Technical Booklet D Structure·Page 36·4B.13
Timber section sizes and standards
Section sizes are either sawn in accordance with BS EN 336 tolerance class 1, or CLS/ALS processed sizes in accordance with BS EN 336 tolerance class 2.
The section sizes given in the tables are either sawn in accordance with tolerance class 1 of BS EN 336, or are CLS/ALS processed sizes in accordance with tolerance class 2 of BS EN 336.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet D - Structure.pdf