How do I fix a leaking waste pipe under a sink?

Finding which of the four usual joints is leaking, and the fixes for each — most need no tools beyond your hands.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Dry everything, run the tap, and watch. Waste leaks come from a loose trap connector, a perished washer, a cracked trap, or the sink seal above. Hand-tighten first — most waste fittings are plastic and are meant to be done up by hand, not with grips.

Waste pipework runs at no pressure, which is why it is a repair most people can do. It also means a leak only appears when water is actually running, so the diagnosis has to be done with the tap on.

Safety

A slow leak under a sink rots the cupboard base and the floor beneath it long before it becomes visible. Check what is under and behind the unit before assuming the repair is the whole job. Waste water is not clean — wear gloves and wash afterwards. If the leak is on a pressurised supply pipe rather than the waste, isolate the water first and treat it as urgent.

Step by step

Clear the cupboard and put paper down.Dry kitchen roll under and around every joint shows exactly where water arrives. Guessing from a puddle on the base is how people tighten the wrong thing.
Run the tap and watch the joints.Fill the bowl and let it drain as well — a leak that only shows under the volume of a full bowl is common and will not appear from a trickle.
Check the four usual places.The sink waste flange where it meets the bowl, the connection between waste and trap, the trap's own screw joints, and where the trap meets the pipe into the wall. Also check any washing machine or dishwasher spigot.
Hand-tighten plastic fittings.Plastic waste connectors seal on a rubber washer and are designed to be tightened by hand. Grips crack the collar or distort the thread, which converts a small leak into a replacement.
Take the joint apart if hand-tightening does not fix it.Unscrew the collar, pull the joint apart and look at the washer. Perished, twisted, missing or fitted the wrong way round covers most cases, and washers cost very little.
Fit the washer the right way round.Most trap washers are tapered and only seal in one orientation, with the taper facing into the fitting. Reversed, they leak however hard you tighten.
Clean the pipe end before reassembling.Limescale, grease and old sealant on the pipe stop the washer seating. Wipe it clean and dry; a smear of silicone grease helps it slide without rolling.
Check the trap body for cracks.Traps crack from being knocked, over-tightened or from something stored in the cupboard pressing on them. A cracked trap is replaced, not repaired — they are inexpensive and standard sizes.
Reseal the sink waste if the leak is above the trap.Water appearing at the top of the waste means the seal between the waste flange and the sink has failed. Undo the backnut, clean off the old sealant, bed the flange on fresh silicone or a new washer, and refit.
Test properly before putting anything back.Fill and drain the bowl twice, then run the tap for a couple of minutes. Dry every joint and leave paper down overnight before restocking the cupboard.

Tips

Photograph the arrangement before dismantling. Trap assemblies come apart easily and go back together wrongly.
Buy a spare washer set when you buy anything for this job. They cost almost nothing and the second trip is the annoying part.
If the cupboard base is soft or swollen, the leak has been going for a while. Deal with the base as well or it will keep spreading.

Common mistakes

Tightening plastic fittings with grips — The collar cracks or the thread distorts, and a joint that needed a new washer now needs a new trap.
Tightening the joint nearest the puddle — Water tracks along pipes and drips from the lowest point, which is frequently not where it escaped. Dry everything and watch instead.

If it doesn't work

Drips only when the bowl is emptied

Cause: A joint that leaks under full flow but not a trickle — Fix: Fill and drain the bowl to reproduce it, then work through the joints under that condition.

Water at the top of the waste inside the bowl

Cause: Failed seal between the waste flange and the sink — Fix: Undo the backnut, clean off the old sealant, rebed the flange on fresh silicone and refit.

Leak from a washing machine hose spigot

Cause: Loose jubilee clip or a perished hose end — Fix: Tighten the clip; if the hose end is split, replace the hose rather than patching it.

Smell as well as a leak

Cause: Trap has been dismantled and not refilled, or is a dry trap — Fix: Run water to refill the trap. A trap with no water in it lets drain gases straight into the room.

Questions people ask

How tight should plastic waste fittings be?

Hand tight. They seal on a rubber washer, not on thread pressure, and using grips cracks the collar. If hand-tight leaks, the washer is the problem rather than the tightness.

Why is my sink leaking only sometimes?

Almost always a joint that holds a trickle but not full flow. Fill the bowl and let it drain to reproduce it, rather than testing with the tap running slowly.