Moisture content (MC) fields in wood are in strong interplay with the surrounding climate, leading to dimensional changes in wooden elements as well as variation of their material behavior. In the European standard for the design of timber structures (EC 5) these effects are considered by assigning wood members to service classes which are solely based on environmental conditions, but independent of the cross-sectional dimensions. Moisture-induced stresses resulting from dimensional changes are not directly considered. In this paper, we compare the EC 5 assignment of service classes to an assignment based on the actual MCs, determined by simulations. Further, we present an approach to assign the service class for solid and glued laminated timber cross sections, depending on the dimensions of the cross section and based on realistic climate simulations for one location. With the proposed approach, the influence of moisture on serviceability design can be taken into account more appropriately, which is important when computing deflections and performing simulations based on 2nd order theory. In addition, a method to estimate MC distributions for larger cross sections solely based on relative humidities is proposed. With these, also moisture-induced stresses and dimensional changes can easily be considered in a static analysis.