Quick answer
Repair under-sink cabinet water damage in this order: poke the board to see whether it is still firm, dry the cabinet for two to three days, find and fix the leak, then repair to match the damage: an overlay panel over sound board, a new base shelf, or a replacement sink base cabinet. Painting over wet board fixes nothing.
Key points
- Swollen particleboard never shrinks back. The poke test tells you in ten seconds whether the board is staying or going.
- Dry the cabinet for two to three days with a fan before deciding anything, because wet board reads worse than it is and hides the leak.
- Every damaged sink base traces to the waste seal, the mixer tails or the dishwasher hose. Fix that first or the repair is temporary.
- The repairs, ranked: seal and move on, a 6 mm overlay panel, a new 16 mm base shelf, or a whole replacement sink base cabinet that unbolts from its neighbours.
- Sudden bursts and slow leaks are different animals to your insurer, so photograph everything with dates before you touch it.
The last owner painted the floor of this sink cabinet. You can tell: white enamel, thick in the corners, brush marks around the waste pipe. Straight through it, the damage has come back anyway. There is a puffed ridge along the front edge, a dark bloom in the back corner, and board that gives under a thumb. Paint is what people do to under-sink water damage when they are selling the house. It holds for about a month.
The sink base dies first in almost every kitchen we open up, because it is the only cabinet with services in it: hot and cold tails, a waste and trap, nearly always a dishwasher hose. If you caught the problem early the fix is small, and the early warning signs are worth learning for next time. This article is for the other case. The board is visibly damaged, and you want to repair it properly, once.
The order matters more than the products. Assess, dry, fix the leak, then repair to match the damage. Run those out of sequence and you will do the whole job twice.
Start with the poke test
Empty the cabinet and press the board hard with a flat screwdriver at the back corners, around the waste pipe, and along the front edge. Sound particleboard pushes back. Board that dents, flakes or feels like wet biscuit is finished, because once water breaks the resin bond, swollen particleboard stays fat and weak no matter how dry it later gets.
Then map how far the water went. Pull the kickboard off and put a torch on the floor under the cabinet: if water has run through the cabinet base onto the structure below, you need to know that before a new shelf goes in. Kickboards clip off and are cheap to replace in their own right, covered in replacing kickboards and toe kicks, so do not be precious about levering one away. Check the cabinet sides where they meet the base too. Swelling that has climbed the side panels changes which repair you are doing.
Dry everything before you decide
Wet board lies. It reads softer than it will be and it hides where the water came from. So strip the cabinet out, sponge up anything standing, wipe the trap and tails dry, and give it two to three days with the doors open and a fan pushing air in. A dehumidifier in the room shortens that in winter. Take the sink and dishwasher out of service while it dries and wash up somewhere else; a cabinet still in use never dries out.
Then poke again. Board that was only surface-wet firms up as it dries and may be worth keeping. Anything still spongy was done before you started. Do not seal, paint or cover anything while there is moisture underneath, because trapped water is how a tidy repair turns into a mould problem inside a closed cupboard.
Find the leak, or the repair is rented
Every damaged sink base has an author. Find it before you rebuild anything, and be systematic instead of hopeful: dry every fitting, lay a paper towel flat across the cabinet floor, then test one service at a time and read the towel.
- Fill the sink to the brim and let it drain in one go. A wet patch under the trap points at the waste and its seals, and a compression ring nipped up a quarter turn is often the entire fix.
- Leave everything dry overnight with the taps untouched. A drip line under the mixer in the morning means the tails or their seats are weeping under mains pressure, and a mixer that leaks at its base is often cheaper to replace than nurse; swapping a kitchen mixer is a short job.
- Run the dishwasher through a full cycle and watch its hose and connection, which live in the same cabinet and cause a share of damage that the sink usually gets blamed for.
If a fitting needs more than a gentle nip-up, that is a plumber's half hour, and it is cheap insurance against reading this article again next winter.
The repair order: assess, dry, fix the leak, then match the repair to the damage.
The repairs, ranked from least to most
Match the repair to what the poke test actually found, not to the worst photo on the internet. There are four honest levels, and the first one is close to free.
Level one: the board dried firm. Spend nothing on joinery. Fix the leak, run a bead of silicone around the pipe penetrations, fit a liner tray, and move on. This outcome is common, and nobody selling cabinets will push it, so consider it pushed here.
Level two: sound but stained or roughened board. Fit an overlay panel, a 6 mm sheet cut to the cabinet floor, dropped over the old base and sealed at its edges. It tidies the surface and adds a wear layer. The rule is that overlay only ever goes over sound board. Capping soft board with a nice panel is the plywood version of painting over it.
Level three: the base shelf is done but the sides stayed dry and square. The cabinet floor is a rectangle of 16 mm board, nothing more. It gets unscrewed or cut out and replaced with new white melamine-faced board, front edge taped, sealed at the pipe holes. A joiner does this in half a day; a confident owner with a jigsaw can too, if the fit around the plumbing does not scare them.
Level four: swelling has climbed the sides or the back. Replace the whole sink base cabinet. A sink base is screwed to its neighbours through the side panels and fixed back to the wall, so with the benchtop supported and the plumbing disconnected it comes out on its own, and a new carcass in moisture-resistant board goes onto the same footprint. Under a laminate top this is routine; a stone top needs propping and more care. Either way the job stays one cabinet wide.
| What the poke test found | The right repair | Typical scope |
|---|---|---|
| Firm board, ugly staining | Fix the leak, seal, fit a liner tray | An afternoon, parts only |
| Sound floor, rough surface | 6 mm overlay panel, edges sealed | An hour once the leak is fixed |
| Soft floor, dry square sides | New 16 mm base shelf | Half a day for a joiner |
| Sides or back swollen too | Replace the sink base cabinet | One cabinet, made and swapped |
And painting over it? Paint has no structure. It hides the evidence, seals damp in, and hands the problem to whoever opens that cupboard in a year. If you are selling, the building inspection will find it anyway. Stay, and you have wallpapered a wound.
Prevention that costs almost nothing
- Fit a liner tray with upstands across the cabinet floor. The next weep runs to the front edge where you see it, instead of soaking quietly into the back corner.
- Re-run the silicone where the sink rim meets the benchtop before it fails, the same discipline as resealing the splashback junction.
- Keep the trap visible. A cabinet packed to the ceiling with cleaning products is a cabinet where a drip gets six months of privacy.
- After any dishwasher or mixer swap, check the cabinet daily for the first week. New connections fail early or not at all.
- In a rental, put the sink base on every inspection checklist. It is what fails first in a rental kitchen, and a tenant will not report a smell.
When is under-sink water damage an insurance claim?
Insurers broadly split water damage into sudden events and gradual damage. A flexi hose that bursts and floods the kitchen on a Tuesday is the first kind, and policies commonly respond to it. The fitting that has wept for eight months is the second kind, and gradual damage is commonly excluded or limited. That surprises people, because finding the damage felt sudden even though the leak was not.
Be honest with yourself about which one you have, then be honest with the insurer, because assessors read swollen board the way you now can. Photograph everything with dates before you touch it, keep the failed fitting, and ring the insurer before any strip-out. Your policy wording decides the rest, and your insurer confirms cover; nothing in an article can.
Getting it fixed without drama
Three photos tell a joiner nearly everything: the cabinet floor from above with the doors open, the underside of the trap, and the space behind the kickboard. Add the cabinet width in millimetres and send it off. We reply within 24 hours with a reference number and will say plainly which of the four levels you are in, including level one, which costs you almost nothing and us the sale. Replacement shelves and one-off sink base carcasses come out of the East Tāmaki workshop, supply-only or fitted, and the leak still gets fixed first.
Frequently asked questions
Can under-sink cabinet water damage be repaired?
Usually, yes. If the board dries firm, you fix the leak and seal the surface. Rough but sound board takes a 6 mm overlay panel. A soft floor gets a new 16 mm base shelf. Only when swelling has climbed the sides does the whole sink base cabinet need replacing, and even that is a single-cabinet job.
How do you dry out the cupboard under a kitchen sink?
Empty it completely, sponge out standing water, wipe the trap and tails dry, then leave the doors open for two to three days with a fan pushing air in. A dehumidifier in the room speeds things up in winter. Dry it before repairing anything, because wet board reads worse than it is and sealing moisture in creates a mould problem inside a closed cupboard.
What usually leaks under the kitchen sink?
Three suspects cover most cases: the waste and trap seals, which show when you fill the sink and drain it in one go; the mixer tails, which weep under mains pressure and leave a drip line overnight; and the dishwasher hose or its connection, which gets blamed on the sink. Test one at a time over a dry paper towel and the towel will point at the culprit.
Can you replace just the bottom panel of a kitchen cabinet?
Yes. The cabinet floor is a rectangle of 16 mm melamine-faced board, and if the sides are still dry and square it can be unscrewed or cut out and replaced, with the front edge taped and the pipe penetrations sealed. It is about half a day for a joiner. If swelling has climbed the side panels, replace the whole sink base instead.
Can you replace one kitchen cabinet without replacing the run?
Yes. Cabinets are individual boxes screwed together through their side panels and fixed back to the wall. With the benchtop supported and the plumbing disconnected, a sink base unbolts from its neighbours and a new carcass goes onto the same footprint. Under a laminate top it is routine; under stone it needs propping and more care. The doors and decor are matched separately.
Does insurance cover water damage under the kitchen sink?
It depends on how the water got there. Sudden events, like a burst flexi hose, are commonly covered. Gradual damage from a slow leak is commonly excluded or limited, even though discovering it felt sudden. Photograph the damage with dates before touching anything, keep the failed fitting, and ring your insurer before any strip-out. The policy wording decides.
Is it OK to paint over a water damaged cabinet floor?
No. Paint has no structure, so soft board stays soft underneath, and sealing damp board also traps moisture and grows mould. Swollen particleboard cannot shrink back; it either dried firm enough to keep or it needs replacing. Paint-over is staging for a sale, and a building inspector will poke straight through it.