You rip a board down the middle and both halves spring apart. You crosscut a plank and it immediately bows. You plane one face and the board curves. The timber looked straight, felt dry, and measured fine — but it was full of invisible stress. Welcome to one of the most frustrating realities of working with wood.
Case hardening (Guide 8) is one source of internal stress — created during drying. But timber also carries stresses that were locked in while the tree was still alive. These growth stresses are built into the wood from the start, and no amount of careful drying will remove them.
This guide explains where internal stresses come from, how they affect your timber, and how to work around them.
Two Sources of Internal Stress
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Image placeholder: Growth stress vs drying stress (at a glance)
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- Simple split graphic: “growth stress (tree)” vs “drying stress (kiln/air dry)”.
Internal stress in timber comes from two fundamentally different origins:
1. Growth stresses (from the living tree)
As a tree grows, each new layer of wood is laid down on the outside of the trunk under longitudinal tension on the outer surface and compression toward the centre. This is how the tree holds itself upright and supports its crown.
These stresses are locked into the wood as it forms. They exist in the standing tree and remain in the timber after felling and sawing.
2. Drying stresses (from moisture loss)
These develop when wood dries unevenly — the surface shrinks before the core, creating the shell-core tension described in Guide 8. Drying stresses can be relieved with proper conditioning, but growth stresses cannot.
The practical difference:
- Drying stresses can be minimised or eliminated with good kiln practice
- Growth stresses are permanent — they're part of the wood's structure
Growth Stresses in Detail
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Image placeholder: Growth stress distribution in a standing tree
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- Log cross-section diagram: tension near bark, compression toward centre.
- Optional: show a saw cut and springing halves.
How they form
When a new wood cell matures in the cambium, it tries to shrink longitudinally and swell transversely as lignin is deposited in the cell wall. But it's bonded to the existing wood beneath it, which restrains this movement.
The result:
- The outermost wood is held in longitudinal tension (it wanted to shorten but couldn't)
- The inner wood is pushed into longitudinal compression (the outer layers are pulling on it)
- There's also a radial component — tension near the bark, compression near the pith
These stresses are in equilibrium in the standing tree. The trunk is stable because the forces balance.
What happens when you saw
Sawing disrupts the equilibrium. Every cut releases stress and the remaining wood redistributes to find a new balance.
This is why:
- A log split in half can spring open or close
- A board sawn near the pith may bow or spring immediately
- Resawing a thick plank into thinner boards can cause them to curve, twist, or crook
- A board that was straight yesterday can move after ripping
The closer to the pith, the greater the compression. Boards containing or near the pith are the most stressed and the most unpredictable.
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Diagram placeholder: Cross-section of a log showing the distribution of growth stress — tension arrows near the outside, compression arrows near the centre. A saw line through the middle, with arrows showing how the two halves would spring apart.
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How Growth Stress Shows Up in Practice
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Image placeholder: Springing after ripping
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- Diagram or photo sequence: straight board → rip cut → halves curve.
Boards that bow or spring when ripped
You rip a straight board down the middle and both pieces curve. This isn't drying stress — it's growth stress being released by the cut. The board was held straight by the balance of tension and compression across its width. Removing material from one side breaks that balance.
Boards that curve after crosscutting
A long board that was straight may develop a bow after being cut shorter, because the remaining piece no longer has the same stress distribution.
Boards near the pith
The pith zone is where growth stresses are highest and most unbalanced. Boards that include the pith (known as boxed heart or boxed pith) are notorious for:
- Splitting along the pith line
- Twisting and warping unpredictably
- Moving after every machining operation
Reaction wood
As discussed in Guide 5, trees that lean or grow on slopes produce reaction wood with abnormal stress patterns:
- Compression wood (softwoods) — on the underside of the lean, under high compression
- Tension wood (hardwoods) — on the upper side, under high tension
Reaction wood amplifies growth stress problems dramatically. A board with reaction wood on one side and normal wood on the other has a severe stress imbalance that reveals itself the moment you saw or plane it.
The Difference Between Growth Stress and Drying Stress
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Image placeholder: Diagnostic cheat sheet
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- Simple visual: “growth stress = worst near pith” vs “drying stress = shell/core pattern”.
It's important to distinguish between the two, because the solutions are different.
| | Growth stress | Drying stress | | — | — | — | | Origin | Formed in the living tree | Created during drying | | Can it be removed? | No — it's structural | Yes — with proper conditioning | | Revealed by | Sawing, ripping, resawing | Sawing, planing, resawing | | Worst near | The pith | The surface (if case hardened) | | Species factor | Worse in fast-grown, leaning, or large trees | Worse in dense, slow-drying species | | Solution | Board selection, staged machining, avoiding the pith | Proper kiln schedule with conditioning |
In practice, a board may contain both types of stress simultaneously. A poorly dried board sawn near the pith of a leaning tree is carrying growth stress, drying stress, and possibly reaction wood stress — a combination that makes it almost impossible to machine to a stable dimension.
Species Prone to High Growth Stress
Growth stress is present in all trees, but some species and growth conditions make it significantly worse:
- Eucalyptus species — notoriously high growth stresses; logs can split violently during sawing
- Beech — moderate to high growth stress, especially in large-diameter trees
- Oak — growth stress is present and can be significant in fast-grown trees
- Ash — moderate growth stress, but reaction wood in leaning trees compounds it
- Tropical hardwoods — many tropical species have very high growth stresses due to fast growth and the mechanical demands of their environment
Softwoods generally have lower growth stresses than hardwoods, but compression wood in leaning softwoods can be severe.
How to Work With Stressed Timber
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Image placeholder: Rough mill → rest → final mill workflow
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- Visual checklist: rough oversize → sticker/rest → re-joint/re-plane.
You can't remove growth stress. But you can manage it.
1. Rough mill, then rest
The single most important practice for dealing with internal stress:
- Rough cut your parts oversize (3–5mm extra in width and thickness)
- Sticker and rest for at least 48 hours — longer for highly stressed species or thick stock
- Re-joint and re-plane to final dimension
This gives the wood time to redistribute stress and move before you commit to final dimensions. If you skip the rest period, the piece may move after assembly.
2. Avoid the pith
Boards containing or immediately adjacent to the pith carry the highest growth stresses and are the least predictable. For critical components:
- Reject boxed-heart boards
- Prefer boards sawn well away from the centre of the log
- If you must use near-pith material, expect it to move and plan accordingly
3. Resaw in stages
If you need to resaw thick stock into thinner boards:
- Don't take it all in one pass
- Remove material from alternating sides
- Let the piece settle between passes
- Accept that resawn pieces will need re-flattening
4. Test before committing
Before milling an entire board to final dimension:
- Crosscut a short sample and observe whether it moves
- Rip a test piece and see if the halves deflect
- Plane one face of a test section and check for cupping
A few minutes of testing can save hours of frustration.
5. Choose your boards wisely
When buying timber, look at the end grain:
- Boards from near the outside of the log (wide ring spacing, bark side visible) tend to have lower net growth stress across the board
- Boards from near the centre (tight rings, pith visible or close) tend to have higher stress
- Boards with asymmetric ring patterns (much tighter on one side) may have reaction wood
6. Design for it
For components where stress release is a risk:
- Use narrower parts — less stress to release across a narrow board
- Laminate — gluing multiple narrow strips together distributes and partially cancels internal stresses
- Orient grain carefully — in panels, bookmatched pieces from the same board will have mirrored stress, which can help cancel cupping
- Allow for adjustment — joints that can be trimmed or fitted after initial assembly give you a margin for movement
When Internal Stress Becomes Unmanageable
Sometimes a board is simply too stressed to use reliably. Signs to watch for:
- The board moves immediately after every cut
- Ripped pieces curve more than 3–4mm over their length
- The board has visible signs of reaction wood (dark, dense latewood bands on one side in softwoods; woolly, tension-wood texture in hardwoods)
- You've rough-milled, rested, and re-machined — and it moved again
In these cases, the honest answer is to set the board aside for a less critical use, or reject it entirely. No amount of skill can force a highly stressed board to stay flat in a precision application.
The Relationship to the Rest of Track 2
Internal stress is the last piece of the movement puzzle:
- Moisture content and EMC (Guides 1–2) determine where the wood is heading
- Movement mechanics (Guides 3–6) determine how much it will change
- Humidity (Guide 7) drives the moisture changes
- Drying stress (Guide 8) adds stress from the drying process
- Growth stress (this guide) adds stress from the tree itself
A board in service is responding to all of these simultaneously. Understanding each one individually is how you learn to predict and manage the combined result.
The Simple Rule
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Every board carries invisible stress from the tree it came from. You can't remove it, but you can manage it: rough mill oversize, let the wood move, then machine to final dimension. And avoid the pith.
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What's Next
We've now covered every aspect of how wood moves, why it moves, and the stresses it carries. In Guide 10 — Dimensional Stability, we'll bring the entire track together: what makes a species "stable", how to compare stability between timbers, and how to choose the right wood for applications where movement must be minimised.
🔗 Knowledge Network
Species Pages
- Eucalyptus spp. — notoriously high growth stresses
- European Beech — moderate to high growth stress in large trees
- European Oak — significant growth stress in fast-grown trees
- Ash — moderate growth stress, compounded by reaction wood in leaning trees
Glossary Terms
- Growth Stress
- Internal Stress
- Boxed Heart (Boxed Pith)
- Reaction Wood
- Compression Wood
- Tension Wood
- Cambium
- Crook
- Spring
- Rough Milling
Calculators
- None for this guide
Categories
- Internal stress
- Growth stress
- Reaction wood
- Timber selection
- Staged machining
- Timber processing
Related Guides
- Track 2 – Guide 8 – Case Hardening and Drying Stress — drying stresses that compound growth stresses
- Track 2 – Guide 5 – Longitudinal Movement (and Why It's Small) — reaction wood and juvenile wood cause abnormal longitudinal shrinkage
- Track 2 – Guide 10 – Dimensional Stability — the full picture of species stability
- Track 1 – Guide 2 – How Trees Grow and How That Becomes Wood — how growth patterns create stress in the living tree
- Track 1 – Guide 5 – Growth Rings Explained — identifying pith proximity and ring patterns
- Track 4 – Guide 1 – How Timber is Sawn — sawing decisions that release or manage growth stress