Cracks in timber members influence the stiffness and load-carrying behaviour but only rudimentary rules are given to evaluate cracked members. Therefore, an investigation to gather information about the most frequent characteristics of cracked timber structures has been carried out. This investigation provides the main characteristics of both the timber elements and the crack distributions encountered. These main characteristics have then been used to define a numerical model in order to investigate the impact of cracks on the stiffness and load-carrying capacity of timber beams. Based on these results, the existing rules considering cracks in timber beams can be evaluated and new rules can be developed.