Damp under the kitchen sink cabinet: where the leak usually comes from

Where the leak usually starts

Under a kitchen sink there are really only four places water can get out: the mixer tap's connections to the two angle valves, the flexible corrugated trap under the plughole, the dishwasher's drain hose where it joins the pipework, and occasionally a crack in the sink bowl itself where it meets the worktop. Almost every damp cabinet traces back to one of the first three.

The pattern of the damage often points to the source before you even touch anything. Staining concentrated at the back near the wall usually means the angle valves or supply hoses. Wetness directly under the plughole points to the trap. A damp patch to one side, often behind where the dishwasher sits, suggests the drain hose connection rather than the sink itself.

It is worth checking the cabinet floor and the underside of any shelf as well as the base, since water can run along a slope in the chipboard before it drips, which makes the source look further away than it actually is.

How to check the mixer tap connections and angle valves

Dry the whole area with a cloth or kitchen roll, including the valves, the hose connections, and the underside of the sink. Then turn on both the hot and cold tap fully for thirty seconds and watch each joint closely with a torch, since a slow drip is easier to see running down a dry surface than to feel with a finger.

The usual culprits are a loose compression nut on a supply hose, a worn rubber washer inside a hose end, or an angle valve that has started to weep from its own spindle. A nut that is simply loose can often be tightened by hand or with a small spanner without needing to replace anything.

If tightening does not stop it, or the valve itself is corroded or the hose has a visible crack, the fitting needs replacing rather than tightening further. This is the kind of small job that fits into a call-out for tap replacement, priced from 55 € per job, and an old angle valve is worth changing at the same time if it looks tired.

How to check the trap and drain connection

The flexible corrugated trap under the plughole is a common failure point, particularly the two threaded connections at either end, which have a rubber seal that hardens and shrinks over years. Dry the trap and the pipe below it, then run water down the sink for a minute and look at both joints and along the length of the corrugated section for weeping.

A loose slip nut can often be hand-tightened, but if the rubber seal inside is cracked or has gone stiff, tightening further just squeezes a bad seal harder and rarely holds. The trap and its seals are cheap, and swapping the whole flexible unit is usually more reliable than trying to nurse an old one along.

Also check where the trap meets the wall outlet, since that connection is fixed and sometimes gets knocked out of alignment when a cabinet is used for storage and the pipe gets nudged.

• Dry everything first — a wet trap makes a new drip impossible to spot
• Run water for at least a minute, not a quick burst
• Check both threaded joints and the full length of the corrugated pipe
• Look at the wall connection, not only the trap under the plughole
• A cracked or hardened rubber seal needs replacing, not just tightening

How to check the dishwasher drain hose

If there is a dishwasher in the cabinet run, its drain hose is worth checking even if the damp patch seems to sit under the sink, since water travels along the cabinet base. Run a dishwasher cycle and watch the point where its drain hose connects to the sink's waste pipe or to a dedicated spigot, which is usually secured with a hose clip.

A hose clip that has loosened over time, or a hose that has been pushed on without one, will leak specifically during the drain phase of the cycle rather than continuously, which is a useful clue that separates it from a supply-side drip that runs whenever a tap is on.

Reseating the hose properly and fitting a new clip is usually enough. If the hose itself is cracked near the connection, which happens as the plastic ages, it needs replacing rather than resealing.

What is urgent and what can wait

A steady drip from a supply-side fitting, meaning anything before the tap or angle valve, should not wait, because that water is under mains pressure and a small weep can become a burst. If you can, shut off the two angle valves under the sink to isolate the tap while you sort out a repair.

A slow weep from the trap or drain hose is unpleasant and will keep damaging the cabinet base, but it is not pressurised and will not suddenly flood the flat, so it is fair to schedule rather than treat as an emergency. Still, chipboard that has already swollen will not recover once it dries out, so it is worth acting within a few days rather than months.

If you live in a flat and water has reached the floor or is tracking towards a neighbour below, that is worth flagging to your apartment association regardless of how minor the leak itself turns out to be, since shared pipework and floor slabs are their concern too.

What you can do yourself and when to call a plumber

Tightening a loose compression nut by hand, reseating a dishwasher hose, wiping up and drying out the cabinet, and swapping a hose clip are all reasonable to do yourself with basic tools, provided the water is off first. None of it requires special equipment.

Once a part is actually worn, cracked, or corroded rather than just loose, replacing it properly is worth having a plumber do, particularly for angle valves, which are awkward to reach and easy to overtighten and crack if you are not used to them. Plumbing call-outs start from 45 € per hour, with a minimum of one hour, and if the cabinet base itself is too swollen to save, a full kitchen installation is priced from 600 € per job.

If you are not sure which of the four points is actually leaking after drying everything and running the taps, that uncertainty is itself a reason to get someone to look, since guessing wrong means the cabinet keeps absorbing water while you replace the wrong part.