Swap a leaking or blocked sink trap for a new one, catch the water, and check every joint for leaks.
SNK-01Clear out and protect the areaEmpty the cupboard under the sink and lay an old towel on the base. Get a bucket or washing-up bowl ready to catch water from the old trap. Have a torch to hand, since under-sink cupboards are often dark.
SNK-02Work out what trap you needLook at the old trap: it's either a U-shaped P-trap or a rounded bottle trap. Measure the waste pipe diameter where it meets the wall — kitchen sinks are usually 40mm, bathroom basins usually 32mm. Buy a like-for-like replacement, or a universal trap kit with reducers if you're not sure.
1 each · Replacement trap (32mm or 40mm, matched to old one)
SNK-03Loosen the old trapPosition the bucket directly under the trap first — it will still hold some water even if the sink looks empty. Unscrew the large slip nuts at each end of the trap by hand, turning anticlockwise. If they're stiff, use grips wrapped in a rag to avoid scratching chrome fittings.
SNK-04Remove and clean the connection pointsLift the old trap away and tip any remaining water into the bucket. Wipe the tailpipe (from the sink) and the waste pipe (into the wall or floor) clean of old gunk, limescale or old sealant. Check both pipe ends aren't cracked or worn, since a damaged pipe end won't seal properly with a new trap.
SNK-05Fit the new trapSlide the new trap into place, loosest first, and hand-tighten each slip nut. Wrap the threaded connections with a couple of turns of PTFE tape if the kit doesn't have built-in rubber washers, then tighten each nut a further quarter turn with grips. Don't overtighten — plastic threads crack easily if forced.
SNK-06Check the fall and the water sealMake sure the trap sits so the pipe runs slightly downhill towards the wall or floor outlet — water needs gravity to help it drain. A trap only works if it holds a small pool of water inside its bend (the 'seal'), which stops smells coming back up, so don't fit it upside down or at an odd angle.
SNK-07Test for leaksPut the bucket back under the trap as a precaution. Run the tap for a minute, then plug the sink and let it drain fully, watching every joint. Nip up any joint that drips with a small extra turn — you're checking for a slow drip, not just a gush.
SNK-08Tidy upDry the pipework and the inside of the cupboard with a towel. Recheck the joints once more after 10–15 minutes, since a small leak can take a few minutes to show. Put your stored items back and dispose of the old trap — most councils accept it in general waste or recycling.