Key takeaways
- A soft or swollen floor in the sink cabinet is particleboard or plywood that took on water from a slow leak, and particleboard does not shrink back flat after it dries.
- In most kitchens the leak comes from one of four places: the supply lines, the faucet base, the drain trap or the dishwasher hookup.
- Find and fix the leak first, then dry the cabinet out for a few days before you decide anything.
- A swollen floor panel on its own can be replaced, but soft sides, a swollen face frame or mold on the back wall mean a new sink base cabinet.
You reach under the kitchen sink for the dish soap and the bottom of the cabinet gives under the bottle. The front edge has puffed up like a paperback left out in the rain, and there's a dark ring where the trash can sits. In a 1968 Anderson Township ranch whose kitchen was redone in the 1990s, there's a good chance that floor is particleboard, and the water came from a slow drip somewhere above it. The drip is the first thing to find, and the cabinet comes second.
What the floor of the sink cabinet is made of
A lot of builder-grade cabinets have a sink base built from particleboard: wood chips pressed together with glue, with a thin vinyl or melamine skin on top. Water that sits at a cut edge, a screw hole or the seam at the back gets into the chips, and the board swells.
Particleboard doesn't fully go back. In a 1973 Forest Products Journal study of commercial particleboards, boards cycled from dry air to damp air, not soaked in water, swelled 5.5 to 15 percent in thickness, and a third to a half of that swelling stayed after the boards dried. A floor that sits under a drip for months can end up crumbling at the edges like wet cake.
An original 1960s sink base may be plywood or solid boards. Those last longer under a leak, and they still rot once they stay wet.
Four places a slow leak under the sink comes from
Slow leaks under a sink start at the joints, and a kitchen sink has four sets of them.
| Leak source | Where the water lands | How to check it |
|---|---|---|
| Supply lines and shutoff valves | Back wall, under the valves | Dry tissue along each line and nut |
| Faucet base | Under the faucet, at the back | Splash the deck behind the faucet, then look up |
| Drain trap, strainer and disposer | Middle of the cabinet floor | Fill the sink, pull the plug, watch the joints |
| Dishwasher hookup | The side next to the dishwasher | Run a cycle and check the hose clamp |
Supply lines. The two hoses from the shutoff valves up to the faucet have a nut at each end, and a nut can weep for months before anyone notices. A shutoff valve that someone turns for the first time in a decade can start weeping at the handle stem.
The faucet base. A rubber gasket seals the faucet to the sink or countertop. Once it flattens, water that splashes behind the faucet runs down through the mounting hole and drips off the underside.
The drain trap. The U-shaped pipe under the sink is held together with slip nuts that work loose over time. It leaks while the sink drains, so the floor under it is dry most of the day. The strainer where the drain meets the sink bowl is sealed with putty or a gasket that can dry out, and a garbage disposer adds two more joints.
The dishwasher hookup. The drain hose from the dishwasher clamps onto the disposer or a fitting on the drain pipe, and the dishwasher's water line may tee off the hot shutoff under the sink too. Whirlpool's guidance is that the drain hose should rise at least 20 inches off the floor in a high loop, or run through an air gap, so dirty sink water doesn't back into the dishwasher. The Insurance Information Institute says to check dishwasher hoses once a year and to replace them every five to seven years.
A weekend check for the sink cabinet
- Empty the cabinet, trash can included, and dry the floor and pipes with a towel.
- Lay a flattened brown paper bag on the floor under the pipes. A drop shows up on it as a dark spot.
- Run hot and cold for a minute each, splash water on the counter behind the faucet, and run a dry tissue along both supply lines and the faucet's underside.
- Fill the sink a few inches, pull the plug, and watch the trap and disposer joints while it drains.
- Run the dishwasher and check the hose connection when it pumps out.
- Look at the bag the next morning. The spot usually lands under the leaking joint, but water can run along a pipe before it drops, so check the joints above and behind it too.
With the cabinet empty, press a thumb into the floor, the bottom of the side panels and the face frame (the strip of wood around the doors). Check the back wall for black spots. A musty smell with nothing visible can mean mold behind the back panel.
If the house has a basement, look at the ceiling under the sink with a flashlight for a stain on the subfloor, the plywood or boards under the kitchen floor. The soft floor next to a bathtub walks through the same basement check for a bathroom.
Fix the leak, then dry the cabinet out
Most of these are small fixes. A loose slip nut can need nothing more than a hand-tightening, and a weeping supply line is an hour's job for a handy person. A flattened faucet gasket or a valve that won't stop weeping is plumber work.
Then dry it. The EPA says to dry water-damaged areas within 24 to 48 hours to prevent mold growth. Leave the doors open and point a box fan into the cabinet for a few days before anything goes back in.
After that, a plastic sink base mat catches the next drip, and a battery water sensor on the floor beeps when it gets wet.
When a repair works and when the sink base needs replacing
A repair works if all three of these are true:
- The leak is fixed and the cabinet has dried out.
- The floor panel is the one part swollen or soft. The sides and face frame are firm and the doors close.
- There's no mold on the back of the cabinet or the wall behind it.
A carpenter can cut out the old floor and set a new plywood panel on cleats. Nobody sees it, so it doesn't have to be pretty.
The sink base needs replacing if any one of these is true:
- The side panels are soft or swollen at the bottom.
- The face frame has swelled and the doors no longer hang or close right.
- Screws pull out of the floor or the back corners crumble.
- There's mold on the back panel, or the drywall behind it is soft or stained.
Replacing one sink base means the sink, faucet and plumbing come out, and the countertop may have to be lifted. The harder part is matching 30-year-old doors, and that's where many couples decide between one new cabinet and new faces on all of them.
Before anyone tears out the cabinet, take photos and call your insurance agent. The Insurance Information Institute says a standard HO-3 policy covers accidental discharge of water from a plumbing system. Ask which side of that line a drip that ran for months falls on.
If the sink base has to go
Bulldog's kitchen work includes new and refaced cabinets, sinks and faucets, and countertops. If the rest of your cabinet boxes are solid, refacing gives them new doors, drawer fronts and veneers. If they're worn or damaged, we'll say so and recommend replacement. Whether your cabinets can be refaced covers how to check the boxes. Many kitchen updates take a few days, and a consultant measures and gives a written quote. The cabinets page shows the door styles and how the boxes are built.
If the cabinet is dry and firm, do the paper bag check, fix the joint it points to, and put a mat down. If the floor by the tub upstairs feels soft too, the signs your shower needs replacing covers the bathroom side.
A new-looking kitchen in days, not months walks through every part of the kitchen in order: what to keep and what to replace.
