Module: Structural characteristics Unit: Strength-reducing factors

Date Published

27 August 2022

Document Type




The growth features present in a piece of timber determine its strength and appearance.

In an ideal world, trees would grow to produce straight-grained and clear timber. Of course this does not always happen. Therefore, in order to make the best use of the available resources, it is necessary to understand and place limits on the natural growth features of timber when it is to be used for structural purposes. The assessment of a piece of structural timber through the application of these limits is known as ‘strength grading’.

Key Information

The assessment of a piece of structural timber is known as ‘strength grading’.

Softwoods may be visually strength-graded into two classes: General Structural (GS) and Special Structural (SS).

Tropical hardwoods are visually strength-graded into a single grade, HS grade.

Temperate hardwoods are graded into General Structural Temperate Hardwood (TH1 and TH2) or Heavy Structural Temperate Hardwood, for larger sections, (THA or THB).

Case studies

The Stonebridge Hillside Hub is a key scheme in the continuing regeneration of the Stonebridge Estate in West London.

The building is organised as a two storey rendered block, containing the students union and associated facilities, interlocking with a four storey timber clad block, housing teaching and administration space.