This thesis describes the development of a method for assessing the long-term behaviour of a plaster gypsum-bound wood wool slab (WWS) for outdoor application under cyclic humidity and drying stress. It is supposed that gypsum crystals are dissolved by the cyclic stress and transported towards the boundary layer between WWS and plaster. Due to the lower porosity in the plaster a recrystallization of gypsum in the boundary layer maybe caused. The gypsum crystals can damage the bond between WWS and plaster, therefore the tensile bond strength could be reduced. A method has been developed that ensures that the drying of plastered WWS occurs only on the plastered side. Therefore a steel frame was constructed, in which the plastered WWS is fitted in for the cyclic stress. The tensile bond strength increased after the cyclic stress. To get a visual assessment of the boundary layer, reflected light microscopy and scanning electron microscopy were performed with sections in the area of the boundary layer between WWS and plaster. The microscopic examination showed that more gypsum crystals were found in the pores of the plaster with increasing number of cycles and therefore the composite action between WWS and plaster was strengthened. Thus, the increase of gypsum crystals in the boundary layer had a positive impact on the tensile bond strength.