How wood moves

The Wood Guide · Calculating with wood

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How wood moves

Wood breathes. It takes up moisture from the air and releases it again, changing size as it does. Ignore that and you get cracks, sticking doors and cupped tops.

Hygroscopic
EMC — equilibrium moisture content
Tangential ≠ radial

wood movement

End grain of an old beam with star-shaped shrinkage cracks (Burg Vischering, Lüdinghausen) — End grain of an old beam with star-shaped shrinkage cracksBurg Vischering, Lüdinghausen

Boards stacked with spacer sticks to dry outdoors (Velden am Wörther See) — Boards stacked with spacer sticks to dry outdoorsVelden am Wörther See

At a glance · wood movement1 / 8

Wood breathes

Wood takes up moisture and gives it back

Wood is hygroscopic: it constantly exchanges moisture with the air. When the air gets more humid, it swells. When it gets drier, it shrinks.

Browse with the arrows or the dashes · movements are exaggerated to make them visible.

Summary

Wood movement in brief

The natural tendency of wood to change size and shape in response to changing moisture content and temperature.

Wood is hygroscopic: it takes up moisture from its surroundings and releases it again. When it absorbs moisture, it swells; when it loses moisture, it shrinks. That continues for as long as the wood exists — a hundred-year-old tabletop still moves with the seasons.

Wood always strives for a balance with the humidity around it. That balance point is called the equilibrium moisture content, or EMC. In a heated house in winter it lies around 6 to 8 percent; in a shed in November it is closer to 16 to 18 percent.

Why it doesn't happen evenly

Wood does not shrink equally in all directions. Tangentially (along the growth ring) shrinkage is about twice as great as radially (across it), and lengthwise it is negligible. It is precisely that imbalance that causes warping and cracking — not the shrinkage itself.

Cause: hygroscopy: wood takes up and releases moisture
Target: equilibrium moisture content (EMC) with the surroundings
Heated indoors: 6 to 8% moisture content
Sheltered outdoors: about 15 to 18%
Tangential shrinkage: about 2× the radial
Longitudinal shrinkage: negligible

To be honest: you can't switch wood movement off, only plan for it. Paint, varnish and oil slow down moisture exchange but don't stop it. Any design that fixes wood rigidly will sooner or later tear itself apart.

What drives it

Where the movement comes from

Moisture contentThe engine behind it all. Wood takes up moisture from the air and releases it; in doing so it swells or shrinks.
Relative humidityThe surroundings determine which way the wood moves. High RH means absorbing and swelling, low RH means releasing and shrinking.
TemperatureWarm air can hold more moisture. Heating in winter lowers the relative humidity and so dries wood out.
Sawing directionFlat-sawn timber moves more than quarter-sawn, because the tangential shrinkage runs across the width of the board.

What you see

The four distortions

Cupping

The board becomes concave or convex across its width. Typical of flat-sawn timber: the growth rings try to straighten out.

Bowing

A curve along the length, usually from uneven drying or a board with cross grain.

Twisting

The board turns around its long axis. Almost always the result of slanting grain in the original piece.

Checking

Occurs when the surface dries faster than the core. The end grain dries fastest — that is where you see the first cracks.

Why end grain cracks first

Wood releases moisture lengthwise ten to fifteen times faster than across the grain. The end grain is therefore a motorway for drying out: it shrinks while the core is still wet, and splits open. That is why the end grain of garden timber is always sealed.

In practice

How to allow for it

Let it acclimatise

Leave wood for one to two weeks in the room where it will end up before you work it. That way it reaches its equilibrium moisture content before assembly, not after.

Let it move

Fix a tabletop with slotted holes or tabletop fasteners, never screwed down rigidly. With a fence or deck, leave gaps between the boards.

Treat all sides

Oil or stain the underside and the end grain too. A board that is sealed on one side and not on the other is guaranteed to warp.

Choose the sawing direction

For work that has to stay flat, quarter-sawn timber is worth the investment — it moves about half as much as flat-sawn.

With garden furniture

Outdoors the humidity changes constantly, so movement is a given. That is why our picnic tables have gaps between the boards and fixings that allow movement. Fine hairline cracks in the end grain are part of it and do not affect the strength.

Quick answer

Frequently asked questions about how wood moves

Can I prevent wood movement?

No, only limit it and plan for it. Well-dried wood that has acclimatised and is given room to move moves little enough not to cause problems.

Why is my new garden table cracking?

Almost always because the end grain dries faster than the core. Fine hairline cracks are normal in solid wood outdoors and do not affect the strength.

What is equilibrium moisture content?

The moisture content at which wood is in balance with the humidity around it — abbreviated EMC. Heated indoors that is 6 to 8%, sheltered outdoors closer to 15 to 18%.

Does pressure-treated wood move less?

Not less, but differently. Pressure treatment protects against rot but not against moisture uptake. Freshly treated wood is also wet and will shrink further after delivery.

Wood movement in perspective

Wood keeps moving for as long as it exists. Good design lets it.

To The Wood Guide Characteristics of wood Maintaining softwoods

Have a question about your project? Just ask — we're happy to help.

Photos via Wikimedia Commons: Burg Vischering, Lüdinghausen — Dietmar Rabich, CC BY-SA 4.0 · Velden am Wörther See — Johann Jaritz, CC BY-SA 4.0

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How wood moves

How wood moves

Wood movement in brief

Where the movement comes from