Due to the increasing demand for long-spanning, material-efficient timber structures, the design of slender components is getting more and more important. While short beams reach the maximum load bearing capacity due to exceeding of the maximum tensile and compressive strength (stress failure), a so-called stability failure usually occurs with slender components. This is characterized by excessive deformations and the resulting additional internal forces (which may exceed the load bearing capacity) due to imperfections and second-order theory effects. For beams of intermediate length there is a transition zone between these two failure modes. For the verification of members subjected to lateral torsional buckling, EN 1995-1-1 provides two design concepts. The first one is the simplified effective length method, the second one is based on second order analysis. However, within these design concepts the material behaviour of timber is considered highly simplified. In this article the material-specific behaviour, in particular the plasticizing under compression and the size effect, is taken into account on the basis of analytical and numerical models. The results are compared to the methods provided by the code.