Embedding nature-based solutions (NBS) in cities is expected to bring quantifiable benefits, including resilience to flooding, drought, and heatwaves, and air quality improvement. Among NBS, green roofs have an important role in temperature regulation in buildings and in lowering the damaging effects of heatwaves on human health. In this paper a spatial microsimulation model is implemented to simulate temperature impacts of green roofs installations in cities and their capacity to attenuate the effects of heatwave episodes. Particularly vulnerable to heatwaves are elderly people with limited mobility, who have limited means to seek cooling and create cooler indoor environments. The model, implemented using the Netlogo platform (version 6.0.4), considers as agents the elderly citizens in a city area and simulates the heatwave-related health impacts, which are measured in mortality likelihood. In particular, the model simulates a generalised 1.5 °C to 3 °C indoor temperature reduction range induced by green roofs (based on inferences from green roof literature) in four different European cities: Szeged (Hungary), Alcalá de Henares (Spain), Metropolitan City of Milan (Italy) and Çankaya municipality (Turkey). The simulation utilises a ceteris paribus modelling approach, meaning that the relationships of the observed phenomenon (mortality induced by heatwaves) with other possible influencing factors (e.g. level of sport and physical activities practiced by people) are not taken into account. In the case of Szeged, Alcalá de Henares, and Çankaya municipality a substantial reduction in mortality is found to occur associated with green roofs roll out. In the case of the Metropolitan city of Milan, green roofs installations show a low mitigation effect in some scenarios. The underlying factor is the temperature threshold parameter of the model, above which heatwave mortality occurs. This parameter was inferred from the literature (Baccini M., et al., 2008) and it resulted to be substantially higher in the Metropolitan city of Milan (31.8 °C) than in the other cities. The simulation helps in obtaining results which are specific to a given city and particular scenarios therein, and provides additional insights, such as expected temperature mitigation effect induced by green roofs under climate change conditions, or the indoor temperature reduction targets that are needed for a particular city to have a maximum desired heatwave mitigation impact. However, the model parameters have to be carefully selected, after an accurate study of the domain literature.