Rising water scarcity and climate change pressures have prompted the EU to push for net-zero water consumption; however, cities struggle with the impact of extreme rainfall events. Adopting sustainable water management strategies is necessary to address the pressing challenges. With roofs covering approximately 20% of urban areas, Blue Green Roofs (BGR) are essential for water harvesting, reducing drainage loads, and enabling water reuse. In this context, this study aims to: (i) assess the summer thermal performance of two commercial BGRs—intensive and extensive—with diverse plant species through the analysis of intralayer temperature variations, and (ii) evaluate the water retention potential of the BGRs under an extreme rainfall event. A monitoring campaign was conducted on two BGR test fields in San Donà di Piave, a flood-prone city in northern Italy. Intralayer temperatures and water levels in the water storage layer have been monitored through sensors providing a high-resolution dataset. Among the plant species tested, Sedum, common in extensive GRs, demonstrated lower intralayer temperatures than lawns used in intensive green roofs. Both BGR systems exhibited a peak attenuation of approximately 9 ℃ of ambient air temperature during the hottest day of August 2021. This study provides valuable insights into BGR technology by comparing the thermal performance of different plant types, considering the contribution of a filled water storage layer, and evaluating its effectiveness in stormwater retention during an extreme rainfall event, thereby underscoring its potential to enhance urban environments.