Structural timber – sizes and permitted deviations

Structural timber – sizes and permitted deviations

Date Published

17 August 2022

Document Type




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.

Key Information

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.

Case studies

The new theatre is small and intimate, a flat stage surrounded by two storeys of galleries framed in oak and lit by candlelight, creating what The Observer has described as ‘a strange play of authenticity and illusion’.

Walking into the new railway station at Abbey Wood, the immediate impression is of the elegant timber roof which arches over the interior, imparting a warm, natural quality to this large public space – more than 25 metres by 25 metres.