Selecting Timber & Boards for a Purpose

Why Material Selection Matters

Choosing the wrong timber for a product can cause joint failure, premature decay, excessive cost or unnecessary environmental damage. Selecting the right material is not guesswork — it is a structured decision that weighs multiple factors: function (what the product must do), aesthetics (how it must look and feel), cost and availability, and environmental and ethical impact. A school stool requires a hard-wearing, splinter-free surface that survives heavy use; a carved sculpture requires workability and grain figure. Understanding these criteria and applying them to a named material is a core exam skill.

Functional Requirements

The function of a product determines which physical and working properties matter most. Functional requirements to consider include:

Strength and hardness — a workbench top must resist indentation and heavy impact. Beech is commonly specified because its fine, even grain and hardness allow it to take repeated blows and be re-surfaced.
Elasticity and toughness — a cricket bat must absorb repeated impact without fracturing. Willow (a hardwood) is uniquely suited because it combines low density with high toughness. Ash provides similar resilience in tool handles and oars.
Moisture and rot resistance — outdoor products must resist biological decay. Western red cedar, larch and oak all have natural oils or tannins that inhibit rot; untreated pine would require a preservative coating.
Ease of machining — MDF, with its homogeneous fibre structure and absence of grain, machines cleanly in any direction, making it ideal for CNC-routed profiles and shaped components in production runs.

Aesthetic Considerations

Timber's natural variation is often a selling point. Grain figure, natural colour and texture contribute strongly to the perceived quality of a product. Oak's distinctive medullary ray figure (silver grain lines visible in quarter-sawn sections) is prized in high-end furniture. Mahogany's rich reddish-brown tone gives a feeling of warmth and luxury. Beech and maple have a pale, neutral appearance suited to contemporary Scandinavian-influenced design. Where a painted or printed finish is required, MDF or birch plywood offer a smoother substrate than most solid timbers. Matching grain across adjacent panels, selecting for consistent colour and avoiding knots are all aesthetic decisions that add value to a product.

Key Fact

In the exam, a selection justification must link a named property of a named material to a specific product requirement. For example: “Ash is selected for the chair leg because its high elasticity allows slight flexing under load without breaking, improving comfort and durability.” Vague answers such as “wood is strong” score nothing.

Cost, Availability and Stock Forms

Timber is sold in standard stock forms and sizes. Specifying standard sizes avoids waste and reduces cost. Common stock forms for timber include:

Planks and boards — sawn to standard widths and thicknesses (e.g. 25 mm, 50 mm, 75 mm thick). Sawn timber is rough-surfaced and sold at nominal dimensions. PAR (Planed All Round) timber is machine-planed smooth and accurate; actual dimensions are slightly smaller than the nominal (e.g. a 50 × 100 mm PAR board may measure 44 × 94 mm).
Mouldings — timber shaped to a decorative profile, such as skirting board, architrave and picture rail. Produced by passing timber through a moulding machine; supplied in standard lengths.
Dowel rod — circular cross-section timber rod in standard diameters (6 mm, 8 mm, 10 mm, 12 mm, etc.). Used as a locating and joining element in furniture construction.
Sheet materials — manufactured boards (plywood, MDF, chipboard, OSB) are sold in standard sheet sizes, most commonly 2440 × 1220 mm, in a range of thicknesses (3 mm to 25 mm for most applications).

Forces, Stresses and Structural Behaviour of Timber

Understanding how forces act on a timber component is essential for selecting the right material and form. Timber behaves differently depending on the direction of loading relative to the grain:

Tension — timber is strong in tension along the grain (pulling apart lengthways); much weaker across it.
Compression — timber is strong in compression along the grain (posts and columns); however, it can buckle if slender.
Bending — a horizontal beam experiences tension on its lower face and compression on its upper face simultaneously. Timber is relatively weak in bending over long spans; the section depth (how tall the beam is) is the critical dimension for resisting bending.
Shear — forces acting parallel to the grain can cause timber to split along the grain; this is why short grain sections in joints are a weakness.

Solid timber is hygroscopic — it absorbs and releases moisture from the surrounding air, causing movement. Tangential shrinkage (across the growth rings) is roughly double radial shrinkage (across the radius), so planks cut tangentially (through-and-through) are more prone to cupping than quarter-sawn boards. Common defects include:

Cupping — the board curves across its width, concave on the face with more growth rings.
Bowing — the board bends along its length.
Twisting — the board spirals along its length, usually from irregular grain.
Knots — where branches join the trunk; all knots interrupt the grain and reduce bending strength. A live knot is held tight by the wood around it; a dead knot is loose and can fall out, leaving a hole.
Shakes and splits — separations along the grain caused by rapid drying or growth stresses.

Designers minimise movement by: specifying kiln-dried timber at the correct moisture content for the end use; using manufactured boards (which are far more stable); designing with lamination (gluing thin layers together so each layer holds the next in shape) to counteract movement; or framing the component so it can expand and contract freely, for example in frame-and-panel construction where a solid panel floats in a grooved frame.

Timber is an anisotropic material, meaning its strength differs depending on the direction of the applied force relative to the grain. Along the grain, timber is at its strongest in both tension and compression; across the grain it is far weaker, which is why short-grain sections in joints are a common point of failure. Designers account for this by orienting components so the grain runs in the direction of the dominant load, and by choosing manufactured boards such as plywood — where cross-laminated veneers share loads in multiple directions — where in-plane strength in all directions is required.

Reinforcing and Stiffening Strategies

When solid timber cannot provide adequate stiffness over a required span, designers use structural strategies:

Lamination — thin strips of timber glued together with the grain of every strip running the same way along the length. Used in laminated structural beams (glulam) and curved furniture components. Reduces movement, increases strength and allows curves to be formed by bending thin laminates in a jig before the adhesive cures.
Manufactured boards — the cross-grain construction of plywood and the uniform density of OSB and MDF give even stiffness in every direction, unlike a solid-timber panel that is stiff along the grain but weak across it. Sheet materials act as rigid diaphragms when fixed to a frame, preventing racking.
Frame construction — using a skeleton of rails and stiles with thin manufactured board panels reduces material use while maintaining rigidity. Flat-pack furniture relies on this principle.