Beyond the surface: a comprehensive hybrid life cycle assessment of embodied greenhouse gas emissions in green and conventional roofs in Melbourne

Green roofs are increasingly recognised as multifunctional solutions to urban environmental challenges; however, the embodied greenhouse gas (GHG) emissions associated with their construction remain an underexplored component of their environmental performance, particularly in the Australian context. This study applies a hybrid life cycle assessment (LCA) approach to quantify and compare the life cycle embodied GHG emissions of conventional and green roof assemblies commonly used in Melbourne, Australia. Extensive and intensive green roofs were assessed over a 50-year lifespan and compared with conventional concrete, flat timber and pitched timber roof assemblies. Embodied emission calculations were performed using material quantities and hybrid life cycle inventory data from the Environmental Performance in Construction (EPiC) database, which combines process-based and environmentally extended input–output data to capture upstream supply chain emissions. Results show that conventional roofs in Melbourne have average annual life cycle embodied emissions of approximately 2.54 kgCO₂-eq/m 2 for concrete roofs and 1.64–1.66 kgCO₂-eq/m 2 for timber roofs. Extensive green roofs increased embodied emissions by approximately 0.97 kgCO₂-eq/m 2 /year, while intensive green roofs increased emissions by approximately 1.99 kgCO₂-eq/m 2 /year, primarily due to substrate and additional system layers. Substrate depth and composition and structural system selection were identified as the main drivers of embodied emissions. The results also show that hybrid LCA produces higher embodied emission estimates than many process-based studies due to more comprehensive system boundary coverage, highlighting the importance of methodological choice in life cycle assessment. This study provides region-specific hybrid LCA data for green roof systems in Australia and identifies key design parameters that can reduce embodied emissions, supporting more informed material selection and low-carbon roof design.