Mechanical properties of secondary lumber of Norway spruce

Extending the service life of building components is essential for a circular economy. Wood, as a renewable raw material, plays due to its mechanical properties and ease of processing a crucial role in this process. Most studies focus on the reuse of building materials. However, it is essential to detect and investigate the use cases in which reuse is impossible due to changing dimensional requirements or damages. This study examines the bending properties of recovered wood, particularly battens with cross-sectional dimensions of 30x50 mm², which were processed from rafters originating from a roof truss deconstructed in southern Germany. The bending tests were performed and interpreted based on the damages of the prior use and the lumber pieces' background information. The visual observation resulted in many fastener holes, mainly derived for battens from the built-in upper layer of the rafters. Even though fastener holes contributed to or were the single cause for the failure of the battens, bending strength around the mean value or even higher was yet achieved for some battens. Developing unique sawing patterns for each rafter by taking into account the location of the pith and the arrangement of knots can enhance the yield. Additionally, introducing a third grade, S7, alongside the existing S10 and S13 grades - similar to the approach used for joists and boards in DIN 4074-1:2012 - could further optimize yield. Although it has been concluded that knots remain even for recovered wood the key sorting criteria, fastener holes, can additionally influence the mechanical properties and, therefore, need to be considered in a standardized strength grading.