Tangential vs Radial Movement

This guide explains how tangential and radial movement work in real boards, how to identify each direction from end grain, and how to choose cuts that reduce cupping and seasonal drama.

Defining the Directions

Tangential vs radial on a log — Clean log cross-section diagram. Arrow for radial (pith → bark) and tangential (along ring curve).

Every piece of wood comes from a cylinder — a tree trunk. That cylinder has a centre (the pith) and concentric growth rings radiating outward.

The two directions of significant movement are defined relative to those rings:

Tangential

Tangential means along the curve of the growth rings — tangent to them.

• On a flat-sawn board, the wide face is mostly tangential

• This is the direction of greatest movement

Radial

Radial means from the pith outward, crossing the growth rings at right angles.

• On a quarter-sawn board, the wide face is mostly radial

• This is the direction of lesser movement

Cross-section of a log showing tangential direction (curving along the rings) and radial direction (arrow from pith outward). Label both clearly.

How to Identify the Direction in a Real Board

End-grain ID (flat vs quarter vs rift) — 3-panel end-grain photos or diagrams. Ring angle labelled on each. Optional: matching face-grain thumbnails.

You don't need lab equipment. You need the end grain.

Look at the end of a board and find the growth ring pattern:

Flat-sawn (tangential face exposed)

• Growth rings on the end grain run roughly parallel to the wide face (or in wide arcs)

• The wide face shows cathedral" or "flame" grain patterns

• This board will move more across its width

Quarter-sawn (radial face exposed)

• Growth rings on the end grain run roughly perpendicular to the wide face (straight vertical lines)

• The wide face shows straight, parallel grain and sometimes ray fleck (medullary ray figure)

• This board will move less across its width

Rift-sawn (intermediate)

• Growth rings on the end grain are at roughly 45 degrees to the wide face

• Movement behaviour is between flat-sawn and quarter-sawn

• Straight grain on the face with minimal figure

Three end-grain cross-sections side by side — flat-sawn, quarter-sawn, and rift-sawn — showing ring orientation relative to the wide face. Below each, show the corresponding face grain pattern.

The Numbers: How Much Difference Does It Make?

T/R ratio visual — Simple bar chart comparing tangential vs radial shrinkage for 3–5 species. Highlight that tangential is typically ~2× radial.

Here are some representative shrinkage values (green to oven-dry) for common species to illustrate the tangential-to-radial ratio:

Key observations:

• The T/R ratio is typically 1.5 to 2.5 across most commercial species

• Some species (like walnut) have a relatively low ratio, meaning more uniform movement

• Others (like beech) have a high ratio, meaning flat-sawn boards will move significantly more than quarter-sawn

These are green-to-oven-dry values. In practice, your boards won't swing through the full range — but the ratio between tangential and radial stays consistent regardless of the MC change.

Why This Matters in Practice

The tangential-to-radial difference has direct consequences for every project.

1. Cupping in flat-sawn boards

Cupping mechanism (bark side vs pith side) — Cross-section diagram showing bark side shrinking more. One real photo of a cupped board if available.

A flat-sawn board has tangential movement across its width and radial movement through its thickness. Because tangential shrinkage is greater, the bark side shrinks more than the pith side.

Result: the board cups away from the bark.

This is predictable and consistent. It's why woodworkers check ring orientation before gluing up panels.

2. Quarter-sawn boards stay flatter

A quarter-sawn board has radial movement across its width. Less movement means less cupping.

This is one of the main reasons quarter-sawn timber is prized for:

• tabletops

• drawer fronts

• door panels

• flooring

• musical instrument soundboards

The trade-off: quarter-sawing produces more waste from the log, so it costs more.

3. Panel glue-ups

When gluing boards edge-to-edge for a panel:

• Alternating the ring direction (bark up / bark down) does not prevent movement — it just distributes cupping into a wavy surface instead of a single cup

• Selecting boards with similar ring orientation and consistent MC produces a more predictable panel

• All-quarter-sawn panels move the least overall

4. Frame and panel construction

Traditional frame and panel design exists specifically because of tangential movement. The panel floats in the frame, free to shrink and swell, while the frame (usually with grain running along its length) stays dimensionally stable.

If the panel is flat-sawn, the seasonal width change will be larger. If quarter-sawn, smaller. Either way, the panel must float.

Estimating Movement in Real Projects

Worked example graphic — Simple graphic: 600mm top, ΔMC = 4%, show flat-sawn vs quarter-sawn movement result.

You can estimate seasonal movement using published shrinkage coefficients and expected MC change.

The basic approach:

1. Determine the direction of movement on your board (tangential or radial, based on ring orientation)

2. Look up the shrinkage coefficient for that species and direction

3. Estimate the MC change your piece will experience (based on the EMC range in its environment)

4. Calculate:

Worked example:

A flat-sawn European oak tabletop, 600mm wide.

• Tangential shrinkage coefficient: approximately 0.00369 per 1% MC change (this is the percentage shrinkage per 1% MC, expressed as a decimal)

• Expected seasonal MC swing: 8% to 12% = 4% MC change

• Movement: 600 × 0.04 × 0.00369 × 600...

Let's simplify. Oak's tangential dimensional change is roughly 0.37% per 1% MC change.

• 4% MC change × 0.37% per 1% MC = 1.48% total change

• 1.48% of 600mm = ~8.9mm

That's nearly a centimetre of seasonal movement. Now compare with quarter-sawn oak (radial, roughly 0.19% per 1% MC change):

• 4% MC change × 0.19% = 0.76%

• 0.76% of 600mm = ~4.6mm

Half the movement — same species, same width, same environment. The only difference is how the board was cut from the log.

The Mill & Merchant movement calculator will do this maths for you. But understanding the principle is more important than the numbers.

Practical Guidelines for Choosing and Orienting Timber

When stability matters most, choose quarter-sawn

• Wide tabletops

• Drawer fronts that must stay flat

• Door panels

• Flooring (less seasonal gapping)

• Instrument tops

When figure and cost matter more, flat-sawn is fine — but plan for movement

• Narrower boards move less in absolute terms

• Multiple narrow boards glued together are more stable than one wide board

• Allow for seasonal movement in fixings and joinery

For the best compromise, consider rift-sawn

• Movement is moderate

• Grain is straight and consistent

• Good for table legs, chair parts, and anything where twist would be a problem

Always check the end grain

Before you buy, before you mill, before you glue — look at the end grain and know what direction you're working with.

Common Mistakes This Knowledge Prevents

• Ignoring ring orientation when gluing up a panel — ending up with a tabletop that cups dramatically in winter

• Using flat-sawn boards for wide, unsupported surfaces — when quarter-sawn would have halved the movement

• Not leaving expansion gaps — especially in flat-sawn flooring, where tangential movement can close gaps and buckle boards

• Expecting all boards to behave the same — two boards from the same plank can have different ring orientations and move differently