Urban green spaces, and green infrastructure more generally, provide multiple benefits that can enhance urban livability and sustainability. These range from the mitigation of air pollution and urban heat island (UHI) effect, to multi-dimensional benefits to human wellbeing and biodiversity. However, the expansion of urban green spaces is not always feasible in many cities. In such urban contexts, there have been proposals to utilize rooftops as green roofs in order to gain some of these benefits. This study spatially identifies areas where roofs have the potential to provide different types of benefits associated with urban green spaces if they are retrofitted with green roofs. Through a GIS-based approach we catalogue available roof space in Sumida ward in Tokyo for green roof implementation, and subsequenlty evaluate the potential of each roof patch to offer four types of benefits if retrofitted with a green roof, namely UHI effect mitigation, air pollution mitigation, and benefits to subjective wellbeing and biodiversity. Approximately 25% of the total roof surface in Sumida ward can potentially be used for green roof implementation. Furthermore, about 5.2% and 59% of this area has a respectively high and moderate potential to provide all four benefits if retrofitted with green roofs. This could increase the extent of green spaces by 10% and 120% respectively across the Sumida ward. In this sense, green roofs can become a major element of green infrastructure with ripple positive effects for urban livability and sustainability through the provision of UHI effect and air pollution mitigation, and benefits to subjective wellbeing and biodiversity.