The investigation of wood mechanical response in outdoor conditions, subjected to environmental changes, long term loadings and time effects, show a complex behaviour of this material. This behaviour, is characterized by a dimensional change which, in most cases, causes initiations and crack propagation until the total collapse of timber structures. This work presents, on the basis of the RDM assumptions of the Timoshenko beam theory, the evolution of notched beam deflection in the time. The model built takes into account the effects of moisture content variations of beam, the crack appeared until its collapse, the intensity of the loading applied and the geometry of the beam. The work is divided in three parts: (i) an experimental study, presenting the specific setup used to recorder the data; (ii) the analytical model based on the classic assumptions of the beam theory starting from the expression of the notched beam’s Young modulus of notched beam (goal of this study); (iii) the validation of the model at the end, shows that the evolution of the experimental data and the analytical data have a good fit.