Green roofs are considered effective in the reduction of atmospheric CO 2 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 CO 2 emissions during their life cycle. Therefore, to assess the CO 2-payoff for extensive green roofs, we calculated CO 2 payback time it takes their CO 2 sequestration and reduction to offset the CO 2 emitted during its production process and maintenance practices. The amount of CO 2 emitted during the production of a modular green roof system was found to be 25.2 kg-CO 2 •m −2. The annual CO 2 emission from the maintenance of green roofs was 0.33 kg-CO 2 •m −2 •yr −1. Annual CO 2 sequestration by three grass species with irrigation treatment was about 2.5 kg-CO 2 •m −2 •yr −1 , which was higher than that of Sedum aizoon. In the hypothetical green roofs, annual CO 2 reduction due to saved energy was between 1.703 and 1.889 kg-CO 2 •m −2 •yr −1. From these results, we concluded that the CO 2 payback time of the extensive green roofs was between 5.8 and 15.9 years, which indicates that extensive green roofs contribute to CO 2 reduction within their lifespan.