Flat roofs, terraces, loggias, shed roofs and vaulted roofs constructed in wood typically all have one thing in common: they contradict a basic tenet of timber construction that holds that the outer surface must diffuse moisture. Nevertheless, current research findings demonstrate how such timber structures can be equipped with adequate moisture protection. Despite the one or the other case of extreme damage – examples of which will be detailed in this article – the satisfactory performance of unventilated flat roofs can be attained through proper planning and subsequently verified. However, it is necessary to dispel a few common misconceptions regarding moisture diffusion verification and develop guidelines for advanced building physics calculations.