Timber identification, classification, defects and material selection
What the exam tests
Identification from visible features, correct use of classification systems (strength, durability, treatability), recognition of defects and defensible selection of material for a service environment.
Core rules
- Botanical softwood/hardwood is not the same as trade softwood/hardwood. Hardwoods are angiosperms with vessels; softwoods are gymnosperms with tracheids and resin canals.
- Ring-porous hardwoods (oak, ash) show earlywood pores in rings; diffuse-porous (beech, maple) have pores distributed evenly.
- Strength classes: softwood C14–C40, hardwood D18–D70 (EN 338). Higher number means higher characteristic strength.
- Durability classes 1–5: 1 is very durable, 5 is not durable. Use class 1–2 for exposed ground contact, or preserve/design to keep timber dry.
- Defects: knots, slope of grain, checks, shakes, splits, wane, bow, cup, twist, spring, insect attack and decay. Knots matter most in tension; slope of grain reduces bending strength.
- Select by service class, exposure, load, stability, appearance, machining, fixings, cost and availability.
Worked example
An external sill sits on a masonry plinth and is wetted. Untreated spruce (durability class 4–5) is unsuitable. Choose a class 1–2 hardwood such as oak, or a preservative-treated softwood with cut ends re-treated, and detail a drip and ventilation. If the sill is fully sheltered, a class 3 timber may be acceptable. The reasoning is exposure first, then durability, then strength and movement.
Traps
Assuming all hardwoods are stronger or more durable; judging a knot only by size and not position; calling timber seasoned without a MC figure; ignoring slope of grain; specifying a durable species but leaving end grain unprotected.