Cornwall, from Truro to the far end of the Lizard, and a same-week survey is usually possible. One last thought for holiday lets and second homes: a small leak under the units can run for weeks between bookings with nobody there to notice the kickboard changing shape, so a quick look behind the plinth on each changeover is a habit worth keeping.
Frequently asked questions
Why is there water under my kitchen units but not under the sink?
Because the sink is only one of several water sources in a kitchen. Appliance hoses, a pipe in the wall behind the units or a pipe under the floor can all put water on the floor without touching the sink cupboard. Water then spreads under the plinth and shows up wherever the floor is lowest.
How do I tell if the dishwasher is leaking under the units?
Turn off its isolation valve and leave it unused for two days with the plinth off and the floor dried. If the floor stays dry, the dishwasher is the likely source. To confirm, pull it forward and run a full cycle while watching the inlet hose, drain hose and door seal.
Can a leak under the kitchen floor come up through concrete?
Yes. Water from a pipe in or under a screed rises through it and appears at the surface as a damp patch, usually at the lowest or least ventilated point, often under the plinth. If the patch is warm, the heating circuit is the prime suspect. If it is cold, think cold supply.
Will you have to take my kitchen out to find the leak?
Almost never. Thermal imaging, acoustic listening and tracer gas locate a leak from the surface, so any opening up is limited to a small area at the confirmed point. Removing a plinth or pulling one appliance forward is usually all the access a survey needs.
Does home insurance cover a leak under kitchen units?
Many UK buildings policies include trace and access cover, which pays for locating the leak and for restoring what had to be disturbed to reach it. It does not normally cover repairing the pipe. Check your own policy wording and keep photographs of the damage from the start.
Find the leak, keep the kitchen
If the sink is dry and the floor is still getting wet, we can trace the source without dismantling the room, and give you a report your insurer can work from.
