Wood Properties

Engineering variability

• Modulus and strength depend on grain direction; longitudinal stiffness greatly exceeds transverse stiffness.
• Moisture content has a major effect below fiber saturation and also drives dimensional change.
• Knots, slope of grain, checks and grading reduce structural-lumber properties relative to clear-wood values.
• Thermal conductivity and heat capacity vary with density, moisture and direction.

Selection note

For structural timber use grade-specific design values from the governing timber code/standard. Clear-wood handbook values are educational/reference data, not allowable stresses.

Do not combine strength from one grade/condition with modulus, thermal data or temperature limits from another without checking compatibility. Where the overview shows a range, it is intended to expose variability—not establish allowable bounds.

Where it is specified

Framing and joinery, roof trusses, floor joists, formwork, cladding, and increasingly mass timber — glulam, cross-laminated timber (CLT) and LVL — in multi-storey buildings where sequestered carbon and speed of erection are the argument.

Timber is specified by strength class (C16, C24 for softwood; D30 upward for hardwood in EN 338) determined by visual or machine grading, not by species alone. The grade reflects knots, slope of grain and density, which are what actually control strength.

Chosen over what

Against steel and concrete: excellent strength-to-weight, fast dry construction, and negative embodied carbon if sustainably sourced. Against that: combustible, dimensionally unstable with moisture, biodegradable, and with far more variable properties.

Against engineered timber: sawn timber is cheaper but limited in span and far more variable. Glulam and LVL disperse defects through lamination, so their characteristic strength is both higher and much more consistent — which is the entire point of engineered wood.

How it fails

Moisture is the master variable. Wood shrinks and swells below the fibre saturation point (about 28–30% moisture content), and it does so anisotropically: roughly 0.1% longitudinally, 4% radially and 8% tangentially. That differential is what causes cupping, checking and splitting, and it is why timber must be installed near its equilibrium service moisture content rather than dried later in place.

Decay and insect attack above about 20% moisture content. Wood kept dry lasts centuries; wood that is intermittently wet does not. Detailing to shed water matters more than preservative treatment.

Creep under sustained load, accounted for in Eurocode 5 by the k_def factor, and markedly worse in variable humidity — the mechano-sorptive effect.

Perpendicular-to-grain weakness. Tension perpendicular to grain is the weakest loading case by a wide margin and is the usual cause of splitting at notches, bolted connections and curved glulam members.

Fire, though large sections behave predictably: timber chars at roughly 0.6–0.8 mm/min and the char layer insulates the core, so mass timber is designed by calculating the residual section after a given fire duration.

Fabrication notes

Connections govern design. Timber members are rarely the limiting element; the joints are. Bolted, dowelled and screwed connections must respect minimum end and edge distances or they split, and modern design increasingly uses self-tapping screws and steel plates precisely to spread load.

Machining is straightforward but grain direction matters — cutting against the grain tears the surface, and tool geometry differs from metalworking practice.

Adhesives for structural use must be certified for service class and durability; the failure of an adhesive in a glulam member is a structural failure.

Preservative treatment to the appropriate use class matters where moisture cannot be excluded, and penetration is species-dependent — spruce treats poorly, pine sapwood well.