Why has my wood floor got gaps between the boards in winter?

Seasonal gapping is wood doing what wood does, and the question is whether the gaps close again in summer — because permanent gaps mean something else.

Difficulty: beginner
Time: 20 min
Read: 4 min

Short answer

Wood shrinks across its width as the air dries, and a heated British house in winter is very dry. Gaps that appear in winter and close in spring are normal seasonal movement. Gaps that stay open all year mean the boards were laid at too high a moisture content, or the room is permanently drier than the wood was acclimatised to.

This is one of the few flooring complaints where the answer is often that nothing is wrong. Wood exchanges moisture with the air until it reaches equilibrium, and the equilibrium point in a centrally heated room in January is very different from the one in a damp house in July. What matters is the pattern over a year.

What you'll need

a hygrometer (optional)
a feeler gauge or a coin, for measuring gap width consistently (optional)
notes or photographs, to compare across seasons

Step by step

Measure and record the gaps now.Photograph the same spot with something for scale, and note the date and the room's humidity if you have a hygrometer. Without a record, next summer's comparison is guesswork.
Check whether the gaps are even or concentrated.Small even gaps between many boards across the floor is classic seasonal shrinkage. One or two large gaps, or gaps in one area only, points at something local — a leak, a heat source, or boards that were never fixed properly.
Measure the room's relative humidity.Wood floors are generally happiest in a moderate humidity band. A heated house in winter can drop well below it, especially with the heating on constantly and little moisture being added.
Look at where the heat is.Gaps concentrated near a radiator, a wood burner or over an underfloor heating manifold indicate local drying rather than a whole-floor problem.
Wait a season before doing anything.If the gaps close in spring, the floor is behaving normally and there is nothing to repair. Filling seasonal gaps in winter means the filler is squeezed out or the boards are stressed when they expand again.
If the gaps do not close, look at the installation.Boards laid before they had acclimatised, or laid at a moisture content much higher than the room's normal, shrink permanently as they come down to equilibrium.
Raise the humidity rather than filling the gaps.Drying out the house is the cause. Modest humidification through the heating season reduces the movement, and it is better for the occupants as well as the floor.
Only fill genuinely permanent gaps, and fill flexibly.For floorboards, slivers of matching timber glued in are the durable answer for large permanent gaps; flexible fillers cope with small ones. Rigid filler in a moving joint is squeezed out.

Tips

Underfloor heating amplifies this considerably, because it dries the floor from below through the whole heating season.
Wide boards gap more than narrow ones, because the shrinkage is a percentage of the width and it all appears at one joint.
Quarter-sawn timber moves less across its width than plain-sawn, which is one reason traditional floors used it.
If the house is dry enough to gap the floor noticeably, it is probably dry enough to affect furniture and instruments too.

Common mistakes

Filling winter gaps with rigid filler — The boards expand again in spring and the filler has nowhere to go. It is either crushed and squeezed proud, or it forces the boards apart elsewhere, creating a new gap in a different place — and now the floor has filler in it as well.
Laying a wood floor without acclimatising it — Boards delivered from a warehouse have their own moisture content. Fixed down before they equilibrate, they shrink in place, and the resulting gaps are permanent rather than seasonal.
Drying a room hard to fix a damp problem while a wood floor is down — Aggressive drying — dehumidifiers running constantly, heaters on the floor — shrinks the boards far past their normal seasonal range, and the shrinkage can be permanent or produce splitting.

If it doesn't work

Small even gaps that close by May

Cause: Normal seasonal movement — Fix: Nothing. Raise winter humidity a little if the gaps bother you, and do not fill them.

Gaps that never close

Cause: Boards laid too wet, or the room is permanently drier than the boards were acclimatised to — Fix: Live with them, or fill large ones with matching timber slivers glued into place. Address the room's humidity first.

One huge gap in an otherwise tight floor

Cause: A board that has come loose, or accumulated movement pushed to one place — Fix: Check the fixings each side of the gap. A loose board's movement all appears at one joint.

Boards cupped as well as gapped

Cause: Moisture imbalance between the top and bottom faces — Fix: That is a different fault and it points to moisture from below — check ventilation of the void and look for leaks before doing anything to the boards.

Questions people ask

What humidity should I aim for?

Wood floor manufacturers usually specify a range for the room, and the middle of that range is the target rather than any single number. The practical point is to avoid extremes: a house that swings from very damp to very dry moves its floor twice as much as one that stays in a moderate band.

Will the gaps damage the floor?

Seasonal gaps are not damage — they are the floor accommodating movement rather than stressing itself. The alternative, a floor with nowhere to move, is what produces cupping, buckling and split boards.

Should I use a humidifier?

A modest one through the heating season helps a wood floor and usually makes the house more comfortable. It is a better answer than filling gaps, because it addresses the cause rather than the symptom.