Can I keep using the sink until the plumber comes?

Not if the leak is at the supply lines or valves, since those stay under pressure all the time. If water only drips while the sink drains, you can close the valves and avoid that sink. Keep the dishwasher off too, because it usually drains through the same pipes.

The cabinet floor is swollen but feels dry now. Can I leave it?

The swelling tells you water sat there, and pressed-wood panels usually stay swollen after they dry. The bigger question is what's underneath. Have the space under the cabinet and behind the toe kick checked with a moisture meter before you cover it or replace the panel.

Why does the cabinet under my sink smell musty?

Something has probably stayed damp. The Smart Home Water Guide lists a musty smell under sinks as a sign of a leak, and the EPA says you may suspect hidden mold when a building smells moldy but you can't see the source. Look for a slow drip, then have the cabinet floor, back panel and toe kick area checked.

Could the water be coming from the dishwasher or the refrigerator?

Yes. Dishwasher supply and drain lines usually connect under the sink, and an ice maker line sometimes branches off there too. The Smart Home Water Guide notes that some leaks only happen while a device is running, so watch the cabinet with a flashlight while the dishwasher fills and drains.

Does insurance treat a slow drip under the sink like a burst supply line?

It may not. The Insurance Information Institute says the common HO-3 policy covers accidental discharge of water from a plumbing system but excludes water seepage, and that policies don't cover damage due to lack of maintenance. Photograph the damage, note when you first saw it and ask your insurer.

Water Under the Kitchen Sink: Cabinets, Floors and Mold

Stop the water and clear the cabinet

Close the valves under the sink. There’s usually one small valve on the hot line and one on the cold line, where they come out of the wall or floor. Turn them by hand until they stop. The Smart Home Water Guide, from the City of Phoenix and other Valley water providers, warns that old or corroded valves can break. If they won’t close, or the leak is upstream of them, close the main shut-off for the house. Our shut-off valve guide shows where that usually is.
Watch for wet electrical. A garbage disposal or dishwasher may be plugged into an outlet inside the sink cabinet. The U.S. Consumer Product Safety Commission warns not to touch wet appliances that are still plugged in. If the outlet or plug is wet, switch off that circuit at the breaker panel from a dry spot.
Empty the cabinet. Set things on towels in another room. Cleaning products and anything soggy go somewhere children and pets can’t reach.
Take photos of the cabinet, the leak point and the floor in front of the cabinets before you wipe anything up. Arizona’s Department of Insurance and Financial Institutions advises taking pictures before any plumber or restoration contractor shows up.
Leave the doors open and put a towel or a shallow pan under the drip.

Find where the water is coming from

If the leak is small and the cabinet is safe to reach, you can narrow down the source before the plumber arrives. Dry every pipe and connection with paper towels, then check one thing at a time with a flashlight. The Smart Home Water Guide notes that leaks may be intermittent, happening only while a water-using device is running, and suggests running devices such as a dishwasher to see if a leak occurs.

Supply lines and valves

The flexible lines that run from the valves up to the faucet are under pressure all the time. A drip at a nut, a crack in a line or a weeping valve stem leaks whether or not the faucet is on, and it can keep going for weeks.

Drain connections

The drain pipes under the sink, including the curved trap, only carry water when something drains. A loose connection there leaks when you run the faucet, empty a full sink or run the disposal, and stays dry the rest of the time.

The faucet and the counter

Water can run down from a loose faucet base or a worn seal and drip from the underside of the counter. Splash around the sink can also get through a gap in the caulk.

The dishwasher

The dishwasher’s supply line and drain hose usually connect under the sink. A leak at either one can run into the sink cabinet, or under the dishwasher and into the next cabinet, without showing in the kitchen.

The refrigerator line

An ice maker or water dispenser line sometimes branches off a valve under the sink and runs through the cabinets to the fridge. These lines are thin and easy to kink or crack when the fridge is pulled out. A drip along that path may show up under the sink, or nowhere you can see.

Sweating pipes and the wall behind

If every connection is dry, look at the wall behind the sink and at cold pipes. A leak inside the wall can wet the back of the cabinet first. Cold pipes can also collect condensation, and the EPA advises acting quickly to dry the surface when moisture collects on pipes.

The Smart Home Water Guide’s checklist for sinks is short: “Check under and around sinks for wet spots, a musty smell or bowed cabinetry.” It also warns that water-using devices “can leak and cause damage to walls and floors, potentially creating an environment for mold or mildew.”

A plumber repairs supply lines, valves, drains and faucets. An appliance technician or plumber handles the dishwasher and the refrigerator line. Neither one dries the cabinets or the floor.

Why the cabinet floor swells

Many sink cabinets have a bottom panel made of particleboard or a similar pressed wood, covered with a thin laminate or vinyl layer. Once water gets past that layer, the board soaks it up, swells and tends to stay swollen and crumbly even after it dries. Plywood and solid wood hold up better, but they can still warp, stain or grow mold.

A swollen panel tells you water sat there. It doesn’t tell you how far the water went.

Water you can’t see: behind the toe kick and under the floor

The toe kick is the recessed strip along the bottom of the cabinets. Behind it is a hollow space between the cabinet bottom and the floor. Water that runs off the cabinet floor collects there and can spread under the neighboring cabinets and the dishwasher, out of sight.

From there it can:

Run under the flooring. For linoleum, ceramic tile and vinyl, the EPA’s cleanup table says to check that the underflooring is dry and dry it if necessary. Under laminate and vinyl plank, water can travel along the seams.
Wick into the wall behind the sink, where the back of the drywall stays wet.
Reach the next room if the kitchen floor runs into it.

Signs to look for: soft or discolored flooring near the toe kick, a floor edge that has lifted or cupped, a swollen baseboard at the end of the cabinet run, and a musty smell when you open the cabinet. A moisture meter, and often a thermal camera, can show what’s behind the toe kick before anything is torn out.

Mold under the sink

A sink cabinet is closed and dark, and a slow drip can run there for weeks before anyone notices. The EPA lists areas inside walls around leaking or condensing pipes, and the back side of drywall, among the places mold can hide. It also says you may suspect hidden mold “if a building smells moldy, but you cannot see the source.”

The EPA advises drying water-damaged areas and items within 24 to 48 hours to prevent mold growth. It says a moldy area smaller than about 10 square feet can usually be handled by the homeowner, but what shows on the cabinet floor may not be all of it. The EPA also warns that investigating hidden mold “will require caution when the investigation involves disturbing potential sites of mold growth,” and suggests an experienced professional if you think you have hidden mold.

Our guide to the signs of hidden mold covers what to watch for, and mold remediation explains what the cleanup involves. If you have health concerns about mold, talk to a doctor.

Dry it or replace it?

That depends on the material, how long it was wet and whether mold has started.

Often dried in place: firm cabinet sides and backs, plywood, solid wood and the subfloor. For wood surfaces, the EPA’s cleanup table says to remove moisture immediately and dry with dehumidifiers, gentle heat and fans. To reach the space underneath, a provider may pull the toe kick, drill small holes or remove the cabinet bottom.
Usually replaced: a pressed-wood cabinet floor that has swollen or crumbles, drywall that’s swollen or has come apart at the seams, and porous materials with mold that can’t be cleaned. The EPA says absorbent materials “may have to be thrown away if they become moldy.”
Sometimes the whole cabinet: if the box itself is damaged, or the flooring under it has to come up, the cabinet may have to come out, and the countertop with it.

Don’t close things up early. A new cabinet floor or a toe kick put back before the subfloor and wall read dry can trap the moisture. The structural drying page explains how a provider measures dryness, and our guide on how long water damage takes to dry explains why there’s no fixed number of days.

Some restoration providers also remove and reset cabinets and countertops. Others stop once everything is dry. Ask which one you’re talking to. Our page on water heater and appliance leaks covers dishwasher and refrigerator line leaks in more detail.

Questions to ask the provider

Where did the water go: under the cabinets, under the dishwasher, under the flooring, into the wall?
Can the cabinets be dried in place? If parts need to come out, which ones, and why?
Do you remove and reset cabinets and countertops, or will I need someone else for that?
How will you check for mold behind the cabinet back and under the floor?
Will you share moisture readings and photos I can give my insurer?
What’s your Arizona contractor license number? Look it up with the Arizona Registrar of Contractors before work starts.

Water damage under the kitchen sink: what to check and do