Water under my kitchen sink — how do I find which joint is actually leaking?
There's water pooling in the cupboard under my kitchen sink, and the cabinet base is starting to get damaged (I know from the threads here that a swollen under-sink cabinet is bad news). The problem is I can't tell where the water's actually coming from — there are several possible culprits down there:
- The tap (leaking down the back)
- The flexible supply hoses to the tap
- The drain trap (P-trap) under the sink
- The waste pipe connection to the wall
How do I work out which joint is leaking before I call a plumber — or before I poke it and make it worse? Is there a sensible way to diagnose it myself? Want to at least know what I'm dealing with (and stop the cabinet rotting).
3 answers
Short answer: dry everything, then test each source separately — a leak under a sink is almost always one of four things, and you find it by isolating water supply vs drainage. Line the cupboard with dry paper/tissue, then watch WHERE it gets wet and WHEN (constant vs only when you run water vs only when it drains). That tells you which joint.
The diagnosis, step by step (turn a torch on and get your head in there): 1. Dry it all and lay dry tissue/paper along the base and around each joint. 2. Do nothing for an hour (taps off). If paper gets wet with everything off → it's the pressurised supply side: the tap tails or the flexible supply hoses (a constant drip even when unused). These fail at the crimped ends and are a very common UAE culprit. 3. Run the tap and watch without draining much → if it's wet around the tap base/back, the tap seal is leaking down. 4. Fill the bowl and let it drain → if it only leaks *while draining*, it's the P-trap or the waste connection to the wall (loose slip-nut, perished seal, or a cracked trap). Drainage leaks only show when water is actually running through.
So the logic: wet with everything off = supply/hose; wet only when running the tap = tap; wet only when draining = trap/waste. That isolates it to one joint.
Quick fixes vs pro: a loose trap slip-nut you can often hand-tighten; a perished flexi hose is cheap to swap (isolate the valve first). But if the joint's corroded, the fitting's cracked, or you can't isolate the supply, don't force it — get a licensed plumber before you snap a fitting and turn a drip into a flood. And dry/ventilate the cupboard fast so the base doesn't rot. Which did yours turn out to be — the flexi hoses catch a lot of people?
The dry-tissue trick is exactly how I found mine — laid kitchen roll down, and it was only the strip under the flexible supply hose that got wet, and only slowly with everything off, which pointed straight at a weeping hose end. Swapped the flexi (cheap, ten minutes, isolate the valve first) and done. Without the tissue I'd have been guessing. The "when does it get wet — off / running / draining" question is the whole diagnosis.
Mine was the trap — only leaked when the water actually drained, which matched the "wet only when draining" clue perfectly. The slip-nut had worked loose; hand-tightened it and it stopped. Worth checking that before assuming the worst. And yeah — get the cupboard dry and aired quickly, because the base board soaks it up and swells fast in this climate. Catching it early is the difference between tightening a nut and replacing the cabinet.
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