Green roofs are considered effective in the reduction of atmospheric CO2 because of their ability to reduce energy consumption of buildings and sequester carbon in plants and substrates. However, green roof system components (substrate, water proofing membrane, etc.) may cause CO2 emissions during their life cycle. Therefore, to assess the CO2-payofffor extensive green roofs, we calculated CO2 payback time it takes their CO2 sequestration and reduction to offset the CO2 emitted during its production process and maintenance practices. The amount of CO2 emitted during the production of a modular green roof system was found to be 25.2 kg-CO2˙m⁻². The annual CO2 emission from the maintenance of green roofs was 0.33 kg-CO2˙m⁻²˙yr⁻¹. Annual CO2 sequestration by three grass species with irrigation treatment was about 2.5 kg-CO2˙m⁻²˙yr⁻¹, which was higher than that of Sedum aizoon. In the hypothetical green roofs, annual CO2 reduction due to saved energy was between 1.703 and 1.889 kg-CO2˙m⁻²˙yr⁻¹. From these results, we concluded that the CO2 payback time of the extensive green roofs was between 5.8 and 15.9 years, which indicates that extensive green roofs contribute to CO2 reduction within their lifespan.
