Water under the bathroom sink: find the source fast

Bathroom vanities are one of the quietest places in the house for water damage to start. The cabinet stays closed, the leak is slow, and by the time there is a musty smell or a soft spot in the base, months have gone by. The good news is that almost every under-sink leak in a bathroom comes from one of five places, and once you know which one, the fix is usually modest.

Empty the cabinet and look

Take everything out. Wipe the cabinet floor completely dry. Bring a flashlight. You cannot diagnose a slow leak through a row of cleaning bottles, and you cannot tell where water is coming from until every surface inside the cabinet is dry at the start.

While the cabinet is empty, put a hand on the base. Soft, spongy, bubbled, or darkened particleboard tells you the leak has been running longer than you thought. Note it and keep going — you need the source before you decide what to do about the damage.

The five places bathroom sinks leak

Shutoff valves. The two small stops on the wall that feed hot and cold to the faucet. They leak at the valve stem, or at the compression fitting where the stop meets the copper coming out of the wall.

Supply lines. The braided or plastic lines running from the stops up to the faucet. Cracked plastic lines and corroded fittings fail more often than people expect, especially on lines older than about ten years. If yours look aged, a set of braided stainless supply lines↗ is cheap insurance.

The p-trap and slip joints. The curved trap under the drain and the nuts holding it together. Slip nuts back off over time. Washers harden and deform.

The pop-up drain assembly. The lift rod that lets you close the drain. It seals to the sink at the drain flange, and again where the rod enters the tailpiece. Either seal can fail — the plumber's putty or gasket at the flange is the most common.

The faucet-to-sink gasket. Sometimes water running off the handles creeps down through the faucet's mounting holes and shows up as a leak below, even though the plumbing itself is fine.

How to pinpoint it

Dry the cabinet thoroughly. Lay a folded paper towel under each suspect: one under each stop, one under the p-trap, one under the pop-up rod entry, one directly under the drain, and one centered under the faucet. Then:

• Run hot, then cold, for a minute each. Let it sit. Check the paper.
• Fill the sink, then drain it. Check again.
• Splash water around the handles. Check the paper under the faucet center.

The pattern of wet paper tells you where the leak is. Wet only under a stop is the valve. Wet only under the trap after draining is a slip joint. Wet only under the pop-up when the sink holds water is the drain assembly. Wet only under the faucet center after handle splashing is the base gasket.

Stopping the damage while you wait for the fix

If you have found the source but cannot repair it this hour, reasonable stopgaps include shutting off the affected valve (hot or cold individually — you can live without one), closing the main while you wait on parts, or using the sink only lightly. A shallow bin or drip tray under the leak catches water and buys the cabinet floor some time.

The repairs most homeowners can handle

Replacing a supply line is one of the friendlier plumbing jobs. Close the stop, unscrew the old line from both ends, hand-thread the new one, snug each end a quarter turn past finger-tight with a wrench, and open the valve slowly while watching for drips.

Reseating slip joints on a p-trap is similar — unscrew, inspect the washer, replace it if it is hardened, reassemble without overtightening.

Replacing a pop-up drain assembly is more involved but achievable. A basin wrench or deep socket helps with the nut holding the drain flange from below. Fresh plumber's putty or a rubber gasket seals the flange to the sink.

Replacing a shutoff valve is the one most homeowners reasonably hand off. Compression-fit quarter-turn stops can be swapped with the house main closed, but if the existing stop is soldered to copper, it involves cutting, fitting, and confirming no drips under pressure. That is a fair line to draw.

When the cabinet base is already damaged

Minor surface staining on particleboard — dry, firm, not delaminating — can be sealed and painted, with a waterproof cabinet liner on top. If the base is soft or crumbling, no coating will save it. Either the base gets cut out and replaced, or the whole vanity does. Replacing a vanity while the plumbing is already apart is often the smarter sequence. If water reached beyond the cabinet, our guide on drywall damage after a leak covers what to check before closing the wall.

If the leak is the faucet itself

If water is showing up around the base of the faucet on top of the sink rather than below it, the o-rings or cartridge inside the faucet are worn. That is a different and usually inexpensive repair — our faucet dripping guide walks through the cartridge swap.

Preventing the next leak

The cheapest leak is the one you catch in the first hour. A small Wi-Fi water leak alarm set↗ tucked into the back of the vanity will ping your phone the moment the cabinet floor gets wet — long before the particleboard swells. Pair it with calendar-reminded supply-line checks every couple of years and most under-sink damage never gets started.

Rough cost ranges

These vary by region, access, and what comes apart easily.

• Supply line: DIY part often $10 to $25.
• Shutoff valve: DIY part often $15 to $30; plumber-installed often $200 to $400.
• Pop-up drain assembly: part often $20 to $60; plumber-installed often $150 to $300.
• Cabinet base replacement: often $300 to $800; full vanity replacement is higher.

A slow under-sink leak rarely gets better on its own. The sooner you pinpoint it, the less of the cabinet you lose.