Discovering a puddle of water inside your under-sink cupboard is a homeowner’s nightmare. Unlike a noisy dripping spout, a leak beneath the sink often goes unnoticed for days or weeks, quietly damaging kitchen cabinets, warping floorboards, and causing mold growth.
If water is pooling around your pipework whenever you turn on the kitchen tap, here are the 4 most common causes and how to fix them.
1. Loose or Damaged Flexi-Tail Hoses
Most modern UK monobloc kitchen taps connect to the mains pipework via braided stainless steel flexible hoses (known as “flexi-tails”).
Over time, everyday tap movement or thermal expansion from hot water can cause the threaded connectors on these hoses to work themselves loose. Alternatively, the internal rubber washers at the ends of the hoses can harden and perish, allowing water to spray or drip down the pipework.
The Fix:
- Isolate your water supply and wipe the hoses dry with a paper towel.
- Use a spanner to check if the compression nuts connecting the flexi-tails to the isolation valves or copper pipes are snug.
- If a hose is corroded or the rubber seal inside the female fitting is split, replace the flexi-tail entirely (never over-tighten them, as this can crush the internal rubber seal).
2. Perished Spout O-Rings (Leaking Down the Tap Body)
Sometimes water appears under the sink even though the pipes themselves are in perfect condition. If the leak only happens when the tap is turned on and swiveled, water is likely seeping past the spout’s internal O-rings.
Water escapes around the base of the tap on top of the counter, then runs straight down the threaded mounting stud and through the tap hole into the cupboard below.
The Fix:
- Unscrew the small grub screw at the rear base of the tap spout.
- Pull the swivel spout upward to detach it from the tap body.
- Slide off the worn rubber O-rings on the internal stem, replace them with new, grease-lubricated O-rings of the same size, and reassemble.
3. A Perished Sink Deck Seal or Gasket
Between the base of your kitchen tap and the top of your sink sits a thin rubber washer or plastic sealing gasket. Its job is to create a watertight barrier so that splashing water from washing up doesn’t slide under the tap base and drip into the cabinet.
If this seal perishes, cracks, or shifts out of place, water pooling on the sink deck will drain directly through the tap mounting hole.
The Fix:
- Loosen the securing back-nut on the threaded stud under the sink so you can lift the tap slightly off the deck.
- Clean away old limescale and grime around the sink hole.
- Fit a fresh rubber sealing ring around the tap base (or apply a thin bead of sanitary silicone sealant around the base) and re-tighten the tap securely.
4. Corroded Tap Tailpipes or Split Brass Housing
In older installations, traditional rigid copper tailpipes were used instead of flexible hoses. Over many years, hard water limescale and chemical cleaners stored under the sink can cause pinhole corrosion in the brass or copper housing.
Similarly, if water froze inside the tap during an unheated winter spell, the internal brass chamber beneath the sink deck may have hairline cracks.
The Fix:
- Inspect the solid brass or copper fittings extending down from the tap body using a torch.
- Pinhole leaks or cracks in the brass body itself cannot be reliably patched with sealant or tape. The entire tap assembly will need to be replaced.
When to Call a Professional Plumber
Under-sink leaks require prompt attention to prevent structural damage to your kitchen units. You should call in an expert if:
- You cannot pinpoint where the water is coming from along the pipework.
- The copper pipework or compression joints under the sink are green, heavily corroded, or seized solid.
- The leak is coming from a cracked tap body that requires a full tap replacement.
