Green roofs might mitigate the urban-heat-islands (UHI) due to an increase of evapotranspiration. However, most green roofs are nowadays extensive ones which are designed as dry site without irrigation, with a thin layer of growing medium and drought-adapted vegetation, especially sedum. Under hot and dry conditions - when cooling is needed - evapotranspiration of such green roofs will decrease rapidly because of a shortage of water. To increase evapotranspiration and thus cooling capacity, supplementary irrigation and plants with high transpiration are needed. But the use of drinking water is not sustainable, due to limited resources. Grey water might be an alternative. In a previous work herbs and grasses tolerant to grey water and with high transpiration rates were selected. In the current study evapotranspiration of extensive roof greenings with three mixtures of these plants compared to common sedum greenings was examined. In a 14-month greenhouse experiment, grey water irrigation reduced evapotranspiration of mixtures of herbs and grasses as well as of sedum only up to 15% compared to tap water. Irrespective of water quality, evapotranspiration of herbs grasses mixtures was more than twice as high as evapotranspiration of sedum. Under field conditions an established herbs grasses roof reached an evapotranspiration of 7.5 L m-2 d-1. Based on the results of the greenhouse and field trials the FAO-56 evapotranspiration model was adapted to non-irrigated green roofs with sedum as well as to irrigated sedum and herbs grasses vegetation, respectively. These models show that under hot and dry conditions evapotranspiration of non-irrigated sedum roofs tend towards zero within a few days, whereas irrigated herbs grasses roofs maintain high evapotranspiration rates. At least under weather conditions in central Europe irrigated roof greenings with herbs and grasses can compensate nearly 100% of global radiation during summer. Thus, this new type of high transpiration green roof can reduce UHI sustainable.