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Grades K–2 · Materials & Matter

Why can a wooden door fit tightly in damp weather?

A wooden door may rub its frame on a damp day. Wood is made of tiny cells. Their walls can take in water from the surroundings, even when the wood is not sitting in a puddle.

P53 visual explanationCell-wall moisture state: wetter. Increasing water in wood cell walls usually expands wood much more across than along the grain. Drying reverses much of the swelling, but exact response depends on wood and history; no universal dimensions are predicted.Wood can swell as its tiny walls take in waterThe size change is exaggerated so it can be seenDrier woodMore water in wallsGrain runs up and downWider across the grainWater entering cell walls can make a wooden door fit tightly.

In the diagramWood can swell as its tiny walls take in waterThe size change is exaggerated so it can be seenDrier woodMore water in wallsGrain runs up and downWider across the grainWater entering cell walls can make a wooden door fit tightly.

Water entering wood cell walls can widen a board across its grain; the drawing exaggerates the change.

As more water enters those walls, they swell. A board usually changes much more across its grain than along it. A small change in width can be enough to make a door fit tightly.

When the walls lose water, the wood can shrink again. Different woods change by different amounts. The drawing makes the swelling extra large so it is easy to see. Wood is not growing a new layer here: the material already in the door is changing size.

About this explanation

Starting knowledge: Compare wider and narrower objects and recognize the grain lines in wood.

Sources

  • USDA Forest Products Laboratory — Water in Wood: A Review of Current Understanding and Knowledge Gaps
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