Sorting sawn timber in terms of strength

Most sawn timber is sorted using modern, highly developed, mechanical strength sorting methods, such as computer-vision measurement, measurement of specific frequency, X-ray measurement and ultrasound measurement. The traditional mechanical strength sorting method is to bend the pieces of sawn timber, based on which the modulus of elasticity can be established and thereby the strength class of the piece of timber. Sawn timber can also be visually sorted according to strength, whereby such parameters as the numbers of knots, and the location and quality of a piece of timber can be checked visually, as well as cracks, warping, distortion and other faults. The thickness of the piece of timber’s growth rings can also be checked.

According to STANDARD EN 338, sawn conifer timber is strength-sorted into strength classes as shown in the following table. Strength classes C14 – C30 can be sorted either visually or mechanically, and strength classes C35 – C50 only mechanically.

Strength classes in accordance with EN 338

All strength classes

C14, C16, C18, C20, C22, C24, C27, C30, C35, C40, C45, C50

The most common strength classes in Finland

C18, C24, C30, C35, C40

INSTA 142 is a pan-Nordic standardaccording to which sawn conifer timber is visually strength-sorted into strength classes as shown in the following table. The INSTA 142 strength classes have been approved to correspond to the C strength classes in accordance with the EN 338 standard.

Strength classes in accordance with INSTA 142

All strength classes

T0, T1, T2, T3

Correspondence with EN 338

T0 = C14, T1 = C18, T2 = C24, T3 = C30