Structural timber – sizes and permitted deviations
Cross section dimensions for structural timber are usually expressed as target sizes, the sizes the saw mills target to achieve. The saw mills would also strive to achieve these dimensions within a certain degree of accuracy, to within the permitted deviations stipulated by European Standards. These sizes as well as the permitted deviations are listed in EN 336.
The standard assumes reference moisture content of 20% for these target sizes and suggests a method to calculate the modified section sizes at other moisture content values. The method is approximate only and disregards the species.
The unit described the target sizes and permitted deviations as used in the UK construction industry. The relevant tolerance classes were introduced and a general method to calculate the section sizes at other moisture contents was proposed. Tables of available timber sizes in the UK market were presented.