Materials, Timber Technology and Material Selection
This section tests your knowledge of timber structure, species, seasoning, defects, grading, moisture content, preservatives, sheet materials, and selection for a given exposure. You should be able to explain why one timber suits a door frame and another suits a concealed ceiling batten.
Core rules
- Heartwood is the inner, darker wood; sapwood is outer and usually less durable. Durability varies by species.
- Seasoning reduces moisture content. Interior joinery commonly needs about 10–12% moisture content; structural timber may be higher but must be stable in service.
- Shrinkage is greater tangentially than radially, causing cup, bow, twist, and spring. Check grain and growth-ring orientation.
- Defects include knots, shakes, checks, resin pockets, and fungal or insect attack. Strength-reducing defects matter in structural members.
- Preservatives protect against termites and decay; cut ends of treated timber should be re-treated with an approved preservative.
- Sheet materials: plywood is strong and stable in both directions; particleboard and MDF are flat but vulnerable to moisture unless protected.
Worked example
A moisture meter reads 19% on teak boards intended for interior door shutters, but the specification requires 10–12%. Do not install the boards. Stack them with stickers in a dry, ventilated workshop and re-check. If they remain high, reject them. The reasoning: installing timber above service moisture content leads to shrinkage, open joints, sticking doors, and surface defects after handover.
Traps
- Accepting green or unseasoned timber because it planes easily.
- Ignoring grain direction when planning for shrinkage and finish.
- Using interior-grade adhesive or board in a wet area.
- Failing to treat cut ends of preservative-treated timber.