Wood moves: why joints shift and boards crack
Wood is the only material in the living space that keeps moving after it has been worked. It takes up moisture from the air and releases it again, and in doing so it changes its dimensions – every year, in every heated room, with every type of wood.
Once you have understood this, you look at a piece of furniture differently. A joint that becomes wider in winter is then not a workmanship fault but proof that someone thought ahead. And a crack in a tabletop almost always has the same cause.
What wood moisture means
Wood always adjusts to the air around it. After a while it reaches a state of equilibrium in which it neither takes up nor releases moisture – the equilibrium moisture content.
In heated living spaces it is usually roughly 7 to 11 per cent. At normal room temperature and about 50 per cent humidity it settles at around 9 per cent, in dry, heated air with about 30 per cent humidity it drops to around 6. Over the year this value shifts: in winter, when the heating is on, the air is drier and the wood releases moisture. In summer it takes it up again.
This movement is the cause of practically everything that stands out about solid wood furniture. It cannot be stopped – only planned for.
Wood moves to different degrees in three directions
This is the real core, and it is rarely explained. Wood does not shrink evenly but depending on direction, and the differences are large.
The ratio between the three directions is about 1 to 10 to 17: along the grain almost no movement, radially ten times that, tangentially seventeen times. Over the entire drying range, wood shrinks radially by 3 to 5 per cent and tangentially by 6 to 10.
An everyday example over the course of a year: with beech, the dimension changes by 0.41 per cent tangentially and 0.20 radially for each per cent change in moisture. For a tabletop one metre (3.3 ft) wide, that amounts to several millimetres over a season – enough to see, and enough for something to crack if the construction does not allow for it.
That is why a board warps instead of shrinking evenly. One side pulls more strongly than the other, and the board follows the stronger one.
How to recognise clean workmanship
This movement leads to a few things a cabinetmaker does that can be seen on the finished piece.
In a glued solid wood panel, strips that are as narrow as possible are used and glued alternately – one side up, the next one down. As a result, the individual strips pull against each other instead of all in the same direction, and the panel stays flat. You can recognise this by the alternating growth rings on the end grain.
In doors, drawer bottoms and back panels made of solid wood, the wood is not fixed but sits in a groove in which it can move. A panel that is glued all round will crack sooner or later.
And before processing, wood is acclimatised: eight to ten days in the room in which it is worked. Skip this, and the later movement is built into the piece of furniture.
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What this means for the living space
A new piece of solid wood furniture comes from a workshop or a warehouse with a different humidity from the home it is delivered to. In the first weeks it adjusts, and that is when most of the movement happens.
In practice this means: not right next to the radiator, not in full sun, and in very dry winters anything that raises the humidity helps. A piece of furniture that stands for months at twenty per cent humidity dries out beyond the point for which it was built.
What, on the other hand, is no cause for concern: a joint that changes by a millimetre over the year. Fine cracks on an end face. A board that feels minimally different to the touch than in summer. That is the nature of the material, not a fault.
Oiled or lacquered: two different principles
A surface can either seal wood or leave it open, and that is a fundamental decision.
Lacquer lies over it as a film. It protects well against liquid and stains, and as long as it is intact, this works reliably. If it is damaged, the spot is exposed, and repairing it almost always means redoing the whole surface.
Oil soaks in and hardens in the fibre. It protects less, but a damaged spot can be re-oiled locally without it being visible. And the surface still feels like wood rather than plastic.
For a piece of furniture that is meant to stay for a long time, oil is usually the easier-care decision – not because it needs less care, but because the care is still possible after ten years.
Veneer is not a compromise
Solid wood is considered the better option, and for a tabletop that you want to be able to sand down, that is true. For large surfaces it is not.
A wide solid wood surface is precisely what moves the most and is most likely to warp. A veneer on a stable substrate stays flat because the substrate does not move. For cabinet doors, back panels and large fronts, that is the technically better solution and not the cheaper one.
The difference that matters is a different one: real wood veneer versus printed foil in a wood look. One is wood, just thin. The other is a picture of wood.
Frequently asked questions
How damp is wood in a flat?
In heated living spaces, wood usually settles at a moisture content of about 7 to 11 per cent, at about 50 per cent humidity around 9. In dry, heated air with about 30 per cent humidity it drops to around 6. The value shifts over the year with the humidity.
Why does a tabletop crack?
Because wood moves to different degrees in different directions – tangentially about seventeen times as much as along the grain. Where the construction does not allow for this movement, tension builds up, and it is released as a crack.
Is a joint that becomes wider in winter a defect?
No. Dry heating air draws moisture out of the wood, it shrinks, joints open. In summer they close again. A construction that allows for this is the well-built one.
Oiled or lacquered – which is better?
Lacquer protects more, but can only be repaired over the whole surface. Oil protects less, but individual spots can be touched up invisibly. For furniture that is meant to stay for a long time, oil is usually more practical.
Is veneer worse than solid wood?
Not fundamentally. For large surfaces it is technically superior, because it does not warp. The important difference is the one between real wood veneer and printed foil in a wood look.
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