Find where a sink waste leaks, drain the trap, fit new washers or sealant, then test the joint properly.
SW-01Clear the cupboard and set up a dry work areaEmpty the cupboard under the sink and lay an old towel on the base. Put a bucket or washing-up bowl underneath the trap to catch any water. Have a torch to hand, as most under-sink cupboards are dark.
SW-02Find exactly where the leak is coming fromDry every joint with kitchen roll, then run the tap and watch closely. Leaks usually come from the compression nuts on the trap, the tailpiece under the plughole, or the overflow connection. Note which joint drips before you take anything apart, so you know what you're actually fixing.
SW-03Try tightening the joint firstCompression nuts sometimes just work loose over time. Hand-tighten the suspect nut, then give it a gentle quarter turn with an adjustable wrench — do not force it. Run the tap again and check. If it still leaks, move on and take it apart properly.
SW-04Stop using the sink and drain the trapTell everyone in the house not to use this sink until you're done. Position the bucket directly under the trap, then unscrew the compression nuts by hand or with the wrench. Tip the water in the trap into the bucket — it will smell, that's normal.
SW-05Take the trap apart and inspect itUnscrew the trap fully and separate it from the tailpiece and the waste pipe. Look at the rubber or fibre washers inside each nut — if they're cracked, flattened, or have a groove worn into them, they need replacing. Clean off any old plumber's putty or limescale from the threads with a cloth.
SW-06Check the sink and overflow connection tooIf the leak was coming from above the trap, check the flat washer under the plughole flange and the overflow gasket. A worn washer here lets water seep down inside the cupboard rather than dripping visibly. Replace any washer that's hardened, flattened, or torn.
SW-07Fit new washers and reassemble the trapPush new washers onto the threads exactly as the old ones sat — check the shape, some are tapered one way. Wrap two turns of PTFE tape clockwise around any threaded metal joints for extra security. Reassemble the trap by hand first to make sure everything lines up before tightening.
SW-08Tighten the nuts correctlyTighten each compression nut by hand until firm, then add a quarter turn with the wrench — no more. Overtightening cracks plastic nuts and squashes washers out of shape, which causes new leaks. If a nut feels tight but still leaks, the washer is the problem, not the tightness.
SW-09Reseal the plughole if neededIf you disturbed the plughole flange, run a thin bead of silicone sealant under the rim before refitting it, then wipe off any excess. Leave it to cure for the time stated on the tube — usually a few hours — before running water past it.
SW-10Test the repair properlyPut dry kitchen roll under every joint you touched. Run the tap for a minute, then let the sink drain fully and check the paper for any damp patches. Leave the cupboard doors open and check again after an hour, since slow drips sometimes only show up once the sink has been used a few times.
SW-11Tidy up and put everything backDry out the cupboard base and check the towel hasn't wicked water further than it looks. Bin the old washers and any water from the bucket. Put your stored items back under the sink once you're confident the joint is dry.
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