Leak under the kitchen sink — how to trace and fix it

Under-sink leaks are usually at one of three points: the tap tails or supply hoses (pressurised water supply), the waste trap and drain connections (waste water when the tap is run), or the sink basket waste seal (water leaking when the sink is full).

Dry everything thoroughly with a cloth and run the tap while watching carefully with a torch. This distinguishes a waste leak (only when running water) from a supply leak (all the time).

Supply hose leaks: the flexible hoses connecting the isolation valves to the tap tails can corrode or develop pin-holes, especially where the armoured sleeve ends. Try tightening the hand-tight connectors. If they are leaking from the body of the hose, replacement hoses are £5–£12 each and screw straight on.

Waste trap leaks: the U-bend and connections are under compression rather than pressure, so they mostly drip when water is running. Check that the compression washers in each joint are seated correctly. Tighten hand-tight, then a quarter turn with grips. Cracked traps are common — a full waste kit (trap + outlets) is £8–£15.

Basin waste/basket waste seal: if water leaks when the sink is full of water, the rubber seal beneath the waste fitting has perished. The waste fitting needs to be removed (from underneath, unscrew the back nut), old sealant cleaned off, and a new rubber washer and fresh plumbers putty or silicone applied.

For leaks at copper compression fittings (olive and cap-nut joints): turn off the supply, dry and sand the outside of the fitting, and apply a specialist pipe repair putty (such as Sylmasta or Plumbers Mait) as a temporary measure. A permanent fix involves refitting the olive or replacing the fitting.