Basic material science for the carpenter: softwood vs. hardwood, heartwood and sapwood, annual rings and knots, and how structural timber is strength-graded — visually or mechanically — and ends up in strength classes such as C18, C24, and C30 to DS/EN 338.
Introduction
Wood is a living, directional material, and a carpenter who understands how it is built up chooses the right wood for the task and avoids the weaknesses that cause it to fail. The moisture article covers how wood moves with moisture; here it is about the material itself — the species, the structure and the grading that determines whether a piece of wood can be used for load-bearing.
Softwood and hardwood
Wood is divided into two large families. Softwood (including spruce and pine) grows quickly, is light and relatively soft and is by far the most used for load-bearing structures in Denmark — rafters, beams, studs. Hardwood (including oak, beech, ash) is usually heavier, harder and stronger, but more expensive, and is used mainly for floors, stairs, furniture and places where durability or appearance matters.
- 01Spruce: the most common construction timber — light, straight and easy to work with; used for most load-bearing frames.
- 02Pine: slightly heavier and more resinous than spruce; the heartwood is more durable and pine is used e.g. for windows and external timber.
- 03Oak: strong, heavy and naturally durable hardwood; for floors, stairs, external details and where durability without impregnation is desired.
- 04Beech: hard and strong but little durable outdoors; mostly for floors, stairs and indoor use.
This is how the stem is built up
If you cut across a trunk, you see the structure in rings from outside to inside: the bark on the outside protects; just inside sits the thin, living cambium, where growth occurs; then the sapwood, and innermost the heartwood around the pith in the centre. The difference between sapwood and heartwood is important for the carpenter.
- 01Sapwood: the outermost, light-coloured wood. It is the tree's "pipe system" that conducts water up the living tree — it therefore absorbs moisture easily and is less durable against rot.
- 02Heartwood: the inner, often darker wood. The cells are no longer active, and in several species (e.g. pine and oak) the heartwood is more durable, drier and more shape-stable than the sapwood.
- 03Annual rings: each ring is one year's growth with light, soft springwood and darker, denser summerwood. Narrow, tight rings often indicate slow, even growth and strong timber.
- 04Pith: the soft centre; boards cut through the pith tend to crack and should be avoided in visible and load-bearing work.
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