Long-term deformations of structural timber elements are strongly influenced by sustained load levels and time- and moisture-dependent phenomena, such as viscoelastic and mechano-sorptive creep, which result from fluctuations in environmental temperature and relative humidity. According to the Eurocode standard [1], the design working life of building structures and their components is prescribed as 50 years. Standard [2] defines the 50-year serviceability requirement for timber beams in terms of the deformation factor kdef, which, in its simplest form, is the ratio of final to initial beam deflection minus one, accounting for both viscoelastic and mechano-sorptive creep contributions. For glued laminated timber, kdef values of 0.6, 0.8 and 2.0 are prescribed in [2], increasing with environmental relative humidity. Deformation factors kdef have been determined in several studies [3-5] based on experimental results of engineered timber products exposed to constant and varying climate conditions. However, these data are typically limited to periods up to 10 years, substantially shorter than the prescribed design working life. Consequently, various methods have been proposed to extrapolate kdef to 50- or 100-year service periods, enabling potential extension of the working life and reuse of structural elements that maintain their load-bearing capacity, serviceability, and durability, in alignment with circular economy principles. In the present study, a hygro-mechanical finite element model is used to predict the long-term deformation of glued laminated timber beams (up to 100 years) based on short-term experimental results (~1 year). Considerable variability in predicted deformation factors is observed, resulting from different relative humidity variations derived from real daily climate recordings and from hygro-mechanical material parameters.