Structural softwood
Modern sawmilling and processing techniques ensure that timber can be machined to accurate sizes.
The key to ensuring accurate sizes is to specify timber at a correct moisture content for the conditions in which it will be used, to ensure that it is delivered at that moisture content and to ensure that it remains at that moisture content.
Moisture content, and its importance to timber is explained in a TRADA Wood Information Sheet Moisture in timber. www.trada.co.uk
Sizes
Sizes for structural timber are given in BS EN 336 Structural timber - Coniferous and poplar - Sizes - Permissible deviations. This includes a National Annex which details sizes customary in the UK. The sizes are quoted as Target sizes at a moisture content of 20%.
To print the tables download the link, right click and select 'Print'.
View table 1 - Sizes of softwood sawn timber.
The cross sectional dimensions (thickness and width) are assumed to increase by 0.25% for every 1.0% increase in moisture content above 20% up to 30%, and to decrease by 0.25% for every 1% decrease in moisture content below 20%.
Two Tolerance Classes are recognised for deviations in thickness and width from the target sizes. Tolerance Class 1 is usually applied to sawn sections and Tolerance Class 2 to machined (processed) sections. The Tolerance Class should be quoted when specifying section sizes; the class references are abbreviated to T1 and T2.
View table 2 - Tolerance classes for structural timber sizes.
Softwood 'machined on the width' may also be used in timber frame construction. This process used to be known as 'regularising'. These target sizes for softwood and hardwood.
View table 3 - Target sizes for timber machined on the width.
The customary lengths of softwood are shown in Table 4.
View table 4 - Softwood lengths.
Note that lengths of 5.70 m and over may not be readily available without finger jointing. Negative length deviations are not permitted.
Timber which is machined on all four faces and has rounded arrases was first imported into the UK from North America surfaced to Canadian Lumber Standards (CLS) or American Lumber Standards (ALS). Surfaced softwood is now readily available from UK and Nordic sources in addition to Canada and the USA.
The sizes are shown in Table 5.