This study evaluated the potential of transforming nonvegetated (grey) roofs into vegetated (green) roofs to increase their water retention capacity and the retention of surface runoff in Taipei City. Of the total area of Taipei City (26 855 ha), we found that 304.5 ha of grey roof could be converted to green roof. Residential areas with higher living standards had a greater potential for grey roof to green roof transformation. The sampled substrate had an average porosity of 59.2% and a bulk density of 1.00 g/cm3. Surface runoff is likely to occur when the level of precipitation exceeds the field capacity of the green roof (13.3 mm/day). The highest percentage of runoff reduction (59.0%) occurred in the cold season. During the rainy months, the runoff reduction ranged from 14% to 28%. The water retention capacity of the green roof was higher during low‐intensity rainfall events than during high‐intensity rainfall events.