How to Reduce Gaps in Wooden Floorboards

Why wooden floorboards develop gaps

Wood is a natural material that responds to the moisture in the air around it. In winter, when heating is on and indoor humidity drops, floorboards lose moisture and shrink slightly. In summer, they absorb moisture and swell. This seasonal movement is completely normal, and in older homes with wide pine boards it can create gaps of 3–5mm or more. Draughts rise through these gaps, making rooms feel colder and increasing heating bills. The key is not to fight the movement, but to manage it gently so your floor stays comfortable and stable.

Before you fill: assess the floor and the cause

Not every gap should be filled. If your gaps close up in humid summer weather, filling them rigidly can cause boards to buckle or crack. Watch the floor over a full season if you can. Check whether the gaps are even across the room or concentrated near radiators, doors, or external walls. Look for signs of structural movement: loose boards, squeaks, dipping, or gaps wider than 8mm. Also consider the subfloor — if it is damp or poorly ventilated, adding filler will not solve the problem. In these cases, a joiner may need to lift boards, check joists, and improve ventilation first. For minor seasonal gaps, however, gentle filling is usually safe and effective.

Control indoor humidity to minimise movement

The most effective long-term solution is to keep your home's relative humidity (RH) stable. Aim for 45–55% RH. A simple hygrometer costs a few pounds and takes the guesswork out of it. In winter, use a humidifier in living areas if RH drops below 40%. In summer, a dehumidifier can prevent boards from swelling too much. Avoid drastic swings: don't dry washing directly on radiators, and use extractor fans in kitchens and bathrooms. Consistent background heating — rather than blasting the heating for short periods — also helps. If you can maintain stable humidity, gaps will be smaller and less variable, and any filler you apply will last much longer.

Choosing the right filler for the job

Rigid fillers such as epoxy or two-part wood fillers are a bad idea for floorboards because they cannot move with the wood. Instead, choose a flexible filler that allows seasonal expansion and contraction. Here are the main options:

• Flexible acrylic sealant: Available in wood tones, it remains slightly elastic and can be painted or stained. Good for gaps up to 5mm.
• Silicone sealant: Very flexible and water-resistant, but cannot be sanded or painted. Best for gaps near skirting boards or in kitchens and bathrooms.
• Wood filler mixed with sawdust: Use a flexible wood filler and mix in fine sawdust from the same boards for a better colour match. Only suitable for small gaps that do not move much.
• Backer rod or foam rope: Push this into deeper gaps first to give the filler something to sit on and reduce the amount of sealant needed.
• Twisted rope or twine: A traditional method for wide gaps in old floors. Push oakum or jute rope into the gap, then seal with a flexible filler. It allows movement and adds insulation.

Whatever you choose, test on a small hidden area first to check colour and adhesion.

Step-by-step: filling gaps without damaging boards

• 1. Clean the gap. Remove dust, grit, and old filler with a vacuum and a thin scraper. The filler needs a clean surface to bond.
• 2. Insert a backer. For gaps deeper than 5mm, push in foam backer rod or rope so the filler sits near the surface.
• 3. Apply the filler. Use a cartridge gun or a small spatula. Work in small sections, pressing the filler into the gap. Do not overfill — leave it slightly proud of the surface.
• 4. Smooth and clean. Wipe away excess with a damp cloth before it sets. For acrylic fillers, a plastic scraper helps create a neat finish.
• 5. Let it cure. Follow the manufacturer's instructions — usually 24–48 hours. Avoid walking on the area or moving furniture across it.
• 6. Sand and finish. Once cured, lightly sand flush with the boards. If you used a stainable filler, apply your floor finish (oil, wax, or varnish) to blend it in.

Aftercare and when to call a professional

After filling, keep monitoring humidity and temperature. Some fillers may shrink slightly over time; if gaps reappear after a season, you can top them up. Avoid scrubbing filled areas with harsh chemicals. If your floor continues to cup, crown, or develop wide gaps despite humidity control, it is worth asking a qualified joiner to inspect the floor. They can check joist moisture, subfloor ventilation, and board fixing. In many cases, a combination of gentle filling and stable humidity will transform a draughty old floor into a comfortable, characterful feature — without damaging the boards you love.