This paper presents a simulation model for fresh water production and derived performance parameters for a water desalination system integrated in a greenhouse roof. Several typical daily weather patterns taken from a 10-year period for three arid regions in Tunisia were analyzed. The simulation model is based on a set of heat balance equations including material layers and a brackish water layer. The equations are solved with the more general PC-program, Engineering Equation Solver. For calculation of solar irradiation, the simulation code TRNSYS (Transient System Simulation) was used in the model. The influence of solar irradiation, optical material properties and process parameters on the fresh water production and total light transmission was studied. The most important conclusion is that the roof-integrated still concept has a fresh water production capacity on a sufficient level for irrigation of plants in greenhouse environments. As about 40% of the solar irradiation has to be transmitted, the thermal efficiency is around one-third of the efficiency for tilted types of conventional stills. Further research and technical development are needed in order to optimize the thermal and optical performance of the concept. The solar energy yield could be improved through better design of the greenhouse shape and more selective absorbers.