Recently, major cities in Korea are suffering from frequent urban flooding caused by heavy rainfall. Such urban flooding mainly occurs due to the limited design capacity of the current drainage network, which increases the vulnerability of the cities to cope with intense precipitation events brought about by climate change. In other words, it can be interpreted that runoff exceeding the design capacity of the drainage network and increased impervious surfaces in the urban cities can overburden the current drainage system and cause floods. The study presents the green roof as a sustainable solution for this issue, and suggests the pre-design using the LID controls model in SWMM to establish more specific flood prevention system. In order to conduct the computer simulation in connection with Korean climate, the study used the measured precipitation data from Cheonan Station of Korea Meteorological Administration (KMA) and the forecasted precipitation data from RCP 8.5 scenario. As a result, Extensive Green Roof System reduced the peak runoff by 53.5% with the past storm events and by 54.9% with the future storm events. The runoff efficiency was decreased to 4% and 7%. This results can be understood that Extensive Green Roof System works effectively in reducing the peak runoff instead of reducing the total stormwater runoff.