Calibration of Drilling Resistance for Sustainable Timber Assessment

Retaining serviceable timber supports longer structural service life and circular use of wood. Quantitative interpretation of drilling resistance requires calibration accuracy and sensitivity to measurement conditions to be considered together. This study combined these aspects for Scots pine, Norway spruce and silver fir. Linear calibrations linked mean drilling resistance (RM) to density and mechanical properties determined for 144 beams. Prediction errors were assessed by leave-one-out cross-validation, and density calibrations were tested for transfer between species. Separate experiments examined drilling orientation and temperature, supplemented by reanalysis of published moisture measurements. RM improved density prediction for all three species relative to a mean-based benchmark, with normalised root mean square errors of 7.4–12.9%. Mechanical prediction accuracy depended on species and the property assessed. Cross-species prediction revealed systematic bias, while differences between measurement conditions corresponded to shifts in estimated density comparable to calibration prediction errors. The combined evaluation of calibration performance and measurement sensitivity provides a practical basis for interpreting drilling resistance in timber assessment. Used alongside visual inspection and structural verification, the results can support targeted repair and retention of serviceable members, contributing to service life extension and preserving timber as a structural resource.