Developing urban areas without considering green spaces enhances cities' vulnerabilities to the effects of climate change. To deal with such challenges, the European Union has promoted integration between buildings and Nature-based Solutions (NBS). Green roofs are a type of NBS and have been identified as a key strategy to enhance urban resilience. Although green roofs are seen as an environmentally friendly strategy to address the environmental challenges faced in urban areas, there is still limited evidence regarding the green roofs' potential environmental impacts throughout their life cycle. This paper addresses this literature gap by comparing the environmental impacts of green roofs and conventional flat roofs, through the Life Cycle Assessment (LCA) methodology in a cradle-to-grave approach. Five scenarios were assessed based on the functional unit of 1 square meter of the roof with a 40-year lifespan: i) two extensive green roofs, ii) two intensive green roofs, and iii) one conventional flat roof. The results showed that the performance of extensive green roofs is better than that of intensive ones, mainly due to the type of vegetation that intensive green roofs hold, and consequently, mainly due to the higher material and watering requirements of intensive green roofs. When comparing the conventional flat roof with the green roof, the conventional flat roof performs better in all impact categories under study, mainly due to the additional technical layers required for green roof construction. In all scenarios, the materials stage is the hotspot for all impact categories, except for freshwater and marine eutrophication for the green roof scenarios, where the end-of-life stage is the hotspot. Sensitivity analyses results indicate that the environmental impacts are not sensitive to the content of organic matter in the substrate. However, the end-of-life of the substrate layer materials has a great impact on the outcomes for some impact categories. The findings of this study suggest that improving waste management strategy may enhance the environmental performance of green roofs compared to conventional flat roofs. Future research should explore the multifaceted potential benefits of green roofs, addressing their social, economic, and environmental aspects, considering optimizing buildings' thermal performance and strengthening urban resilience under specific climatic and economic realities.
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