Two table tops, both solid wood, both beautifully made. One stays flat for decades. The other cups, twists, and opens gaps at every joint within a year. The difference isn’t craftsmanship — it’s species selection. Some timbers are inherently stable. Others fight you from the moment they leave the kiln.
In Track 2, we explored why wood moves: moisture content, equilibrium moisture content, tangential and radial shrinkage, and the physics of dimensional change. That track treated all wood as if it behaved the same way.
This guide applies that knowledge to specific species. How much does each species actually move? Which ones are stable enough for wide panels and tight joinery? Which ones demand extra caution? And what numbers on the data sheet tell you what you need to know?
What “Stability” Means
In timber science, dimensional stability refers to how much a species changes in size in response to changes in moisture content.
A stable species changes very little when humidity fluctuates. An unstable species changes a lot.
Stability is not a single number. It depends on:
- Total shrinkage — the maximum possible dimensional change from green to oven-dry
- Tangential shrinkage — movement parallel to the growth rings
- Radial shrinkage — movement perpendicular to the growth rings
- T/R ratio — the ratio of tangential to radial shrinkage
- Movement in service — the actual dimensional change expected within a normal indoor humidity range
All of these vary by species, and all of them matter.
Shrinkage Values: The Raw Numbers
Shrinkage is measured as the percentage change in dimension from green (above fibre saturation point, ~28–30% MC) to oven-dry (0% MC).
| Species | Tangential (%) | Radial (%) | T/R ratio | Volumetric (%) | Stability rating | | — | — | — | — | — | — | | Western Red Cedar | 5.0 | 2.4 | 2.1 | 6.8 | Excellent | | Teak | 4.0 | 2.2 | 1.8 | 7.0 | Excellent | | Black Walnut | 7.8 | 5.5 | 1.4 | 12.8 | Good | | American Cherry | 7.1 | 3.7 | 1.9 | 11.5 | Good | | Honduras Mahogany | 4.1 | 3.0 | 1.4 | 7.8 | Excellent | | Douglas Fir | 7.6 | 4.8 | 1.6 | 11.8 | Moderate | | Scots Pine | 7.7 | 4.0 | 1.9 | 12.0 | Moderate | | European Oak | 10.0 | 4.4 | 2.3 | 15.4 | Moderate–Poor | | European Ash | 8.0 | 5.0 | 1.6 | 13.2 | Moderate | | Hard Maple | 9.9 | 4.8 | 2.1 | 14.7 | Poor | | European Beech | 11.8 | 5.8 | 2.0 | 17.3 | Poor | | Ipe | 8.0 | 5.9 | 1.4 | 13.0 | Moderate (but very dense) |
The range is significant. Beech shrinks tangentially more than twice as much as teak. In a 300 mm wide panel, that difference translates to millimetres of movement — enough to open joints, crack finishes, or distort frames.
Understanding the T/R Ratio
The T/R ratio (tangential shrinkage divided by radial shrinkage) is one of the most underrated numbers on a species data sheet.
As we covered in Track 2 (Guide 4 — Tangential vs Radial Movement), wood always shrinks more tangentially than radially. But how much more varies enormously.
Why the T/R ratio matters
A high T/R ratio means the wood shrinks much more in one direction than the other. This unequal movement is the primary cause of:
- Cupping in plain-sawn boards (the face parallel to the growth rings shrinks more than the face perpendicular to them)
- Distortion in panels and wide boards
- Differential stress at joints between tangential and radial surfaces
A low T/R ratio means the wood shrinks more evenly in both directions. This produces:
- Less cupping in plain-sawn boards
- More predictable behaviour in joinery
- Better dimensional consistency across all cut orientations
T/R ratio guidelines
- Below 1.5 — Excellent. Very even movement. Minimal distortion. (Walnut ~1.4, Honduras Mahogany ~1.4, Ipe ~1.4)
- 1.5–2.0 — Good to moderate. Normal cupping expected in plain-sawn boards. (Cherry ~1.9, Douglas Fir ~1.6, Ash ~1.6)
- Above 2.0 — High distortion potential. Plain-sawn boards cup significantly. Quarter-sawing strongly recommended. (Oak ~2.3, Beech ~2.0, Maple ~2.1, Cedar ~2.1)
<aside> 📌
Practical rule: Species with T/R ratios above 2.0 are much better behaved when quarter-sawn. Quarter-sawing presents the radial face (the less-moving face) to the surface, and movement in service becomes more symmetrical and predictable.
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Movement in Service: The Number That Matters Most
Total shrinkage values (green to oven-dry) describe the maximum possible movement. But in real life, timber doesn’t cycle between green and oven-dry. It cycles within a narrow range of moisture content dictated by the environment.
Typical indoor ranges
- UK / Northern Europe: Indoor RH typically cycles between ~40% in heated winter and ~65% in summer. This corresponds to EMC of roughly 8–12%.
- Mediterranean: Indoor RH may range from ~35% to ~70%. EMC: ~7–13%.
- Tropical: Indoor RH may stay consistently high, ~60–80%. EMC: ~11–16%.
- Dry continental / air-conditioned: Indoor RH can drop as low as ~20–30% in winter. EMC: ~4–7%.
The movement in service is the dimensional change that occurs across the EMC range the timber actually experiences.
This is a far more useful number than total shrinkage, because it describes what the timber will actually do in the building or piece of furniture.
Estimating movement in service
A useful rule of thumb:
$$ \text{Movement (mm)} = \text{Width (mm)} \times \frac{\text{Shrinkage %}}{100} \times \frac{\Delta MC}{\text{FSP}} $$
Where:
- Width = the dimension in the direction of movement (tangential or radial)
- Shrinkage % = the tangential or radial shrinkage value for the species
- ΔMC = the change in moisture content the timber will experience
- FSP = fibre saturation point (~28–30%)
Example: A 200 mm wide plain-sawn European oak panel (tangential shrinkage 10.0%) in a UK interior where MC cycles between 9% and 12% (a 3% swing):
$$ 200 \times \frac{10.0}{100} \times \frac{3}{28} = 2.1 \text{ mm} $$
The same panel in teak (tangential shrinkage 4.0%):
$$ 200 \times \frac{4.0}{100} \times \frac{3}{28} = 0.86 \text{ mm} $$
Teak moves less than half as much as oak. Over a 600 mm table top, the difference is roughly 6 mm vs 2.5 mm — enough to matter at every joint.
What Makes a Species Stable?
Several factors contribute to dimensional stability:
1. Low total shrinkage
The most obvious factor. Species with less total shrinkage simply move less, regardless of the conditions.
Cedar, teak, and mahogany all have low total shrinkage. This is partly why they’re prized for boat building, outdoor furniture, and musical instruments.
2. Low T/R ratio
As discussed, a low T/R ratio means more uniform movement and less distortion. Walnut and mahogany combine low total shrinkage with a low T/R ratio — the ideal combination.
3. Extractive content
Some extractives — particularly oily ones — reduce the wood’s ability to absorb and release water vapour quickly. This dampens the moisture cycling that drives movement.
Teak’s exceptional stability is partly due to its oily extractives, which slow moisture exchange with the environment. The wood doesn’t react as quickly to humidity changes, smoothing out the peaks and troughs of movement.
4. Density
As noted in Guide 1, denser species generally have higher total shrinkage because they contain more cell wall material. This is why beech (720 kg/m³, 11.8% tangential shrinkage) moves far more than cedar (370 kg/m³, 5.0%).
But density is not the only factor — teak (640 kg/m³) is significantly denser than cedar but has even lower tangential shrinkage (4.0%). Extractives override the density effect in this case.
5. Cell wall structure
The arrangement and chemistry of the cell wall — particularly the cellulose microfibril angle and the proportion of hemicellulose — influences how much the wall swells and shrinks with moisture. Species-specific differences in wall architecture contribute to the variation in shrinkage values.
The Most Stable Species
These species are consistently chosen for applications where minimal movement is critical:
**Teak (Tectona grandis)**
- Tangential: 4.0% | Radial: 2.2% | T/R: 1.8
- The gold standard for stability in a naturally durable hardwood
- Oily extractives reduce moisture exchange
- Prized for boat building, outdoor furniture, and precision joinery
**Honduras Mahogany (Swietenia macrophylla)**
- Tangential: 4.1% | Radial: 3.0% | T/R: 1.4
- Exceptionally low T/R ratio — moves almost equally in both directions
- The traditional choice for fine furniture, pattern making, and boat interiors
- Now heavily restricted (CITES Appendix II); African and plantation alternatives are common
**Western Red Cedar (Thuja plicata)**
- Tangential: 5.0% | Radial: 2.4% | T/R: 2.1
- Lowest total shrinkage among commonly available softwoods
- Higher T/R ratio is its one weakness, but low total movement compensates
- Ideal for cladding, shingles, and anywhere dimensional change must be minimised
**Black Walnut (Juglans nigra)**
- Tangential: 7.8% | Radial: 5.5% | T/R: 1.4
- Higher total shrinkage than the three above, but an excellent T/R ratio
- Moves more, but moves evenly — less distortion, less cupping
- A superb furniture timber that accommodates movement predictably
**Sapele (Entandrophragma cylindricum)**
- Tangential: 5.4% | Radial: 3.6% | T/R: 1.5
- A practical mahogany alternative with good stability
- Interlocked grain complicates planing but doesn’t affect stability
The Least Stable Species
These species move significantly and require careful design:
**European Beech (Fagus sylvatica)**
- Tangential: 11.8% | Radial: 5.8% | T/R: 2.0
- The most movement-prone common European hardwood
- Excellent workability and hardness, but demands tight moisture control
- Wide beech panels in fluctuating environments are a recipe for problems
**Hard Maple (Acer saccharum)**
- Tangential: 9.9% | Radial: 4.8% | T/R: 2.1
- High total movement and a high T/R ratio
- Beautiful timber for floors and furniture, but needs careful acclimation and well-designed joinery
**European Oak (Quercus robur / petraea)**
- Tangential: 10.0% | Radial: 4.4% | T/R: 2.3
- One of the highest T/R ratios among common species
- Quarter-sawn oak is dramatically more stable than plain-sawn — historically, this was well understood by makers of fine furniture and cooperage
**Hickory (Carya spp.)**
- Tangential: 10.5% | Radial: 7.0% | T/R: 1.5
- Very high total shrinkage despite a reasonable T/R ratio
- Tough and strong, but moves a lot
How Sawing Pattern Affects Stability
The relationship between sawing pattern and stability was introduced in Track 2 and Track 4 will cover it in detail. But the key point bears repeating here:
Plain-sawn (flat-sawn)
- The wide face is approximately parallel to the growth rings
- Movement across the face is tangential — the maximum direction
- Boards cup toward the bark side as they dry
- In species with high T/R ratios, cupping can be severe
Quarter-sawn
- The wide face is approximately perpendicular to the growth rings
- Movement across the face is radial — the lesser direction
- Boards remain flatter and cup less
- The most stable orientation for any species
Rift-sawn
- Growth rings at 30–60° to the face
- A compromise between plain-sawn and quarter-sawn
- More stable than plain-sawn, less stable than quarter-sawn
<aside> 💡
For species with T/R ratios above 2.0 (oak, beech, maple), quarter-sawing is the single most effective thing you can do to improve stability. It doesn’t change the wood — it changes which face is presented to the environment.
</aside>
Designing for Stability
Species selection is the first step. Design is the second.
Allow for movement
Every solid-wood assembly must accommodate dimensional change:
- Table tops: Attach to the apron with slotted holes, buttons, or figure-eight fasteners — never glue the top directly to the base frame
- Frame and panel: The panel floats in the groove, free to expand and contract without stressing the frame
- Floorboards: Leave expansion gaps at walls and around fixed objects
- Drawer fronts: Size with seasonal movement in mind; a drawer that fits perfectly in summer may bind in winter (or vice versa)
Use stable species for critical dimensions
Where tight tolerances matter — doors, drawers, instrument bodies, precision joinery — choose a stable species:
- Walnut, mahogany, cherry, or cedar for panels and wide components
- Oak is fine for structural frames and narrow components but problematic for wide plain-sawn panels
Quarter-saw wide boards
For species with high T/R ratios, quarter-sawing wide boards pays for itself in reduced callbacks, repairs, and customer complaints.
Control the environment
The timber only moves if the humidity changes. In workshops:
- Acclimate timber to the target environment before working it
- Store finished components in conditions similar to their destination
- Advise customers to maintain reasonable indoor humidity (ideally 40–60% RH)
Use engineered alternatives where appropriate
Plywood, MDF, and other engineered panels are dimensionally stable because their cross-laminated structure cancels out directional movement. For applications where movement cannot be tolerated (large flat panels, carcasses, substrates for veneering), engineered materials are often the pragmatic choice.
Stability and the Species Database
On Timber Logic, every species page will include:
- Tangential shrinkage (%)
- Radial shrinkage (%)
- Volumetric shrinkage (%)
- T/R ratio
- Movement in service estimate for typical indoor conditions
- Stability rating (Excellent / Good / Moderate / Poor)
These values, combined with the movement calculator, allow users to predict exactly how much a component will change in a given environment — taking the guesswork out of design.
Modified Timber: Engineered Stability
As covered briefly in Guide 4, modified timber products can dramatically improve stability:
Acetylated timber (Accoya)
Acetylation reduces moisture uptake by ~80%. The wood barely responds to humidity changes. Shrinkage and swelling are reduced to a fraction of the unmodified values. For applications demanding near-zero movement (windows, doors, precision external joinery), Accoya is effectively dimensionally stable.
Thermally modified timber
Thermal modification reduces equilibrium moisture content and total shrinkage. The improvement is significant but less dramatic than acetylation. Stability improves by roughly 30–50% depending on the treatment schedule.
Engineered wood products
- Plywood — cross-laminated veneers cancel directional movement. Extremely stable in both width and length.
- MDF / HDF — homogeneous fibre composition means near-zero directional variation. Ideal substrates for veneering.
- CLT — cross-laminated panels are stable in-plane, though thickness movement still occurs.
Media and Image Recommendations
- Comparison chart: tangential shrinkage of common species
- Horizontal bar chart ranking species from lowest to highest tangential shrinkage — teak at the bottom, beech at the top
- Diagram: cupping in plain-sawn vs quarter-sawn boards
- Cross-sections showing growth ring orientation and the resulting cup direction and magnitude
- Photo: seasonal movement in a table top
- A wide solid-wood table top showing gaps at the breadboard ends in winter (dry) vs tight in summer (humid)
- Diagram: movement in service calculation
- Worked example showing the formula applied to a 200 mm oak panel, with the EMC range and resulting movement clearly labelled
- Photo comparison: flat-sawn beech vs quarter-sawn beech
- Same species, same conditions — one cupped, one flat. Visual proof that sawing pattern matters.
The Key Idea
<aside> 💡
Species differ enormously in how much they move. The key numbers are tangential shrinkage, radial shrinkage, and the T/R ratio. Low values mean a stable timber. High T/R ratios mean cupping and distortion. Choose stable species for wide panels and precision joinery. Quarter-saw unstable species. And always design to accommodate the movement that will inevitably occur — because no species is truly zero-movement.
</aside>
What’s Next
In Guide 10 — Choosing the Right Timber for the Job, we bring everything together. Density, hardness, stiffness, durability, workability, extractives, toxicity, and stability — all the properties we’ve covered in this track — are weighed against each other in real-world decision-making. How do you balance competing requirements to select the best species for a specific application?
🔗 Knowledge Network
Species Pages
- Teak — excellent stability, oily extractives slow moisture exchange
- Honduras Mahogany — excellent stability, T/R ratio 1.4
- Western Red Cedar — lowest shrinkage among common softwoods
- Black Walnut — good stability, excellent T/R ratio (1.4)
- American Cherry — good stability
- Sapele — good stability, practical mahogany alternative
- European Oak — moderate–poor stability, high T/R ratio (2.3), benefits most from quarter-sawing
- European Beech — poor stability, highest movement among common European hardwoods
- Hard Maple — poor stability, high T/R ratio (2.1)
- European Ash — moderate stability
- Douglas Fir — moderate stability
- Scots Pine — moderate stability
- Ipe — moderate stability, very dense
- Hickory — high total shrinkage despite reasonable T/R ratio
Glossary Terms
- Dimensional Stability
- Tangential Shrinkage
- Radial Shrinkage
- T/R Ratio
- Volumetric Shrinkage
- Movement in Service
- FSP (Fibre Saturation Point)
- EMC (Equilibrium Moisture Content)
- Accoya
- Acetylation
- Thermal Modification
- Plywood
- MDF
- CLT
Calculators
- Movement Calculator
Related Guides
- Track 2 – Guide 4 – Tangential vs Radial Movement
- Track 2 – Guide 6 – Shrinkage and Swelling
- Track 2 – Guide 10 – Dimensional Stability
- Track 3 – Guide 10 – Choosing the Right Timber for the Job
- Track 4 – Guide 2 – Plain Sawn vs Quarter Sawn vs Rift Sawn
- Track 3 – Guide 7 – Resin and Extractives
Fact-Check Report — Guide 9: Stability Differences Between Species