Water desalination in a greenhouse roof means that solar energy is absorbed for evaporation from a thin, flowing layer of water, Earlier a quite detailed simulation model was developed for analysis of the thermal and optical characteristics of this desalination system concept. This paper describes laboratory experiments with a small roof module and presents measurements compared to simulations obtained in order to validate the thermal part of the model. Equations, parameters and simplifications used in the model are briefly described. The laboratory work has been carried out with artificial light from a solar simulator operated in a well controlled thermal environment. The validation has an emphasis on the main parameters with impact on the water production capacity. These are the light absorptance and temperature of the absorber, the water flow rate and temperature, the inlet water temperature and the water storage volume. The main conclusion is that good agreement is obtained between simulated and measured variations of water production values for variations of these parameters. Concerning absolute values the simulated values are somewhat overestimated and possible factors behind this are discussed. Based on the simulation results, the presentation also includes some more conceptual discussions concerning system designs and operation methods. The most important indication is that geothermal water at elevated temperatures combined with this roof technology is the alternative with the highest water production capacity. A final, overall statement is that the model has a considerable potential for development into a more general system analysis tool.