Every winter, the same messages: “gaps have appeared between my boards”. Then in spring they close again. It is neither a manufacturing defect nor an installation error: it is wood doing what wood has always done. Here is the mechanism and what can be done about it.
Wood is always seeking equilibrium
Wood is hygroscopic: it exchanges moisture with the surrounding air until it reaches equilibrium. When the air is humid it absorbs and swells. When the air is dry it gives up its water and shrinks.
These movements happen almost entirely across the width — an 18 cm board moves visibly, its length barely changes. That is why gaps appear between boards and never at the ends.
The figures to remember
| Relative humidity | What happens |
|---|---|
| Below 35% | Marked shrinkage, visible gaps, risk of splits within the board itself |
| 40 to 60% | The comfort zone — the wood is stable, and so are your airways |
| 60 to 70% | Slight swelling, the gaps close again |
| Above 70% | Significant swelling, risk of cupping and lifting |
A hygrometer costs about fifteen euros. It is the only way to know where you stand, and the most profitable purchase for anyone who owns a wood floor.
Why winter dries so much
It is counter-intuitive: it is raining outside, and yet the indoor air is dry. The explanation lies in the physics of air.
Outdoor air at 5 °C and 80% relative humidity holds little water in absolute terms. Warm that same air to 21 °C without adding anything, and its relative humidity falls to around 25-30%. Heating does not remove water, it increases the air's capacity to hold it — the result for the wood is the same.
The worst-affected homes are those with underfloor heating, a wood stove or electric convectors, and effective ventilation.
What to do when it is too dry
- A humidifier, evaporative or ultrasonic, in the main rooms. That is the effective solution. Set it to 45-50%, no more.
- Turning the heating down one degree has a real effect on relative humidity.
- Plants and drying laundry indoors help a little, without being enough in a well-ventilated home.
- Never block the ventilation to keep humidity in: you would create a far worse problem than a few gaps.
What to do when it is too humid
- Check the ventilation. A clogged extract vent no longer pulls. The sheet-of-paper test is enough.
- Air the rooms for ten minutes twice a day, even in winter, including when it is raining.
- Look for a cause before buying a dehumidifier: a leak, rising damp, a screed still drying in a recent build.
Gaps that come back every winter: normal or not?
A joint that opens by less than a millimetre in winter and closes again in spring is normal behaviour for wood, especially solid boards and wide formats. It is neither a defect nor grounds for a claim.
What does deserve examination:
- gaps of several millimetres that never close again;
- splits within a board rather than between boards;
- pronounced cupping, meaning boards that are dished or domed;
- boards lifting at the perimeter — a sign that the expansion gap has been filled or that the skirtings are blocking the movement.
What limits the phenomenon from the start
- Engineered boards move considerably less than solid: their cross-laminated layers block shrinkage. It is the strongest argument in their favour.
- Narrow boards move less in absolute terms than wide ones — 0.5% shrinkage over 12 cm is 0.6 mm, over 22 cm it is 1.1 mm.
- Serious acclimatisation before laying, in the real conditions of the home. It is the leading cause of failure.
- A perimeter gap respected, never filled with a rigid sealant nor blocked by skirtings screwed through the floor.