The object of research is the module of deformation of a composite material during bending under force and medium load. Method. Mathematical modeling of the rheological model. Results. The relationship between the determination of the long-term strength of a compressed wood bar and the deformation of structures with the simultaneous manifestation of power and environmental resistance is established. Equations are written for determining the long-term modulus of deformation of wood, considering the influence of importance based on processing the results of the experiment. Using different values measured in bending tests, deflections and deformations associated with the deformation characteristics of the structure material under various stress-strain states, analytical dependencies were obtained that can be used in the calculations of timber structures.