Water Leak Under the Sink: Causes, Checks & Fixes

Stop the water first and keep it away from electrics

Use the nearest safe isolation valve or main stopcock. If water is reaching sockets, lights, appliances or the consumer unit, keep clear and isolate the affected electrical circuit only if it is safe to do so.

MOST LIKELY CAUSES

Start with these

Trap or waste joint Loose nuts, damaged washers and misalignment cause drips.

Tap tail or flexi hose Braided hoses and connections can leak under pressure.

Isolation valve The spindle, compression nut or body may leak.

Appliance hose connection Washing-machine and dishwasher hoses may drip into the cabinet.

◫ Symptom variations Match when and where the leak appears. ⌄

Leak only when sink drains
Constant drip with taps off
Cabinet base is wet
Leak starts when tap is opened
Water appears after appliance use

✓ Quick checks Simple checks before dismantling anything. ⌄

Dry the area so fresh water reveals the true source.
Check whether the leak is constant or only appears during use.
Identify whether the water is clean supply water, heating water or waste water.
Close the nearest service valve or main stopcock if the leak is pressurised.
Protect floors, cabinets and electrical equipment from further water.

If the leak stops: keep the source isolated until a lasting repair is made.

If water continues: check tanks, heating circuits, shared supplies or call a plumber urgently.

1 Step 1 — Dry and observe Follow this check before dismantling plumbing. ⌄

Process

Empty the cabinet, dry every pipe and place tissue around suspected points.

Success looks like: The first wet location identifies the source.

Escalate when: Water may track from higher fittings.

2 Step 2 — Separate supply and waste faults Follow this check before dismantling plumbing. ⌄

Process

Run the tap into a bowl without using the sink waste, then test drainage separately.

Success looks like: The leak is identified as supply-side or waste-side.

Escalate when: If both remain dry, check appliance connections.

3 Step 3 — Isolate the leaking supply connection Follow this check before dismantling plumbing. ⌄

Process

Close the local service valve or main stopcock if a pressurised hose or valve leaks.

Success looks like: The leak stops.

Escalate when: A waste leak may only need the sink left unused.

⌁ Common causes explained Why these leaks develop. ⌄

Leaks under sinks commonly come from the trap, waste joints, tap tails, isolation valves, tap base, overflow or washing-machine and dishwasher connections.

Water often travels along pipes, cabinets, joists and surfaces before it drips. The visible wet patch may not sit directly below the actual failed joint or fitting.

⌘ Advanced diagnostics Plumber-level checks and repair work. ⌄

Inspect trap washers and waste alignment.
Check tap tails and flexi-hose condition.
Test isolation valve gland and compression joints.
Inspect sink overflow and tap-base seals.
Check appliance supply and drain connections.

▣ Parts that may need replacing Common components linked to this leak. ⌄

Trap washer or trap
Waste fitting
Tap tail
Flexible hose
Isolation valve
Hose washer or clip

! When to call a plumber Conditions that should end basic troubleshooting. ⌄

The leak cannot be isolated.
Water is reaching electrical equipment.
The leak is concealed inside a wall, ceiling or floor.
A ceiling is bulging, sagging or unsafe.
A main stopcock, buried supply or heating circuit is involved.
The leak returns after a minor adjustment.

? Frequently asked questions Common questions about plumbing leaks. ⌄

Why does the drip appear away from the damaged joint?

Water can track along pipes and surfaces before falling at a lower point.

Should I keep tightening a leaking nut?

No. Over-tightening can distort olives, seals and fittings and make the leak worse.

Can a small drip cause serious damage?

Yes. Slow leaks can damage cabinets, ceilings, flooring and hidden timber over time.