WIS 2/3-28 Introducing wood
Timber can be used ‘in the round’ but the majority of structural and constructional timber is sawn into rectangular sections.
The pattern of sawing affects the appearance and behaviour of the sawn sections. The appearance is influenced by the angle of grain exposure on the surface of the board, while the behaviour, particularly in terms of dimensional stability and distortion, is affected by internal stresses during drying.
The figure of some hardwoods eg oak and ash is enhanced by quarter sawing. It may be used for converting some hardwoods because of their higher value but such conversion is at the expense of yield since it produces narrower boards and costs are increased by the repeated turning of the log during sawing.
Softwood logs are usually converted directly to the section sizes required in construction and joinery and efficiency of out-turn is obtained by maximising yields in these sizes.
Structural softwoods are available in a wide range of cross-sectional sizes and lengths. Basic lengths, between 1.8m and 7.2m are measured in increments of 300mm. Standard dimensions of sawn softwood are between 75 and 300mm in width and between 22 and 300mm in thickness. Larger sections are sometimes available or can be formed by pairing sections together or by laminating. Smaller sections are also available and where necessary can be cut from sections within the standard range. Each time a section is cut, up to 4mm may be lost as a result of the saw cut.
Sawn timber sections have fairly rough and slightly irregular surfaces which may be further machined to improve smoothness or dimensional accuracy.
The chart shows typical sawn sizes of softwood.
Machined and surfaced timber. It has become common practice to machine (by planing or fine sawing) the top and bottom surfaces of members such as floor joists to ensure an even depth and thus flat floors and ceilings. Fully planed timber sections have a smooth, well presented finish and are appropriate where high levels of dimensional accuracy are required or where finished components will be handled. Timber machined or surfaced is commonly reduced by 3mm for sections of 150mm or below, or 5mm for larger sections.
Timber known as CLS or ALS (Canadian/American Lumber Sizes) is surfaced on all four sides and has rounded arrises. It is produced in a limited range of sizes and is widely used in the timber frame industry. It originated in North America but is now widely produced in Europe and in the UK.
The majority of hardwoods used by the building industry are for joinery. However, some are used structurally where longer lengths, increased spans, greater durability or their decorative value are required. Structural hardwoods are available in long lengths. Most hardwoods are imported in specific thicknesses but random widths which are within an agreed and accepted range. Standard hardwood sizes are given in BS 5450.