Roof underlay systems constitute the rainproof layers of pitched roofs. Thus, roof underlay systems are essential for moisture protection of ventilated roof constructions. In the current project, the longevity of underlay systems is examined and the existing test method is redesigned. In addition to the rainproofness of underlay systems with membranes, the aging of these systems should be implemented in the new test method. Furthermore, a model for the prediction of climatic conditions within the ventilation layer resp. on top of the roof underlay is developed based on laboratory and field tests. Thus, realistic hygrothermal simulations of ventilated roofs will be possible.