Timber Construction: Types, Properties, and Material Grading Standards

Timber as a Construction Material

1. Types of Construction Timber

Hardwoods (e.g., Mvuli, Meru Oak, Mahogany, Eucalyptus)

• Difficult to work because of hardness; may require pre-drilled holes for jointing with nails.
• Useful for severe environmental exposure conditions, offering better resistance to wear, insect, and fungal attack than softwoods.
• Valued for aesthetic appeal.

Softwoods (e.g., Cedar, Podo, Pine, Cypress, Grevillea Robusta)

• Some types are commonly available in stockyards and are generally cheaper than hardwoods.
• Preferred in construction because they are easier to work and joint with nails.
• Have straighter grains than hardwoods.
• More liable to insect and fungal attack than hardwoods and require intensive preservation treatment.

Available Sizes: Timber sizes are generally available in multiples of 25mm up to 300mm size (e.g., 100 x 50mm, 150 x 50mm, 100 x 100mm, 200 x 50mm, etc.).

2. Advantages Over Other Construction Materials

• High strength-to-weight ratio.
• Some softwoods are commonly available locally, improving supply chain efficiency.
• Construction time is faster, and the structure can be loaded immediately after assembly.
• Aesthetically appealing as a natural material.
• Useful for temporary construction—easily dismantled and reusable if care is taken.

3. Disadvantages Over Other Construction Materials

• Cost and availability, especially concerning hardwood species.
• Less durable than concrete or steel if not properly preserved.
• Combustible nature requires fire safety considerations.
• Liable to natural and seasoning defects, which make strength and quality highly variable.
• May require specialized skills in joinery and installation.

Timber Strength Properties and Grading

4. Strength Properties

4.1 Material Grading

Timber strength is assessed through grading processes to ensure structural integrity.

Visual Stress Grading (BS EN 518:1995 & KS02 – 771: 1999)

The following features are assessed in individual timber pieces during visual stress grading:

• Knot Area: Assessed as a percentage of the cross-section area and its location within the cross section.
• Fissures and Resin Pockets: Evaluated based on the location and size of the fissure in relation to the cross-section dimensions.
• Slope of Grain: Measured relative to the member's longitudinal axis.
• Wane: The size and location of wane (the presence of original rounded surface) in the cross-section of the member.
• Rate of Growth: Indicated by the spacing of growth rings (typically wider in softwoods).