What's leaking under there? A field guide

Under-sink leaks are rarely dramatic. The catastrophic burst is the exception; the rule is a slow seep that runs down a pipe, follows the cabinet floor to the lowest corner, and quietly delaminates the particleboard for a month before anyone notices a smell. Here are the usual suspects, in rough order of likelihood.

1. The P-trap slip nuts

The curved trap under the drain is held by hand-tightened slip nuts, and they loosen with vibration, temperature swings, and every time someone stores a stockpot too aggressively. Symptom: drips directly below the trap after running water. Test: fill the sink, pull the stopper, and watch the trap with a dry paper towel in hand. A quarter-turn by hand usually fixes it.

2. Supply line connections

Two braided lines feed the faucet, and their compression fittings weep at the shutoff valve or the faucet stem. Symptom: moisture high in the cabinet, often on the lines themselves. These fail slowly and then, occasionally, all at once; supply line bursts are one of the most expensive residential water claims. Replace lines older than ten years.

3. The faucet base and sink rim

Water pooling on the counter finds the faucet mount or a failed rim seal and drips onto the cabinet ceiling, then rains down the walls. Symptom: moisture at the top of the cabinet, not the bottom. Test: run a bead of water around the faucet base and watch from below with a flashlight.

4. The disposal

Garbage disposals leak from three places: the sink flange above, the dishwasher hose barb on the side, and, terminally, the body seams below. A body leak means replacement, not repair.

5. Everything you store down there

The most common "leak" is not plumbing at all. It is the dish soap refill lying on its side, the surface cleaner with a cracked trigger, the flower vase that never fully dried. Storage spills outnumber plumbing failures in most cabinets, which is an argument for a contained floor rather than more vigilance.

That is the design brief behind the Basin Mat: a silicone floor with a raised lip that holds up to 2.5 gallons, so the slow version of any of these failures stays on the mat, visible and wipeable, instead of in the particleboard. Whatever the source, the first symptom becomes a puddle you can see instead of a smell you can't place.