Why Are Gaps Appearing Between My Floorboards?

Small gaps that open up between floorboards, especially over winter when indoor heating dries the air, are usually completely normal seasonal wood movement — timber naturally shrinks slightly as humidity drops and expands again as humidity rises in summer. Larger, permanent, or worsening gaps are a different matter and can point to a subfloor or joist problem.

Why wood floors move with the seasons

Solid wood is hygroscopic, meaning it absorbs and releases moisture from the surrounding air. Central heating in winter dries indoor air significantly, causing boards to shrink slightly and small gaps to appear between them; in summer, as humidity rises, the wood absorbs moisture again and the gaps typically close back up, at least partially. A small amount of seasonal gapping (often described as hairline to around 1-2mm) is expected and not a fault.

When gaps are a bigger problem

Gaps that are consistently wide (several millimetres or more), that don't close up in summer, or that keep getting worse year over year usually point to something beyond normal seasonal movement: the boards may have been laid before fully drying out and acclimatising, the subfloor or joists beneath may have shrunk or moved, or the property's heating and humidity levels may be unusually extreme for the wood species used.

Does it matter if I don't fix it?

Small seasonal gaps are mostly cosmetic and don't affect the floor's structural integrity. Wider, permanent gaps can let more draughts through from below (relevant for older floors being refinished anyway) and can collect dirt and debris, but they're rarely a sign of a serious structural issue on their own — though persistent, worsening gaps combined with other signs like sagging are worth having checked.

How to fix gaps between floorboards

• Flexible wood filler or rope caulk: suitable for small, stable gaps — designed to move slightly with the wood rather than crack out.
• Wood fillets/slivers: thin strips of matching wood glued into wider, more consistent gaps, then sanded flush — a more time-consuming but longer-lasting fix, often done alongside sanding and refinishing.
• Full re-lay: only worth considering if gaps are severe and widespread, since it's a significant undertaking compared with filling.

Common mistakes

Filling gaps with a hard, rigid filler before the driest part of the year (when gaps are widest) often means the filler gets compressed and can crack or push out again once the wood expands in summer. It's generally best to fill gaps in late winter, when boards are at their most contracted, using a flexible product designed for the purpose. If you're planning to sand and refinish anyway, gap-filling is usually done as part of that same process — see our guide to sanding and refinishing costs.

How much gap is too much?

As a rough guide, gaps up to around 1-2mm that open in winter and close in summer are typically normal seasonal movement. Gaps consistently wider than this, or that don't visibly close up during the warmer, more humid months, are worth a closer look — either from a flooring professional or, if you suspect a structural cause, a builder who can check the joists and subfloor beneath.

Does it affect heating bills?

Small individual gaps make a negligible difference to heating costs, but a floor with many gaps across its full area can allow a meaningful amount of draught through from an unheated void below, particularly on a suspended timber ground floor. If you notice cold draughts specifically at floor level in a room with gappy floorboards, addressing the gaps (or checking underfloor insulation and ventilation) can make a genuine difference to comfort, separate from the cosmetic appearance.

Do all wood species gap the same amount?

No — different timber species move by different amounts with humidity changes. Wider boards also tend to show more visible movement per board than narrower ones, even at the same percentage of shrinkage, simply because there's more material to shrink. If you're choosing new solid wood flooring and are concerned about gapping, narrower boards or a more dimensionally stable species can reduce how noticeable seasonal gaps are likely to be.

A dehumidifier in summer and a humidifier in winter can help stabilise indoor humidity year-round, reducing the extent of seasonal gapping in a wood floor, though this is more of a comfort and cosmetic measure than something most households find necessary purely for their flooring's sake.

Underfloor draught-proofing strips, fitted along skirting boards where they meet the floor, can also help reduce draughts associated with gappy floorboards without needing to treat every individual gap between boards.

Related reading: laminate flooring lifting.