The aim of this study is to validate different analytical relations used in hygrothermal simulations for the material properties. Therefore, a valida tion experiment on four types of flat roofs has been set up at the laboratory. All rele vant material properties of the individual material layers were determined before hand. During the experiment, temperatures, heat fluxes and moisture contents were measured. The measured data were then compared with numerical simulation results. By repeating the simulations with different expressions for the vapour per meability and for the sorption isotherm, the influence of approximate analytic rela tionships was investigated.