TIMBER SIZES AND SURFACE FINISHES

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. 100 50mm and 225 75mm (4" X 2" and 9" X 3" 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. 225mm is not the same as 900. Planed timber has been variously described as, nominal, regularised and wrought, e.g. 100 X 50mm sawn becomes 97 X 47mm when planed and is otherwise known as ex. 100 50mm, 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 applies to sawn timber, e.g. 100 X 75mm.
T2 applies to planed timber, e.g. 97 X 72mm.

Further example ~ a section of timber required to be 195mm planed X 50mm sawn is specified as: 195 (T2) 50 (T1).

Target sizes for sawn softwood (T1) ~
50, 63, 75, 100, 125, 150, 175, 200, 225, 250 and 300mm.

Target sizes for planed/machined softwood (T2) ~
47, 60, 72, 97, 120, 145, 170, 195, 220, 245 and 295mm.

Ref. BS EN 336: Structural timber. Sizes, permitted deviations.