What are the characteristics of wood?
The Wood Guide · Calculating with wood
Wood knowledgeThe characteristics of wood
Every knot is a branch that was once there, every line of grain a year of growth. The "imperfections" are exactly what sets wood apart from any factory-made material.
- Knots and grain
- Heartwood and sapwood
- Hygroscopic
Three woods, five tests
Spruce, acacia and teak: all three are wood, yet completely different. Put them side by side in the same tests and you see why. In the price too.
- ·
Use the arrows or bars to browse · figures: CIRAD Tropix, at 12% moisture · our acacia comes from Vietnam.
The characteristics in brief
Wood is not a homogeneous material but a grown structure, and that explains almost everything about how it behaves.
A sheet of steel is the same everywhere. A board is not: it has a top and a bottom, an inner and an outer face, a direction in which it is strong and a direction in which it splits. Understand that, and you understand why a tabletop warps and why a screw will not hold in the end grain.
What follows are the seven characteristics that together determine how a wood species looks, feels and behaves.
A knot is not a defect. It is the place where a branch grew out of the trunk, with its own fibre direction and a darker colour. In structural timber it is a weak spot and gets graded out; in a tabletop it is exactly what makes the piece unique.
- Knots
- the bases of former branches
- Grain
- the pattern of growth rings on the surface
- Growth rings
- one per growing season: pale earlywood, dark latewood
- Heartwood and sapwood
- dead, protected inside; living, vulnerable outside
- Density
- 150 kg/m³ (balsa) to 1,100+ (ipe)
- Hygroscopy
- wood takes up moisture and releases it
To be fair: all these properties together make wood unpredictable compared with sheet material. Two boards from the same tree can move differently, colour differently and react differently to stain. If you want a perfectly uniform result, don't choose solid wood.
The natural fingerprint
Where the branches were
- Formed where a branch grew out of the trunk
- Usually darker than the surrounding wood
- Vary widely in size and shape from species to species
- A sound knot stays put, a loose knot falls out
- A weak spot in structural timber, character in furniture wood
The picture of growth
- Shows the growth rings and the growth pattern
- Flat-sawn gives flames, quarter-sawn gives stripes
- Wide rings = fast growth = lighter, softer wood
- Narrow rings = slow growth = denser, stronger wood
- Determines how stain and oil adhere
How you saw a log determines both the look and the behaviour. Flat-sawn (tangential) gives the familiar cloudy flame pattern but moves more; quarter-sawn (radial) gives a calm striped pattern and stays much flatter. For a tabletop that has to stay flat, quarter-sawn is the better choice — and the more expensive one.
The measurable characteristics
| Characteristic | What it tells you | Practical consequence |
|---|---|---|
| Density | Amount of wood fibre per volume | Determines weight, hardness and durability |
| Hardness (Janka) | Resistance to indentation | Predicts whether a floor or tabletop will dent |
| Moisture content | Water in the wood, as a percentage | Determines whether it will still move after installation |
| Durability class | Resistance to rot, EN 350 | Tells you how long it lasts outdoors |
| Bending strength | Resistance to bending | Determines the span of beams |
| Shrinkage percentage | Change in dimensions on drying | Explains cracking and warping |
Colour, smell and feel
Colour
Determined by the substances in the heartwood: tannins in oak, oils in teak, pigment in padauk. Almost every species changes colour under the influence of light.
Smell
Cedar and western red cedar smell of their essential oils, teak has a leathery scent, oak smells sour because of its tannins. The smell fades as the wood ages.
Lustre
Some species have a natural silky lustre — jarrah, paulownia, ave — that only really comes into its own after finishing.
Warmth
Wood feels warmer than stone or metal because it conducts heat poorly. The effect is strongest in light, porous species.
Frequently asked questions about the characteristics of wood
Why does wood have knots?
A knot is the base of a branch that has grown into the trunk. Every branch leaves one behind, with its own fibre direction and usually a darker colour.
What is the difference between grain and growth rings?
Growth rings are the growth layers themselves; the grain is the pattern you see on the surface when those rings have been sawn through. How you saw determines the look: flames or stripes.
Why does wood move?
Because it is hygroscopic: it takes up moisture from the air and releases it again, swelling and shrinking as it does. See how wood moves.
What makes a wood species durable?
The substances the tree stores in its heartwood — tannins, resins and oils — plus its density. See the durability classes.
The characteristics in perspective
Every property comes from how the tree grew. That is both the charm and the difficulty.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Laburnum anagyroides — Per Grunnet, CC BY-SA 4.0 · Pinus strobus — Philipp Zinger, CC BY-SA 3.0