Hydrologic Performance of an Extensive Green Roof under Intense Rain Events: Results from a Rain-Chamber Simulation

The water storage capacity of a green roof generates several benefits for the building contermi-nous environment. The hydrologic performance is conventionally expressed by the runoff coeffi-cient, according to international standards and guidelines. The runoff coefficient is a dimension-less number and defines the water retention performance in a long period. At the scale of single rain events, characterized by varying intensity and duration, the reaction of the green roof is scarcely investigated. The purpose of this study is to highlight how an extensive green roof – hav-ing a supposed minimum water performance, compared to intensive ones – responses to real and repetitive rain events, simulated in a rain chamber with controlled rain and runoff data. The ex-periment provides, through cumulative curves graphs, the behaviour of the green roof sample during 4 rainy days. The simulated rain events are based on a statistical study (summarized in the paper) on 25 years of rain data for a specific location in North-Italy characterized by an av-erage rain/year of 1,100 mm. The results prove the active response of the substrate, although thin and mineral, and so quit draining, in terms of water retention and detention during intense rain events. The study raises questions about how to better express the water performance of green roofs.