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A damp cabinet under the kitchen or bathroom sink is easy to ignore until the base swells and the smell arrives. The good news is that there are only four places the water can come from, and telling them apart takes a paper towel and a little patience.
Dry everything first
Empty the cabinet, dry every pipe and fitting with a towel, and lay dry paper towels on the cabinet floor. Then test one thing at a time, so that you know which test made the towels wet.
Test 1: the drain
Fill the sink halfway, then pull the plug and watch the pipes underneath. Water appearing while the sink drains means the leak is in the drain side: the tailpiece straight below the sink, the U-shaped trap, or the joint where the pipe enters the wall.
- A loose slip nut. The large plastic or chrome nuts hand-tighten. Give each a further quarter turn. Do not use a wrench on plastic nuts, which crack.
- A worn washer. Unscrew the nut, slide it back, and look at the tapered washer inside. If it is flattened, hardened or cracked, replace it. A slip joint washer assortment covers the usual sizes. Fit the washer with its taper facing the joint.
- A corroded metal trap. Chrome traps rust through from the inside. Replace the whole trap with a plastic one. It comes as a kit, with the nuts and washers.
- The sink strainer or drain flange. If water seeps around the drain opening itself, the putty under the strainer has failed. Resetting it is a longer job, though still one a patient beginner can do.
Test 2: the supply lines
With the sink empty, turn the faucet on full for a minute, hot and then cold. Water on the towels now points to the supply side: the two flexible lines running from the shut-off valves up to the faucet.
- Tighten the nuts at each end, a quarter turn at a time. Hold the valve steady with a second wrench so that you do not twist the pipe behind it.
- Replace the line if the leak continues, or if the line is more than ten years old, or if it is the grey plastic type. Braided stainless supply lines cost little. Turn the valve off, unscrew the old line, screw on the new one and tighten it snugly.
- A dripping shut-off valve. Old valves leak at the stem when they are turned. Tightening the small packing nut behind the handle slightly often stops it. If it does not, the valve needs replacing, and that is a plumber's job unless you can shut off the main and are confident with fittings.
Test 3: the faucet itself
Run the faucet again and look up at its base from inside the cabinet. Water dripping from the underside of the faucet body means it is leaking at the base or through a worn cartridge and running down. See our guide to dripping faucets, or, if the faucet is old, consider replacing it.
Test 4: splashing and condensation
If the towels are dry after all three tests and the cabinet still gets damp, the water may be coming from above: splashes at the back of the sink finding a gap in the caulk, or a sprayer hose that leaks only when it is pulled out. In a humid basement bathroom, cold pipes also sweat. Foam pipe insulation stops that.
Repairing the damage
Dry the cabinet fully with a fan before you put anything back. Swollen particleboard does not recover, but a sheet of cabinet liner protects what is left and makes the next leak obvious. A water leak alarm on the cabinet floor costs very little and will save you from the slow leak that goes unnoticed for months.
When to call a plumber
- The leak is at the joint where the drain pipe enters the wall
- The shut-off valve leaks or will not close
- The pipes are corroded, or the cabinet floor is rotten through
- The leak is from a dishwasher or disposal connection that you cannot trace
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