The one catch is the trap. That curved pipe holds water on purpose, so a cracked or loose trap can keep dripping for a while after the faucet is off. Timing points you in the right direction; where the water first shows up settles it.
A paper towel test you can do tonight
Empty the cabinet, wipe every pipe and valve dry, and lay dry paper towels flat on the cabinet floor: one under the shutoff valves, one under the trap, one under the disposal if you have one. Then:
- Leave the sink alone for an hour or two. A wet towel under the valves, with dry pipes above the drain, points to a valve, hose or supply connection.
- Next, run the faucet for a minute with the drain open. If water now appears at a slip nut, the trap or the base of the strainer, it's on the drain side.
- Fill the basin partway and let it drain. Some drain joints only leak under a full rush of water.
- If it only gets wet during a dishwasher cycle, tell the plumber that. The dishwasher's supply line and drain hose both run into this cabinet.
- Water running down from the faucet base or the sprayer hose when the tap is on is a faucet leak, which ends up under the sink but isn't the drain.
Run your finger along each joint once the area is dry. The first wet spot is usually the source, because water travels down and along pipes before it drips. The Building America leak guide lists the hot and cold faucet hoses and valves, and the dishwasher and disposal connections, among the most common leak points in a house. The EPA WaterSense checklist adds rust around joints as a clue under sinks.
Use your water meter to confirm a supply leak
A drain leak doesn't move your water meter. A supply leak does. The Philadelphia Water Department says the small blue dial on the meter moves whenever water flows through it, even for a tiny leak. Most Philadelphia meters sit in the basement against the wall closest to the street. Make sure nobody is using water and nothing is running, then watch the dial. WaterSense suggests a slower version: write down the reading, use no water for two hours and check that it hasn't changed.
If the dial moves and the sink cabinet is wet, close the sink's two shutoff valves and check again. If the dial stops, the leak is on that sink's supply side. If it keeps moving, there's another leak too, such as a toilet, the water heater or the line coming in from the street. Our guide on who pays for a leaking water service line in Philadelphia covers that last case.
When to stop and shut the water off
PWD's advice for a puddle under a sink is to turn off the supply valve and call a plumber. Turn the valves by hand only. If one is stuck or crusted, don't force it, because old stop valves can break. Use the main shutoff instead and tell the plumber the fixture valve wouldn't close. If water has reached the disposal's plug or an outlet in the cabinet, don't touch it until the power to that circuit is off.
A uniform film of beads on the cold pipe in humid weather may be condensation, not a leak. EPA's mold and moisture guide points to high humidity as the cause and pipe insulation as the fix. Leak drips form at a joint; condensation covers the whole cold pipe.
Drying out the cabinet
EPA says to dry water-damaged materials within 24 to 48 hours to keep mold from growing. Pull out wet items, leave the doors open and soak up standing water. Particleboard cabinet floors swell and crumble when they stay wet, so note any soft spots for the plumber. If the water came from the drain and smells like sewage, treat it as contaminated: EPA's remediation guide says not to blow fans across it until you know the water is clean, and to bring in a professional for contaminated water.
What to tell the plumber
- Supply or drain, based on your towel test and meter check, and where the first wet spot was.
- Whether the sink's shutoff valves closed and whether that stopped the meter.
- Ask what part failed, whether the shutoff valves themselves should be replaced while they're there, and how they'll check that the leak is gone before closing up.
PWD points Philadelphia homeowners to the L&I contractor search to confirm a plumber holds a journeyman or master license. If the fix turns into new piping or a larger job, check which plumbing jobs need an L&I permit. If the faucet also has weak flow, our guide to low water pressure at one faucet covers the usual causes.