How to Fix Swollen, Cupped and Warped Wood Floors
To fix swollen, cupped or warped floorboards, dry the floor completely, wait for the boards to settle, and only then sand, repair or replace what hasn't recovered. Excess moisture is the biggest threat to a wood floor, whether it comes from a plumbing leak, a flooded washing machine or just persistently high humidity, and the boards respond by changing shape. Minor movement often reverses if you catch it early, but badly buckled boards have to be replaced. This guide explains what each type of movement means, how to dry wet floorboards, how to fix the damage and how to stop it coming back. If water has only just got onto or under the floor, start with our guide on what to do when water gets on or under a wood floor.
Why are my floorboards swelling, cupping or lifting?
Wood expands when it absorbs moisture and shrinks as it dries, and solid hardwood shows this most. When the moisture level is too high for the floor, the boards change shape in recognisable ways:
| What you see | What it's called | Likely cause |
|---|
| Board edges curl up higher than the middle | Cupping | Moisture below or between the boards, such as a slow leak, damp subfloor or humid room |
| The middle of the board bulges above the edges | Crowning | Moisture on the surface, or a cupped floor that was sanded before it dried |
| The floor swells and lifts in the middle of the room, or boards lift off the subfloor | Buckling or bulging | Heavy or prolonged moisture, or no expansion gap left along the walls |
| Boards swell so tight that joints squeeze together | Swelling | The wood has absorbed water and expanded |
Left alone, the moisture can also cause staining, a musty smell and eventually permanent structural damage. Cupping and crowning are both forms of water damage, but the cause isn't always a leak, which is why finding the real source matters.
How to dry wet floorboards
Drying is the most important step, and it works best when you do it quickly and patiently. For water under the boards, treat the void as part of the job, because the underside and the subfloor dry last.
- Stop the source of the water and remove standing water with a wet vacuum or towels.
- Open windows and doors to bring in air, and run fans so that air moves across the floor.
- Run a dehumidifier in the room. It pulls moisture out of both the air and the wood.
- If water is trapped under the floor, lift a few boards or the skirting, if you can do it safely, so air can circulate in the void and dry the underside.
- Check progress with a moisture meter, comparing readings with a dry area of the floor.
- Keep going until the readings match the dry reference and the boards have stopped moving.
Don't use direct heat, such as heaters or heat guns, because forcing the drying can crack the boards. Drying usually takes days to a few weeks, and the floor often looks worse before it starts to flatten. Dehumidifiers also help to prepare a room before sanding or restoration work, but they're a short-term tool and not a permanent fix for a damp room.
Can cupped or swollen floorboards be fixed?
Often, yes, and the first treatment is just time. Minor cupping can reverse on its own once you remove the moisture source and the floor dries, so give it a few weeks before deciding it needs work. Wait until the floor is fully dry and flat before you sand it, because sanding a damp, cupped floor can leave the boards crowned once they dry out. If the boards stay cupped or crowned after drying, they usually need sanding and refinishing, and boards that are too far gone have to be replaced.
| Condition | What to do |
|---|
| Boards are flat but stained | Treat the stains, then sand, re-stain and seal |
| Light cupping that eases as the floor dries | Wait for it to flatten, then sand lightly and refinish if needed |
| Cupping or crowning that stays after drying | Sand the floor flat and refinish it |
| One or two lifted, warped boards | Remove and replace the affected boards, then sand the whole floor so the finish matches |
| Badly buckled boards | Replace them, because buckled boards can't be relaid |
How to repair water-damaged floorboards
Light damage with no warping
If the boards are flat but stained, you're in a good position. Many stains can be lifted with oxalic acid crystals, which are a wood bleach, followed by sanding, re-staining and sealing. Wear gloves and eye protection when you use it, test a small area first and follow the manufacturer's neutralising instructions. Our range of wood stains and floor lacquers covers the refinishing.
Replacing damaged boards
A plank that has lifted can often be taken out with a pry bar or cut out with a saw, then swapped for a matching board. A solid board is normally fitted tongue to groove and stapled or nailed down, and engineered boards are usually glued. Fill any gaps with a suitable wood filler. For parquet, damaged blocks can be lifted, replaced and re-glued, and our guide to parquet floor restoration explains how.
Sanding and refinishing after replacement
Once the damaged boards are replaced, the whole floor usually needs sanding and refinishing so the repair blends in. Sanding after water damage can be more aggressive than usual, because the boards have already moved and need more work to bring them to a uniform surface. If you're doing the work yourself, our floor sanders and buffers cover the machines, and a professional can handle larger or more complex jobs.
Why do floorboards cup? Causes to check
Cupping isn't always a plumbing problem, and it isn't always the homeowner's fault. Find and fix the moisture source, or the damage will come back. These are the usual causes:
- Leaking pipes or walls, including a small plumbing leak or a crack that lets in rainwater.
- Appliances that leak, such as dishwashers, washing machines and fridges.
- A faulty damp-proof course, which lets moisture creep up into the floor from below.
- Moisture trapped in a wooden subfloor or in the void beneath the boards.
- Poor ventilation, which traps humidity in the room.
- Concrete that wasn't fully dry when the wood was laid, so the floor sits on a damp base.
- Over-wet mopping or steam mops, which push water into the joints.
When installation is the cause
Sometimes the problem starts on day one. One of the most common reasons for bulging floors is that the installer didn't leave an expansion gap along the walls. The usual gap is around 10 to 15mm, but follow your floor's installation instructions. Another is that the moisture level of the subfloor or the room wasn't tested before the wood went down. Every supplier provides installation guidelines, and a fitter who ignores them has set the floor up for failure. If you suspect the installation was at fault, raise it with your fitter, who may be obliged to put it right, and check the terms of your floor's guarantee, because moisture damage is often excluded.
Where wood floors don't belong
Wood isn't suitable for every room. Bathrooms, damp basements and rooms with constant moisture problems are hard on timber, and no sanding or finish can overcome a damp environment for long. A good fitter should check the moisture levels of a space before installation, and if the readings are too high, the answer is a different floor, not a bigger dehumidifier. For damp rooms, consider engineered wood or LVT and SPC, and see our guides to the best flooring for kitchens and the best bathroom flooring. Save real wood for the rooms where it will thrive.
How to stop floorboards cupping again
- Monitor humidity with a hygrometer, and spot changes before the boards start to move.
- Keep rooms ventilated, and use a dehumidifier in damp spells.
- Make sure the subfloor and room are moisture-tested before any wood goes down.
- Check the floor was fitted with the right expansion gaps and acclimatised as the supplier advises.
- Clean up spills immediately, and mop with a barely damp mop.
- Fix leaks promptly, and check hoses and seals on appliances regularly.
A good installation from a trusted fitter matters as much as ongoing care, and our floor fitting service covers London. Prevention is always cheaper than repair.
Frequently Asked Questions about Cupped and Water-Damaged Floorboards
Can swollen or cupped floorboards go back to normal?
Minor cases often do. Once the moisture source is fixed and the floor has dried, mild cupping can flatten again over a few weeks. Persistent cupping needs sanding, and buckled boards need replacing.
How do you dry wet floorboards?
Stop the water, remove the surface water, then use fans, ventilation and a dehumidifier to dry the room. If water is trapped underneath, lift a few boards or the skirting so air can reach the void, and check progress with a moisture meter.
How long should I wait before sanding a cupped floor?
Wait until the moisture readings match a dry part of the floor and the boards have stopped moving, which can take weeks. Sanding too soon can leave the boards crowned once they finish drying.
What's the difference between cupping and crowning?
With cupping, the edges of a board rise higher than the centre. With crowning, the centre bulges above the edges. Both are signs of moisture problems and need attention before they worsen.
Why is my wood floor bulging upwards?
The most likely causes are heavy moisture and a missing expansion gap along the walls, which gives the swelling floor nowhere to go. Lift a skirting board to check the gap and have the moisture level tested.
Can buckled floorboards be saved?
Usually not. A board that has buckled badly can't be relaid and has to be replaced, although boards that are only cupped may recover.
Can you sand a water-damaged wood floor?
Yes, once it's fully dry and flat. Light damage can usually be sanded and refinished, while severe damage may need aggressive sanding or board replacement first.
Is a cupped floor the fitter's fault?
Sometimes. If the fitter ignored moisture testing or the supplier's guidelines, or left no expansion gap, they may be responsible. If a leak or high humidity caused it, the cause lies elsewhere, so find the source first.