How to Fill Gaps in Wooden Floors
The two main methods, and why gaps can return
Gaps between floorboards are especially common in older homes, where boards have shrunk and moved over many decades. They let in cold draughts from under the floor, trap dust and crumbs, and can make an otherwise beautiful floor look tired. Filling them makes a real difference, but it is important to know what to expect.
Resin and sanding dust
Resin filling uses a clear binder mixed with the fine dust from your own floor, so the colour matches closely. It is spread across the floor, worked into every gap and sanded smooth once it has hardened. It works best on fine gaps, cracks and holes rather than wide openings.
Good colour match because it uses your floor's own dust.
Quick and cost-effective across a whole room.
Best for gaps of a few millimetres.
Can crack or fall out as boards expand and contract.
Reclaimed wood strips
Wider gaps are better filled with slivers: tapered strips of matching timber glued and fitted into place. After the glue sets, the strips are trimmed and sanded level. Because the slivers are wood, they take the finish in a similar way to the floor and can look very natural. It is labour-intensive, so it is priced higher than resin filling.
Seasonal and structural movement
This is the part many people do not expect: wooden floors are never completely still. Timber absorbs moisture in humid summer months and releases it when the heating is on in winter, so boards swell and shrink with the seasons. Structural movement in joists and the building adds to this, particularly on suspended timber floors.
Because of this, any gap filler, resin or timber, can crack, shrink, loosen or work out over time. It is a normal characteristic of timber floors rather than a fault in the work. That is why gap filling is excluded from our service guarantee, which covers the sanding and finishing we carry out. We will always explain this before you decide.
Helping filler last longer
Keep indoor humidity steady and avoid extreme heating swings.
Fix loose boards before filling so they move less.
Match the method to the gap size.
Consider fitting insulation under suspended floors where possible.
The order of work
In most cases, gaps are filled during a floor restoration, not as a separate job. The floor is first sanded with a coarse abrasive to flatten the boards and remove the old finish. After that, gaps are cleaned and filled, and the floor is sanded again once everything has set. Sanding after filling leaves the repairs level and ready to take the same finish as the surrounding timber.
Filling at the right stage gives a much neater result than trying to fill a floor that has already been finished. It also allows us to check each gap closely and choose the right method for it, rather than treating the whole floor the same way.
Keeping the warmth in
Beyond the look of the floor, filling gaps can make a room feel noticeably warmer. Suspended timber floors in older homes often have a ventilated void beneath them, and cold air can rise through every gap. Sealing the gaps cuts draughts, but the space below must still be ventilated through the airbricks to avoid damp problems.
The essentials at a glance
Resin mixed with your floor's own sanding dust is ideal for narrow gaps, cracks and nail holes.
Reclaimed timber slivers are stronger and more natural-looking in gaps wider than a few millimetres.
All wooden floors move with seasonal humidity and structural movement, so any filler can crack or loosen in time.
Because of this natural movement, gap filling is excluded from our service guarantee.
Securing loose boards, keeping humidity steady and ventilating the void below help the filling last.
Our recommendation
If most of your gaps are narrow, resin is often the best value. For wider gaps, slivers tend to look better and hold up better. Many floors benefit from a mix of both. Find out more about our gap filling service, and we are happy to look at your boards on a free survey in Croydon.