The paper describes a method for computational assessment of the long-term performance of timber beams in variable climates. The method is based on a cross-section analysis, in which the moisture content and longitudinal stress-strain distributions are calculated as function of time and ambient conditions assuming asynimetrical mechanical behaviour in the tension and cornpression sides of the beam. The method is implemented as a non-linear FE-program, which combines the moisture transport and structural analysis, which is based on a non-linear model of longitudinal creep in wood. The calculated stress-strain state is monitored against a failure criterion, which is based on a definition of a local damage variable and on its integration over the volume in the fashion of the Welbull theory of weakest link to give a global damage parameter. Computational results include long-term deformation (deflection) and time-to-failure (long-term capacity) predictions. Examples of time-to-failure calculation are given.