Strength graded timber

Constructional strength graded timber is used in accordance with its typical strength.  For economical purposes, constructional strength graded timber is divided into classes. Strength of the timber may be estimated visually, by so called mechanical breaking way or ultrasound.

Timber is a natural product of a tree. Consequentially, quality differences of strength graded timber are dependent on tree species, their genetics and conditions of environment and growth. Differences of tree quality do not depend only on tree species and types, specific part of a tree is also important: through the cross section, along the axis of the trunk, etc. Sawing process, during which the sawn timber is being manufactured from round timber, is destroying natural structure of a cultivated timber. Therefore, strength qualities of an unsorted timber may vary at a degree of the strongest piece being 10 times stronger that the weakest one.


Minimal requirements for estimating strength categories in a standard of EN 14081:2007


Standard EN 1912C14C16C18C22C24C27C30C35C40
Bending parallel to grain N/mm2 14 16 18 22 24 27 30 35 40
Tension parallel to grain N/mm2 8 10 11 13 14 16 18 21 24
Compression parallel to grain N/mm2 16 17 18 20 21 22 23 24 26
Modulus of Elasticity KN/mm2 7 8 9 10 11 12 12 13 14
Characteristic density Kg/m3 200 310 320 340 350 370 380 400 420


Thickn., mmWidth, mm
45 70
45 95
45 120
45 145
45 170
45 195
45 220
45 245


Length: 3,0 – 6,0 m

* In addition to these standard sizes, we can produce the strength graded timber products according to the given sizes and drawings. 


Without consideration of the sorting method, it is necessary to estimate strength category and other important information of a constructional timber and to mark each plank. Accuracy of visual strength grading has some minuses. Considering the fact that sorting decision depends on the decision of a sorter, we can never be completely objective. In order to use better timber qualities, mechanical sorting was started being used in Australia, USA, and Great Britain since 1960 and later in other countries. New machines with greater predictable accuracy were started to be made, because of the increasing interest in mechanical sorting according to strength, assurance of quality and increasing demand of a high quality timber.  


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