The strength properties of wood have a great variability within the same species and the same growing region. In this regard, defect parameters do not fully characterize the strength and deformability of structural timber. Deterioration of timber quality requires a more reasonable use of resources and accurate assessment of the strength properties of output timber. To evaluate the strength properties of timber planks and beams produced from small size wood assortment, we have adopted a special parameter called "relative aggregate knot volume at destruction section". This paper introduces the regression models demonstrating the correlation between strength properties of timber planks and aggregate knot volume at destruction section. We also provide recommendations for grading timber planks by strength classes and give a comparative analysis of timber output by grades under applicable national standards and regulations, thus demonstrating the possibility of attracting additional sawn timber resources for production of load-bearing engineering structures.