Roof greening provides a promising solution to urban flooding because of the commonly assumed effect on rainwater runoff reduction; however, few studies have investigated this effect in humid subtropical areas using long-term monitoring data. In this study, we evaluated two typical types of green roofs, extensive and intensive, in Nanjing City for their capacity to reduce runoff based on empirical hydrological equations calibrated with field measurement data collected over a whole year circle. We analyzed the variations in rainwater retention, evaporation, and runoff with respect to season and rainfall types. Based on paired rainfall-runoff data of 77 events over the year, the curve numbers (CN) of the two green roofs were calculated, and the annual runoff reduction of the city-scale roof greening was assessed. The results showed that annual runoff reduction rates of the extensive and intensive green roofs were 42% and 60.7%, respectively. The runoff reduction magnitude was highest in spring, followed by winter, autumn, and summer, with respective average reduction rates of 78.6%, 47.5%, 33.2%, and 32.9% for the extensive green roof and 98%, 84.3%, 49.5%, and 48.1% for the intensive roof. Soil played a dominant role in rainwater retention, accounting for 52% and 62% of the total runoff reduction for the extensive and intensive green roofs, respectively. Precipitation and rainfall intensity were key factors affecting runoff effects, both being negatively correlated with the runoff reduction rate (P < 0.01). Antecedent soil moisture was negatively correlated with the runoff reduction rate for the extensive green roof (0.01 < P < 0.05) but showed no significant correlation for the intensive green roof. The CNs of the extensive and intensive green roofs were assessed to be 92 and 88, respectively, compared with the value of 98 for conventional impervious roofs. A hypothetical 60% coverage of extensive and intensive greening on all building roofs in the main city of Nanjing might lead to an annual runoff reduction of 2.8×10⁷ m³ and 4.2×10⁷ m³, respectively. These results can provide a scientific basis for urban stormwater management and sponge city construction.