Why is moisture content critical in timber selection and what effect does it have on dimension and strength?

• A. Moisture causes swelling/shrinkage; higher moisture reduces strength and stiffness; use kiln-dried timber or stable moisture conditions; consider equilibrium moisture content.
• B. Moisture has no effect on dimension or strength.
• C. Higher moisture increases modulus of elasticity.
• D. Only affects color.

Answer: (A)

Moisture content is crucial because timber is hygroscopic, meaning it absorbs or loses water from the surrounding air. When moisture rises, wood swells across the grain; when it falls, wood shrinks. This dimensional movement can cause gaps, misaligned joints, or warping if the timber isn’t kept within a stable moisture level. In terms of strength, higher moisture content tends to soften the cell walls, reducing stiffness and overall strength, while keeping the timber at a stable, appropriate moisture content helps maintain predictable performance. That’s why kiln-dried timber or conditioning to the expected environment (aiming for the equilibrium moisture content) is recommended. Variety claims that moisture doesn’t affect dimensions or strength or only changes color aren’t accurate, because moisture directly influences both size and mechanical properties of wood.