Why Wood Moves

Change the moisture around a perfectly flat board and it will cup, bow, twist, or do all three — not because it is defective, but because it is wood.

This guide explains what “wood movement” actually is at a physical level: bound water entering and leaving the cell wall below fibre saturation point.

What "Wood Movement" Actually Means

Wood movement is the dimensional change — shrinkage or swelling — that occurs when moisture content changes below the fibre saturation point (FSP).

Recall from Guide 1:

• Above FSP (~30% MC), water is mostly free water sitting in cell lumens. Removing it doesn't change the size of the wood.
• Below FSP, water is bound water held within the cell walls themselves. Removing or adding it changes the physical size of those walls.

When bound water leaves the cell wall, the wall gets thinner. When bound water enters, the wall swells. Multiply that across millions of cells and you get measurable, predictable dimensional change in the whole board.

This is not damage. It is normal material behaviour.

The Mechanism: What Happens Inside the Cell Wall

Wood cell walls are made primarily of cellulose microfibrils embedded in a matrix of hemicellulose and lignin.

Water molecules bond to the hemicellulose and the amorphous regions of cellulose within the wall. As moisture is absorbed:

• water molecules push between the cellulose chains
• the cell wall physically expands
• the expansion is perpendicular to the microfibrils, not along them

As moisture is lost:

• water molecules leave the spaces between chains
• the cell wall contracts
• the contraction is again perpendicular to the microfibrils

This is why wood moves far more across the grain than along it — the microfibrils run roughly parallel to the length of the cell, so the expansion happens sideways.

The Three Axes of Movement

Wood is anisotropic — it behaves differently in different directions. Movement occurs in three axes:

1. Tangential (across the growth rings)

This is the largest movement direction.

• Typical tangential shrinkage from green to oven-dry: 5–10% depending on species
• This is the direction that causes the most visible problems

2. Radial (along the growth rings, toward the centre)

This is roughly half of tangential movement.

• Typical radial shrinkage from green to oven-dry: 2–6% depending on species

3. Longitudinal (along the grain)

This is very small — typically 0.1–0.3%.

• For most practical purposes, wood does not change length
• There are exceptions (reaction wood, juvenile wood), but the general rule holds

The ratio matters enormously:

This difference in movement by direction is the root cause of cupping, warping, and most joint failures.

Why the Difference Between Tangential and Radial?

Two main factors explain it:

1. Ray cells

Rays run radially — from the centre of the tree outward. They act as a kind of internal restraint, resisting movement in the radial direction.

2. Cell geometry

The way cells are arranged around the growth rings means that tangential expansion compounds across many cells, while radial expansion is partly restrained by alternating earlywood and latewood layers with different properties.

The result: tangential movement is always greater than radial. Always. In every species.

How Movement Creates Real Problems

When you understand the three axes, common defects suddenly make sense.

Cupping

A flat-sawn board cups because the tangential face (near the bark side) shrinks more than the radial face (near the pith side). The board curves away from the bark.

Bowing

Bowing (lengthwise curve) is usually caused by uneven drying — one face losing moisture faster than the other, or a moisture gradient from end to end.

Twisting

Twisting often results from spiral grain — where the wood fibres don't run perfectly straight. As the board shrinks, the spiral grain causes opposing corners to lift.

Cracking and checking

When the surface of a board dries faster than the core, the surface tries to shrink while the core resists. The result is tension at the surface that can exceed the wood's strength — causing surface checks or end splits.

Movement Is Ongoing, Not a One-Time Event

A common misconception: once wood is dried, it stops moving.

It doesn't.

Wood moves every time its moisture content changes, and MC changes every time the surrounding humidity changes.

In a centrally heated UK home:

• a solid oak tabletop 500mm wide might move 4–6mm across the seasons
• a wide pine floorboard might move 3–5mm

These are not trivial amounts. They are the reason traditional joinery uses floating panels, slotted fixings, and expansion gaps — not because the craftsman was being cautious, but because the wood demands it.

Moisture Content Change Drives Movement — Not Absolute MC

The amount of movement depends on:

1. The change in MC (how many percentage points the wood gains or loses)
2. The species (each species has its own shrinkage coefficients)
3. The direction (tangential, radial, or longitudinal)

A board sitting stable at 12% MC in an unheated shed will not move. But bring it into a heated home at 7% EMC and it will lose 5 percentage points of MC — and shrink accordingly.

The formula for estimating movement is straightforward:

Where:

• \Delta W = change in width
• W = original width
• \Delta MC\% = change in moisture content (as a decimal)
• S = shrinkage coefficient for the relevant direction

You don't need to memorise this — we'll cover the maths in later guides. But the principle is clear: more MC change = more movement.

Can You Stop Wood Movement?

No.

You can slow it with finishes (which reduce the rate of moisture exchange), but you cannot stop it.

You can reduce it by:

• choosing quarter-sawn boards (radial face exposed, less movement)
• choosing species with lower shrinkage coefficients
• keeping environmental humidity as stable as possible

But the only way to truly eliminate movement is to use engineered wood products (plywood, MDF, etc.) where cross-lamination or fibre randomisation cancels out directional movement.

For solid wood, the answer is never "prevent movement." It's design for movement.

Common Mistakes That Ignoring Movement Causes

• Gluing a solid panel into a rigid frame — the panel can't shrink, so it cracks or the frame breaks.
• Screwing a tabletop directly to the aprons — the top can't expand across its width, so it cups or splits.
• Butting solid wood tight to a wall — when humidity rises, the wood expands with nowhere to go.
• Using a flat-sawn board where a quarter-sawn board was needed — more cupping, more seasonal drama.
• Assuming planed timber won't move — planing removes wood, not physics.

The Simple Rule