The Bottom of the Sink Cabinet Is Swollen

Before the board: stop the water, then find it

Sanding a swollen floor over a joint that is still dripping just gives the water a fresh surface to soak into. So the board waits. The first hour is about the plumbing, and the order matters, because a slow leak is easiest to find while the cabinet is still wet and hardest once everything has dried out.

1. Empty it and photograph it.Take everything out, mop up standing water with old towels, and take wide and close photographs before you change anything — the floor, the back panel, the pipes, and the floor in front of the toe kick. If this ever becomes an insurance conversation, the photographs of the damage as you found it are the ones that count.
2. Dry the obvious, then make the leak show itself.Wipe every pipe, nut and hose bone dry, and lay a sheet of dry paper towel flat on the cabinet floor under each joint. Run the tap hot and cold for five minutes. Fill the sink, pull the plug, and let a full basin drain. Run a short dishwasher cycle. Then look at the paper.Read the paperA dot directly under a joint is that joint. A wet line that starts at the back panel came from above it — the tap’s deck seal, or the seam between an undermount bowl and the stone — and ran down the inside of the back. Water always shows up at the lowest point it can reach, not where it started.
3. Check the usual suspects, top to bottom.The seal under the tap base, and the rim or seam of the sink. The two supply connections and their shut-off valves. The trap’s slip nuts and washers. The dishwasher’s drain hose where it meets the sink tailpiece or the disposer, and the loop that should lift it up under the counter. The disposer’s body and its outlet. Any filter housing or tank. Run a dry fingertip under each one; a joint that is weeping feels cool and slightly tacky before it ever looks wet.
4. Decide whose job the fix is.A trap slip nut that needs a quarter-turn, or a washer that has slipped, is yours. Anything carrying water under pressure — a supply line, a valve, a compression fitting that keeps weeping — belongs to a plumber; a supply line that lets go while you are out is a floor and a ceiling, not a cabinet. The things that are not yours to fix are the same list the trade uses.
5. Look under the box, not just in it.Flip base cabinets stand on adjustable legs behind a clip-on toe kick. Pull the toe kick off — it unclips — and shine a torch underneath. If water has run out of the back of the cabinet it will have gone down to the floor under the box, and on a cabinet that sits directly on the floor that is the part you would never see.

Then dry it properly: doors open, a fan blowing straight into the cabinet, a dehumidifier in the kitchen if you have one, and two to three days of patience. Do not seal, paint or cover anything while the board is still damp — you will be trapping the water you are trying to get rid of. Wet board feels noticeably cooler to the back of your hand than dry board in the same room; when the floor feels the same temperature as the side panels, it is dry enough to judge.

Why the board swells where it does

A Flip cabinet panel is a sandwich: a ¾″ (18 mm) core of plywood or particleboard with a melamine face fused to both sides and a PVC band on the edges that show. The melamine is a cured resin skin, not a coat of paint, and water sitting on it does nothing. The core underneath is wood — and wood drinks. So the board does not fail where the water sat. It fails where the water found a way past the skin.

• The holes you cut. The supply and drain holes in the back panel, and any hole drilled for a dishwasher hose or a filter line, are raw core with no skin at all. They are the most common entry point by a wide margin, which is why the swelling so often starts as a ring around a pipe.
• The edges nobody sees. The floor’s back edge and the joints where the floor meets the side and back panels are where a puddle sits longest, and a factory leaves some hidden edges unbanded on purpose. Water wicks in along the joint and lifts the floor from the back corners forward.
• Every screw hole. A screw driven through the floor — for a tray, a pull-out, a bracket — opens a little channel straight into the core. One slow drip landing on a screw head can swell a circle the size of a saucer around it.

What happens next depends on the core. Particleboard is wood chips pressed together with resin; when water reaches it, the chips swell and the bonds between them let go, and that swelling does not go back when the board dries. The tell is a dome or a ridge that stays put, and board that feels like cork under your thumb. Plywood takes on less water, swells less and recovers more of its shape as it dries, but a long soak can separate its layers, which shows as a blister you can press down and watch rise again. Either way the melamine face is just a passenger: it rides up on the swelling core and cracks or lifts where the curve is tightest. On the low-emission page there is a drawing of exactly where a panel is open to the air — it is the same map water uses.

A lip on hard board: flatten it, seal it, protect it

This is the repair most people need, and it is genuinely an afternoon. The swollen lip will not shrink back, so the aim is to take it down level with the sound board around it and close the edge it came through.

1. Make sure it is dry.A lip that is still damp will keep moving after you flatten it. If in doubt, give it another day with the fan.
2. Take the ridge down.Wrap 120-grit paper around a flat block and sand only the raised part, keeping the block flat on the sound floor either side so you cannot dish it. On a sharp ridge, a wide chisel held bevel-down and pushed, not tapped, shaves it faster. Stop when the block rocks no more. You will cut through the melamine on the ridge; that is expected.What you will seeParticleboard under the skin looks like coarse, pale grey-brown chips; plywood shows thin layers. If what you uncover is dark, crumbly or smells sour, it is wetter and softer than it felt — stop and treat it as grade C.
3. Seal everything raw.Every surface you sanded, and the edge of every pipe hole, gets sealed. Two coats of a water-based primer or sealer brushed on is enough; on a straight edge, an offcut of iron-on edge banding is better still, because it is the same material the factory used. Seal the pipe holes all the way round, including the back face if you can reach it.
4. Close the gaps around the pipes.A bead of flexible sealant where each pipe passes through the back panel stops water running down a pipe and into the hole, and keeps draughts and insects out of the cabinet as a bonus.
5. Put a tray down.A full-width under-sink tray or a moulded mat with a raised edge turns the next slow weep into a puddle you can see instead of a stain you find a year later.

Do not paint the whole floor to hide the patch. Melamine is engineered to reject coatings, and paint on it peels at the first scrape of a bottle — the painting page explains why. The tray covers the repair anyway.

A soft or bowed floor: give the cabinet a new one, on top

When the floor panel itself has gone soft, the instinct is to cut it out. On a cabinet that stands on legs, hold that thought. The adjustable legs under a base cabinet are fixed to the underside of its bottom panel, so the floor of the box is not just a shelf — it is what the whole box stands on, and what carries the countertop above it down to the legs. Cut it out and you have removed the thing the legs are attached to.

The repair that respects that is an overlay: a new, sealed floor laid on top of the old one, which stays where it is doing its structural job.

1. Make a template.Cut a piece of cardboard to the inside of the cabinet, notching it around the pipes and anything fixed to the back. Cardboard is free and forgiving; you can try it in, trim it, try it again.
2. Cut the new floor from it.A ¼″ to ½″ panel is plenty — exterior-grade plywood or a melamine-faced offcut. Thinner keeps the door clearance and the trap height almost unchanged; check that the bottom of the trap still clears the new floor by a comfortable margin.
3. Seal it on every face and edge before it goes in.Two coats on the edges especially. This is the whole point: the new floor should be the most water-resistant thing in the box.
4. Flatten the old floor enough to sit on.Knock down any dome with the sanding block so the overlay lies flat and does not rock. It does not need to be pretty, only dry and flat.
5. Drop it in, and keep it removable.Lay it loose, or hold it with a few short screws near the front where you can reach them. A floor you can lift out is a floor you can look under next time something drips.
6. Seal the perimeter and add the tray.A thin bead of flexible sealant where the overlay meets the side and back panels stops water getting underneath it. Then the tray on top.

Swollen into the sides: replace the box, not the kitchen

Once the swelling has reached the side panels or the joints, the box has lost the thing that held it square, and no patch will give it back. The good news is that a kitchen is a row of separate boxes. You are replacing one of them, and on frameless cabinets the parts that people actually see — the doors and the false front — are separate from the box and can usually go straight back on.

• Order the same cabinet. The code of your sink base — SB36, SB30 and so on — is on your design drawing and in the saved design. The same code is the same width, height and depth, so the countertop, the neighbours and the plumbing all line up as before.
• Keep your doors. The interior of every box is the same white melamine, and each door comes off by loosening two screws per hinge. Rehang your existing doors on the new box and the colour question disappears, because nothing you look at has changed.
• Seal the new box before it goes in. Cut the pipe holes in the back panel between the mounting rails — never through a rail — using a cardboard template of the pipes, and seal their edges while the box is still on the floor. You are fixing the cause, not just the casualty.

How the old box comes out depends on the sink, and it is worth being honest about this before you start.

1. Shut the water off and disconnect.Close both supply valves, disconnect the supply lines, the trap and the dishwasher drain, and unplug a disposer at its outlet. Have a bucket and towels under everything; traps hold water.
2. Free the box from its neighbours and the wall.The dishwasher is usually screwed to the side of the sink base through its side brackets — undo those and slide the dishwasher forward a few inches. Back out the screws that join the sink base to the cabinets either side (they are driven sideways through the side panels from inside the box) and the screws through the back mounting rail into the wall.
3. If the sink drops in from above, lift it out.Cut the sealant around the rim, release the clips underneath, and lift the sink out through the countertop. With the bowl gone, the box can come forward and the new one go back in, and the countertop never moves. This is the straightforward case.
4. If the sink is undermounted, stop and look.An undermount bowl is glued and clipped to the underside of the stone and hangs down inside the cabinet, so a whole box cannot slide out past it. The old box usually comes out in pieces, and the new one has to be fitted around the bowl — or the stone shop supports or lifts the top. If the sink is held up from below by the cabinet itself, on a cradle or on rails fixed to the side panels, as heavy fireclay and cast-iron sinks must be, the cabinet is load-bearing and the sink has to be propped before anything is undone.Honest scopeThis is the one job on this page where a finish carpenter, or the stone shop, earns their fee. A cracked slab is not repairable and you cannot re-order half of one.
5. Set the new box on the line.Slide it in, turn its legs up until the top of the box meets the underside of the countertop and the bubble sits dead centre both ways, clamp its front edges flush with the neighbours and screw them together sideways, then screw through the back mounting rail into the studs. The base install section is the same sequence, done once.
6. Reconnect, then watch it.Doors back on, dishwasher brackets back in, plumbing reconnected — the trap yourself, the supply lines yourself only if you are confident, otherwise the plumber who fixed the leak. Put dry paper towel on the new floor and check it after an hour, a day and a week.

Mould, and the musty smell that will not leave

A sink cabinet that smells musty after the leak is fixed and the board is dry is telling you the water went somewhere you have not looked. Melamine itself gives mould very little to live on; the raw core of a swollen panel, the paper face of the drywall behind the cabinet, and the subfloor underneath it give it plenty.

• Look behind. Shine a torch through the pipe holes at the wall behind the back panel. Dark staining or a soft, bulging patch of drywall means water ran down behind the cabinet.
• Look under. With the toe kick off, look at the floor beneath the box and at the bottom of the back panel from below. Discoloured flooring, cupped boards or a laminate floor lifting at its joints mean the water got under the cabinet and into the floor.
• Clean what you can see. A small patch of mould on a melamine face wipes off with a mild detergent solution and a cloth; dry it properly afterwards. Do not soak raw board with water or bleach — you are adding the problem back in.
• Know where your job ends. Mould on drywall, in the subfloor, or over more than a small patch, and any smell that comes back after cleaning, is for a remediation contractor and possibly your insurer. The same goes for anyone in the house with asthma or another breathing condition.

One smell is not damp at all. A brand-new cabinet that has been shut for a week can smell sharp when you open it, and that is the finish and the board airing out, not mould — the low-emission page covers why a closed box concentrates it and how to air it. Musty and earthy means water; sharp and chemical means new.

Make the next leak a puddle you can see

Leaks under a sink are not rare events; they are a maintenance item. The aim is not to make the cabinet waterproof, which it cannot be, but to make sure the next drip lands on something that shows it and does not drink it.

• Seal every cut before the cabinet goes in. Pipe holes, notches, a trimmed back panel: a few minutes with primer or a banding offcut while the box is still on the floor. It is the single biggest difference between a sink base that lasts and one that does not.
• Tray, then alarm. A full-width tray with a raised lip under everything, and a battery water-leak alarm sitting in it. The alarm costs very little and turns a slow weep into a beep the same day.
• Check the joints on a schedule. A week after install, again after a month, then whenever you change the bin bags: a dry fingertip round every nut. Slip nuts on a trap settle in the first weeks of use.
• Keep the valves reachable. Plan the bin, the filter and the cleaning caddy so you can get a hand to both shut-off valves without emptying the cabinet. In a leak, that is the difference between seconds and minutes.
• Mind the dishwasher’s drain loop. The drain hose should rise up under the countertop before it drops to the sink connection. Without that loop, dirty water can siphon back, and a hose that sags across the cabinet floor is one more thing lying in the wet.
• Keep bottles in a caddy. Cleaning bottles drip from the cap. A caddy keeps those drips off the floor and lets you lift everything out in one go to look.

If you are planning the kitchen rather than repairing it, the sink guide has the decisions that keep the inside of this cabinet sane — disposer, trap height, filters — and pipes where the cabinet goes covers cutting the back panel neatly in the first place.

When it is not the cabinet any more

Sometimes the cabinet floor is the least of it. These are the signs the water went further, and that the cabinet repair should wait until somebody has looked at the house:

• The kitchen floor has moved. Hardwood cupping or crowning in front of the sink, laminate or vinyl plank lifting at the joints, or tile that sounds hollow when you tap it with a knuckle.
• There is a stain downstairs. A ceiling mark under the kitchen means the subfloor has been wet long enough to pass water through.
• The wall is soft. Drywall that gives under a thumb behind the cabinet, or paint blistering on the wall beside it.
• The leak was in a pressurised line. A supply line or valve that has been weeping under pressure can put out far more water than a drain, and much of it goes where you cannot see.

In any of those cases, photograph everything, call your insurer before you throw anything out, and get the plumber, the flooring and, if needed, a remediation contractor in before the new cabinet goes in. Putting a fresh box over a wet subfloor is how you end up doing this page twice.