Trace a kitchen sink leak to its exact source, step by step, before you call a plumber or buy parts.
LK-01Clear the cupboard and set upTake everything out from under the sink so you can see all the pipework. Lay an old towel on the cupboard floor and put a torch and a small bowl within reach. Good light matters more than anything else for this job.
LK-02Dry every pipe and joint completelyWipe the waste trap, all pipe joints, tap tails and isolation valves bone dry with kitchen roll. This matters because any new wetness that appears afterwards tells you exactly where the leak starts. Don't skip a joint just because it looks dry already.
LK-03Check for loose connections by handGently try to turn every waste joint, tap flexi hose and isolation valve nut by hand. Plastic waste fittings only need to be hand-tight, so if one turns easily it may be the whole problem. Don't force anything with a spanner yet.
LK-04Run the tap and watch the waste trapTurn the tap on and let water run through the plug and down the waste for a minute. Watch and feel every waste joint below, especially where the trap meets the sink and where it meets the pipe going into the wall. A leak that only appears now is a waste leak, not a supply leak.
LK-05Check the tap tails and supply pipesWith the tap still running, feel around where the flexible tap hoses meet the tap body and where they meet the isolation valves. Test hot and cold separately by running each tap on its own. A drip here points to the supply side, not the waste.
LK-06Check any dishwasher or washing machine connectionIf a dishwasher or washing machine takes its water or waste from under the sink, run a short cycle and check its connections the same way. If there's a socket or plug near any dampness, don't touch it until you've dried the area and are sure no water is reaching it.
LK-07Test the overflow if the sink has onePut the plug in and fill the sink until water reaches the overflow slot. Check under the sink for drips from the overflow pipe or its joint. This is a common leak that only shows up when the sink is fuller than normal.
LK-08Isolate the water if you can't stop the leakIf you've found a leak you can't tighten away, turn off the isolation valve under the sink for that pipe only, using a screwdriver or your fingers depending on the type. If there's no isolation valve or the leak is worse than expected, turn off the main stopcock, usually under the kitchen sink or near the front door.
LK-09Note what you found and tidy upWrite down exactly which joint, hose or fitting is leaking and what it looks like, so you know what part to buy or what to tell a plumber. Dry everything out again, put your things back under the sink, and keep the area ventilated until it's fully dry.