The grading rules for Canadian strength graded timber are written, interpreted and maintained by the National Lumber Grades Authority (NLGA). For solid timber (i.e. timber without finger-jointing) they are:
Canadian visually graded structural timber:
NLGA ‘National Grading Rule for Dimension Lumber’ (NGRDL)
Canadian machine graded structural timber:
NLGA SPS – 2 ‘Special Products Standard for Machine Graded Lumber’
Canadian timber may also be graded to GS and SS grades, either in Canada or in the UK, in accordance with BS 4978: ‘Visual strength grading of softwood. Specification’.
The construction industry within Europe is now subject to the EU Construction Products Directive under which graded structural timber is required to comply with EN 14081-1 ‘Timber structures – Strength graded structural timber with rectangular cross-section – Part 1: General requirements’. NLGA NGRDL and SPS-2 both incorporate, within an EU Export Annex together with the necessary compliance statement, the requirements of EN 14081-1 where these exceed the NLGA requirements.
For the express purpose of simplifying supply to specification of structural timber the strength class system was developed. European strength class requirements are stipulated in EN 338 ‘Structural timber – Strength classes’, with the assignment of individual grades, species and species combinations to strength classes given in EN 1912 ‘Structural timber – Strength classes – Assignment of visual grades and species’.