A cabinet floor that has gone soft and dark under a kitchen sink has been wet for a long time. Panels do not swell from one spill. They swell from water arriving, drying partway, and arriving again, until the panel stops being a panel.
What the stain is not is a map. Where the board is darkest tells you where water pooled, and water pools at the low point of a cabinet floor, not under the thing that dropped it. That is why the most common wrong repair here is a new trap on a leak that was never in the drain.
What the swelling is telling you, and what it is not
The bottom of a sink base is usually a manufactured panel — particleboard or MDF under a melamine skin, sometimes plywood in a better box. The skin is the only water resistance it has, and the skin stops at the edges. Water gets under it at a cut edge, a screw hole, the notch cut for the drain, and the core takes it up. Plywood swells and mostly dries back. A chip core does not; the resin bond releases and the panel goes to the texture of oatmeal. Which one you have decides whether this is a cleanup or a rebuild, and what a cabinet box is made of is the short version of why.
Before you touch anything, trace the darkening. A stain that fans forward from the back wall came from the back. A stain that starts in a front corner, right behind the face frame, came down the inside of the cabinet front. A ring under the trap came from the trap. It is crude, and it stops working once the panel is soaked through, but it is free.
Dry it out, then lay paper
You cannot read a wet cabinet. Everything looks like the source. So the first move is not a repair, it is a reset.
- Empty the base completely — everything out, including the trash pull-out.
- Dry it. Doors open, a fan pointed in, until the panel is dry to the touch and stays dry overnight.
- Lay plain paper across the whole base. Printer paper, or paper towel laid flat. Tape a strip up the back wall and one down the inside of the front. Paper takes a mark where a drop lands and keeps it. A damp panel just spreads.
- Wait, and use nothing. If paper marks with no fixture run at all, the leak is on the pressure side and it runs whether you use the sink or not: a supply riser, a shutoff, a braided line, or the small tube feeding a fridge or a filter that also lives in that cabinet.
- If the paper stays clean, run one thing at a time, replacing the paper between each. Faucet hot. Faucet cold. Sprayer pulled to full extension and held. Both bowls filled to the rim and released. Disposal. Dishwasher, a whole cycle, with nobody touching the sink.
Pressure side or use side is the first fork. Running the fixtures separately splits the rest. Everything below is how to read the mark.
From above: the faucet base, the deck plate and the sprayer hose
This is the one that gets misdiagnosed, because the water lands nowhere near the faucet.
A faucet passes through the deck and is pulled down from underneath by a nut or a bracket, sealed by a gasket or by putty. That seal hardens, the nut backs off, and the shallow puddle that stands around the base of every kitchen faucet finds the gap. It never drips straight down. It runs down the outside of the faucet shanks, along the underside of the sink deck, then along whatever is closest — a supply line, the sprayer hose — and leaves at the lowest point of that path. Usually the back of the cabinet, at the wall. It reads exactly like a leak in the wall.
The deck plate on a three-hole install does the same thing on a bigger scale. It covers two open holes, it is sealed underneath by a bead someone put on years ago, and water that gets past that bead has two holes to go through.
A pull-down sprayer adds a hose and two more joints, one at the wand and one where the hose meets the valve body, plus the counterweight riding on the loop. Those only see pressure while the trigger is held. Then the hose runs wet and drips off the bottom of the loop. The parts that wear on a faucet are covered in what to look for when you choose one; the hose and its quick-connect are near the top of that list.
How you tell
Dry the underside of the deck. Wrap a paper towel snugly around the faucet shanks, above the mounting nut, and another around the sprayer hose at the bottom of its loop. Then, without running the faucet at all, pour a cup of water around the base of the faucet the way it gets there in real life. If the towel on the shanks marks, the deck seal is gone, and no work on the drain will change anything.
Then run the faucet with the drain plugged and the bowl filling, so nothing goes through the trap. Wet shanks now, after a dry result on the pour test, points at the faucet body or the supply connections instead. Last, pull the sprayer out to full extension, hold it, and look at the counterweight.
The drain side, dry every time you look
The second family only leaks while something is draining, which lasts seconds. By the time you get a flashlight into the cabinet it has stopped and the drips have run off to the low corner. This is why people tell us the cabinet is dry every time they check and the floor keeps getting worse.
Three joints do most of it. Slip joints — tailpiece to trap, trap to arm — are a plastic washer squeezed by a nut, and a washer that has taken a set weeps a few drops per use. A disposal has its own set: the mounting flange at the sink, which leaks when the bowl holds standing water; the gasket at the discharge tube; and the inlet nipple where the dishwasher hose lands. A disposal wet at the seam near the motor housing has failed internally, and that unit gets replaced, not repaired.
The dishwasher drain hose is the sneakiest of the three. Its pump runs several times in a cycle, under real pressure, usually when you are not in the room. A loose clamp at the disposal nipple puts a little water in the same corner every load and nothing at any other time.
Behind all of it is the branch drain in the wall — the trap arm connection, the sanitary tee. If the mark is at the back wall and only appears when the sink drains, that is a plumber's job, and it involves opening the wall to see. We do not pretend otherwise.
How you tell
Fill both bowls to the rim and pull the plugs together. A running tap barely loads a drain; a full bowl released loads it properly, and slip joints that hold under a trickle let go under that. Then run the disposal with water. Then fill a bowl and let it stand, which is the flange test. Then run the dishwasher all the way through with the sink untouched.
Read the paper between each. Under the trap is a slip joint. Directly under the drain opening is a basket strainer or the disposal flange. At the back wall, with the towels on the trap dry, means the wall.
The rim: caulk under a drop-in, clips under an undermount
The third family never involves plumbing at all. It is the joint between the sink and the counter.
A drop-in sits on the counter with a bead of silicone under the rim, and that bead is all that stands between the countertop cutout and the water you push around while washing up. When it fails, water goes under the rim, over the raw cut edge of the substrate, and down the inside face of the cabinet front. Front corner wet, back dry, plumbing dry. On a laminate top that cutout edge is bare particleboard, and swelling at the counter edge around the sink is the same water on its way through.
An undermount hangs from the underside of the top on anchors set into the stone, with clips drawn tight and silicone as the seal rather than the support. Its failure is invisible from the room: the bead releases somewhere along the back, water runs over the cut edge and down the outside of the bowl, and drips off the low point of the bowl. If the reveal has changed, or the sink moves when you push up on it from below, clips have let go — and that is the counter fabricator's work, not a plumber's, because the anchors are epoxied into stone. How the two mounts differ, and why the counter usually decides which one you have, is in the sink post.
How you tell
Dry the counter, dry the cabinet, lay fresh paper, and never turn the faucet on. Pour water slowly along the rim joint, a cup at a time, working around the sink, front edge first — that is where dishwater actually goes. If paper marks with the plumbing untouched, you have found it. On an undermount, feel the underside of the counter around the cut edge with a dry hand after the bowl has stood full, and push up on the bowl to check for movement.
What is a Saturday and what is a phone call
Some of this is genuinely a homeowner's morning. A slip nut that needs a new washer and a snug. A braided supply line swapped at the shutoff. A hose clamp on a dishwasher drain. Cutting out a failed bead under a drop-in rim and replacing it, which is tedious rather than difficult.
The line we would draw is the shutoff valve. If the stop under the sink will not turn, or starts weeping at the packing nut the moment you touch it, stop. That is no longer a supply line job. It is a job that begins with shutting the house off at the main, and it belongs to a plumber. So does anything at the branch drain, anything inside the wall, and any leak you cannot make happen on demand with the paper test. A disposal wet at its own body seam is a replacement. An undermount that has dropped goes back to whoever fabricated the top.
We are a remodeling contractor. Plumbing work goes to a plumber, and we say which parts those are before anything is quoted rather than after.
When the cabinet floor itself is gone
Fixing the leak and drying the box is the whole job when the panel is plywood and the swelling is mild. When a chip core has gone spongy, drying brings nothing back.
The awkward part is that a cabinet bottom is captured. It sits in dados or on cleats between the two gables with the face frame across the front, so it does not lift out. There are three honest ways forward. Scribe a new panel over the top of the old one, which is a patch — it hides the damage, it raises the floor inside the cabinet, and it does nothing at all if the source is still live. Cut the failed bottom out in pieces and fit a new one, which is real work in a cramped box with plumbing in the way. Or replace the sink base, which means the countertop comes up over that bay, and on a stone top that brings a fabricator, a lift and possibly a seam decision.
Under all of it sits the question nobody can answer from the room: how far the water went. The toe kick, the subfloor under the cabinet, the bottom plate of the wall behind. A soft spot in the subfloor is a different job again, and a floor that is no longer flat has to be dealt with before anything new goes over it. It has to be opened up to know. Anyone quoting the repair without pulling the cabinet or cutting the toe kick is guessing, and we would rather say that than write a number.
If the run is coming out anyway
Sometimes the sink base is the worst of a kitchen that was already on the list, and once the counter is off that bay the conversation stops being a repair. Independent research on this market — Angi, Homeyou and Block Renovation — puts Indianapolis kitchen projects at roughly $18,000–$24,000, with a full spread of about $8,000–$40,000, and one national dataset puts the average Indianapolis kitchen at about $25,000, roughly 46% below the national average.
That is a decision, not a leak, and the two are worth keeping apart. Stop the water first. The kitchen remodeling page covers what actually moves the number if you get there.
Mold, and what we do not do
Wet panels in a dark closed box grow things, and people ask us about it constantly. We do not test for mold, we do not identify it, and we do not remediate or abate it. Nothing we say about a swollen panel is a judgment about the air in your house.
If that is the worry, the answer is testing by someone who does it for a living, and the guidance to read is the EPA's and your county health department's rather than a contractor's. What we do is the building side: find the water, stop it, rebuild what it destroyed. It is the same division we describe in what has to happen when mold turns up before framing.
The short version
Dry the cabinet, lay paper, run the fixtures one at a time. A mark with nothing running is the pressure side. A mark at the back wall after the faucet runs is usually the faucet deck, not the wall. A mark that only appears when something drains is a slip joint, a disposal gasket, or the dishwasher hose. A mark at the front corner with the plumbing dry is the rim.
Then fix the water before you fix the wood. A new cabinet bottom under a live leak is money spent twice.
If you want someone to pull the cabinet and look at what is under it, ask for an estimate and we will walk it with you.
