Wood science: timber structure, growth, seasoning and moisture behaviour
What the exam tests
This domain asks you to connect tree growth and cell-wall structure to the way timber behaves in service: shrinkage, movement, drying defects and equilibrium moisture content (EMC). Expect calculations of moisture content (MC) and questions on why flat-sawn boards cup and shrink more in width than thickness.
Core rules
- The vascular cambium adds sapwood; inner sapwood converts to heartwood. Sapwood is wetter and less durable; heartwood is darker and often more durable.
- Softwoods have tracheids; hardwoods have vessels and fibres. The cell wall is cellulose microfibrils in a lignin/hemicellulose matrix.
- Timber is anisotropic: shrinkage is greatest tangentially, then radially; longitudinal shrinkage is small (about 0.1–0.3%).
- MC (%) = (wet mass − oven-dry mass) / oven-dry mass × 100.
- Fibre saturation point is about 28–30% MC. Above it, no shrinkage; below it, wood shrinks as it dries and swells as it gains moisture.
- EMC depends on relative humidity and temperature, not on species alone. Seasoning to the service EMC is essential.
- Air drying is slow and gentle; kiln drying is faster but can cause case hardening, checking, collapse and honeycombing if schedules are too aggressive.
Worked example
A 150 mm wide flat-sawn board is at 18% MC and is to be used at 10% MC. Tangential shrinkage from green to oven-dry is 8%. Assume FSP = 28%. Movement ≈ 150 × 0.08 × (18 − 10)/28 = 150 × 0.08 × 0.286 = 3.4 mm. The board loses about 3.4 mm in width. Quarter-sawn timber would move less across the width because radial shrinkage is lower.
Traps
Using the wet mass as the denominator; treating EMC as a fixed number; ignoring the difference between flat-sawn and quarter-sawn; confusing case hardening with surface checking; assuming longitudinal shrinkage is significant.