Rethinking green roofs- natural and recycled materials improve their carbon footprint

Green roofs are promoted to deliver climate regulation and urban heat island mitigation and many more. Ecosystem services of green roofs are discussed generally positively, but their footprint from production and demolition processes have not been fully addressed. In this study, a life cycle analysis (LCA) was conducted to assess the possibility of creating of a carbon-neutral green roof and to evaluate and compare the global warming potential (GWP) of two green roofs: 1) a conventional green roof (GR-c) with expanded clay, pumice, and compost in the substrate and a polypropylene drainage, and 2) an eco-friendly green roof (GR-a) with recycled bricks and compost in the substrate and a cork drainage. The LCA refers to a functional unit (FU) of a 218 m² green roof (substrate depth of 9 cm; lifespan of 40 years). The results showed that the use of a brick substrate can reduce the GWP to 3139 kg of CO₂ eq/FU (- 50%) and the use of cork drainage to 441 kg of CO₂ eq/FU (- 69%). Apart from production, demolition is a key process  to be improved in future, accounting for 32% (GR-c) and 55% (GR-a) of the GWP. Once produced, a green roof can take up 783 gCO₂/(m²⋅a) because of plant uptake. To become CO₂-neutral, a GR-c and GR-a would have to last 88 and 53 years, respectively. Furthermore, the GWP was influenced by green roof maintenance and plant CO₂ uptake. We conclude that recycled bricks and cork are promising green roof materials.