Timber Properties

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Timber (or Lumber in North America) refers to wood at any stage after the tree has been felled. It is suitable for building or carpentry, and for various engineering, and other purposes. Timber as a building material possesses a number of valuable properties, such as low heat conductivity, amenability to mechanical working, low bulk density and relatively high strength.

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The quality of timber must be ensured before using it for a specific purpose. The following are the physical and mechanical properties of timber that should be examined :- colour, appearance, hardness, specific gravity, moisture content, grain, shrinkage and swelling, strength, density, toughness, elasticity, warping, durability, workability, soundness, abrasion and defect content.​

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Colour - Colour contributes to the 'personality' of wood. Most timbers vary in colour and will change with use, the application of finishes, and exposure to light. Appearance is also determined by knots, figure and grain pattern, as well as exposure to weathering.

Density - The density is a very important factor affecting the strength of wood. The densest woods are generally the strongest. The density is also related to the stiffness and hardness e.g. both these properties increase as the timber density increases. Density varies depending on species and its moisture content.

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Moisture Content - Timber has high moisture content when first processed from the tree. The moisture content is reduced prior to the timber being used as a structural material. The drying of timber can be achieved using either of two methods :-

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• Air seasoning - the timber is stacked in open sided weatherproof containers so that the air can freely circulate to all surfaces of the timber to promote drying; this process can take a considerable time and uses a considerable amount of space. 

 

• Kiln drying - the timber is dried out in heated, ventilated and dehumidified ovens; this process requires special equipment and high energy use; however the process is controlled and rapid.

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Poor control of the drying process can result in a range of defects including bowing, cupping, twisting and cracking.

Surface Texture - Texture is the wood property that determines the condition of the surface and its stability. It plays an important role in deciding how a wood is finished. Surface texture is classified as fine, medium or coarse.

 

Directional Properties/Anisotropy - The properties of timber are different in different directions. The elastic modulus and strength of wood is much greater in a direction along the grain than across the grain. If the grain is sloping at an angle to the direction of loading this can also have a marked effect of the strength of the timber.

 

An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e. non-permanently) when a stress is applied to it.

Stress/Strain Relationship - At low stress levels the stress is proportional to the strain. The strain is greater for a given stress at high moisture contents. However timber does have significant creep characteristics i.e. over time at high constant stress values the strain will increase. In calculating strength of structures the duration of the loading is one of the factors to be considered.

Stress is caused due to strain, and strain is caused due to applied force. Hence we can say that applied force first, then strain, then stress follows.

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Thermal Properties - Timber is a poor conductor of heat. The denser timbers are better conductors than the lighter timbers. Timber expands when heated but this is offset by the reduction in section resulting from the loss of moisture.

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Fire Resistance - Timber compares favourably with other construction materials in terms of fire resistance. A small timber section may ignite easily and reduce to ash. However larger sections have significant resistance to fire because the surfaces initially in contact with the flames burn to charcoal which resists the further spread of the flame.

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Durability - Timber does not corrode. It is, however, subject to weathering, chemical attack, and fungal attack.The durability of timber varies depending on the wood species

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Treatability - Refers to how easily timbers can be penetrated with preservatives applied by vacuum pressure processes.

 

Wood Grain - In its simplest aesthetic meaning, wood grain is the alternating regions of relatively darker and lighter wood resulting from the differing growth parameters occurring in different seasons (i.e. growth rings) on a cut or split piece of wood.

Causes including fungus, burls, stress, knots, and special grain alignments. 

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The two basic categories of grain are straight and cross grain. Straight grain runs parallel to the longitudinal axis of the piece. Cross grain deviates from the longitudinal axis in two ways, spiral grain or diagonal grain. The amount of deviation is called the slope of the grain.

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Grain alignment must be considered when joining pieces of wood, or designing wooden structures. For example, a stressed        span is less likely to fail if tension is applied along the grain, rather than across the grain. Grain direction will also affect the type of warping seen in the finished item.

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Grain pattern density determines strength. A piece of timber with a tight pattern is stronger than one with a loose grain pattern.