A sink waste usually leaks because a seal has worn or moved, a plastic nut has worked loose or cracked, or the waste fitting in the plughole is no longer sealed to the sink. Leaks from the waste tend to show up while water is draining, rather than when the sink is not being used.
The waste is made of several parts joined together, so the leak can come from more than one place.
The Parts of a Sink Waste
The waste fitting sits in the plughole. It is held in place by a back nut under the sink and sealed with washers or sealant. If this seal fails, water can creep out around the underside of the plughole.
Below that is the trap, a curved section that holds a little water to stop smells coming up. Traps have screw-on nuts with rubber or plastic washers. Many also have an overflow pipe connecting the overflow slot in the sink to the waste.
Finally, a waste pipe carries water to the wall or to the drain. Its joints may be push-fit or compression, each with its own seals.
Why Waste Joints Start to Leak
Plastic nuts can loosen slowly as pipes expand and contract with hot and cold water. Items stored in the cupboard can also knock the trap out of line.
Washers and seals age. Rubber can harden, flatten or split, and grease or grit can stop it sitting properly. Overflow pipes are easy to forget and can leak at either end.
A partial blockage can also make a waste joint leak. When water backs up, it puts extra pressure on joints that are not designed to hold it. If the sink is also draining slowly, the blockage may need dealing with first through a blocked sink visit.
Signs to Look Out For
Drips from the trap when the sink empties, damp around the back nut under the plughole, or water collecting where the waste pipe meets the wall all suggest the waste. A musty smell in the cupboard may mean a slow leak has been there for a while.
These signs do not necessarily mean the waste is the only source. A tap connector nearby can drip onto the trap and make it look wet.
Safe Steps and Repair
Put a bowl under the trap and avoid using the sink until it is checked. You can gently hand-tighten a plastic nut that is obviously loose, but stop if it does not move easily. Avoid tools on plastic waste fittings, as they crack easily.
Most waste leaks are fixed by renewing a washer, re-seating a joint or replacing a trap, rather than the whole sink. If yours keeps dripping, our Oxford sink repairs can check every joint from the plughole to the wall and renew the parts that have failed.