Structural softwood cross sectional size has established
terminology such as, sawn, basic and unwrought as produced by
conversion of the log into commercial dimensions, e.g. 100¾50 mm
and 225¾75 mm (4^00 ¾ 200 and 9^00 ¾ 300 respectively, as the nearest
imperial sizes).Timber is converted in imperial and metric sizes depending on its
source in the world. Thereafter, standardisation can be undertaken
by machine planing the surfaces to produce uniformly compatible
and practically convenient dimensions, i.e. 225 mm is not the same
as 9^00. Planed timber has been variously described as, nominal,
regularised and wrought, e.g. 100¾50 mm sawn becomes(^97) ¾47 mm when planed and is otherwise known as ex.
(^100) ¾50 mm, where ex means out of.
Guidance in BS EN 336 requires the sizes of timber from a supplier
to be redefined as `Target Sizes' within the following tolerances:
T1 ~ Thickness and width100 mm, † 1 to+3 mm.
Thickness and width > 100 mm, † 2 to+4 mm.
T2 ~ Thickness and width100 mm, † 1 to+1 mm.
Thickness and width > 100 mm, † 1.5 to+1.5 mm.
T1 applies to sawn timber, e.g. 100¾75 mm.
T2 applies to planed timber, e.g. 97¾72 mm.
Further example ~ a section of timber required to be 195 mm
planed¾50 mm sawn is specified as: 195 (T2)¾50 (T1).
Target sizes for sawn softwood (T1) ~
50, 63, 75, 100, 125, 150, 175, 200, 225, 250 and 300 mm.
Target sizes for planed/machined softwood (T2) ~
47, 60, 72, 97, 120, 145, 170, 195, 220, 245 and 295 mm.
Ref. BS EN 336: Structural timber. Sizes, permitted deviations.
Timber Sizes and Surface Finishes