mpervious surfaces in urban areas are increasing as a result of urbanization and development. Roof surfaces constitute a large portion of impervious cover in cities. Within the scope of the sustainability principle and urban policies in the world, water retention benefits of the green roofs to the urban environment have been assessed with various academic studies. In recent years, green roof systems are being used to reduce stormwater runoff in urban areas. In the research, main aim was to determine the rainfall-runoff relationship of a typical extensive green roof with 50 mm thick substrate and to perform comparisons with an unvegetated roof based on field measurements. Hydrological performance of green roofs such as water retention and runoff delay were investigated in local climate of Bahcekoy, Istanbul. Through comparative field measurements at Istanbul University Faculty of Forestry Landscape Architecture Department, Green Roof Research Station (IUGRRS), stormwater retention performance of green roofs was investigated in terms of sustainability. From the derived data, moisture changes in the substrate, outdoor environment interactions and the runoff characteristics of the green roofs were evaluated. According to rainfall characteristics, a typical extensive green roof system delayed the runoff between 1 to 23 h in several rain events and prevented the runoff between 12.8% and 100%. Overall, green roof systems may provide an alternative and sustainable solution for the impervious roof surfaces in cities.