2.1 Why Wood Doesn’t Shrink Uniformly (The Root of All Deformation)
Wood is an anisotropic material.
That’s a fancy word for “properties change depending on direction”.
The three key directions relative to the tree:
- Longitudinal (up and down the trunk)
- Radial (pith → bark)
- Tangential (around the tree’s circumference)
If wood shrank the same in all directions, timber would behave like a polite engineering plastic.
But it doesn’t. It’s a moody, uneven, biologically structured mess of:
- hollow cellulose tubes (tracheids, fibres)
- lignin “glue”
- rays (little radial strips)
- annual ring boundaries
- earlywood and latewood
- residual stresses from wind and gravity
- water distributed in wildly different parts of the structure
This unevenness is why:
- Boards cup
- Shelves sag or crown
- Doors twist
- Planks bow
- Frames rack
- Table tops ripple
The unequal shrinkage is the culprit.
Let’s put numbers to this again, but with a movement lens this time.
Typical shrinkage from saturated → oven dry:
- Tangential: 6–12%
- Radial: 3–6%
- Longitudinal: 0.1–0.3%
Already, we see the shape of things to come:
Tangential shrinkage is roughly double radial.
This one fact predicts almost every warp in the workshop.
Everything that follows is merely that inequality playing out.
2.2 Why Flatsawn Boards Cup (Nature’s Preferred Warp)
Take a flatsawn board — the growth rings are roughly parallel to the face.
A flatsawn board wants to cup because:
- The tangential direction is on the face, and tangential shrinkage is large.
- The radial direction is through the thickness, and that shrinkage is small.
Picture the rings like elastic bands in the board.
As MC drops, the tangential bands tighten more than the radial ones.
Result:
- The board pulls its edges upward or downward depending on which face dries faster.
- The growth rings try to straighten themselves.
- The board becomes a shallow U or ∩ shape.
Rule of thumb for flatsawn stock:
Flatsawn = movement drama
Beautiful figure; terrible emotional stability.
Practical points:
- The wider the board, the stronger the cupping force.
- Alternating grain orientation in glue-ups helps but doesn’t eliminate it.
- Using narrower boards reduces cupping amplitude.
- Breadboard ends don’t stop cupping; they restrain it.