Everybody kind of likes the idea of green roofs, but water professionals are not always 100% confident we understand them well enough to promise they will meet water quality and flooding regulations. But the studies are gradually trickling in. Here is a new one from Ecological Engineering:

Increasing recognition is being given to the adoption of green roofs in urban areas to enhance the local ecosystem. Green roofs may bring several benefits to urban areas including flood mitigation. However, empirical evidence from full-scale roofs, especially those that have been operational for more than several years is limited. This study investigates the hydrologic performance of a full-scale extensive green roof in Leeds, UK. Monitoring of the green roof took place over a 20 month period (between 30th June 2012 and 9th February 2014). The results indicate that the green roof can effectively retain and detain rainfall from the precipitation events included in the analysis. Retention was found to correspond significantly with rainfall depth, duration, intensity and prior dry weather period. Significant differences in retention values between the summer and winter seasons were also noted. Regression analysis failed to provide an accurate model to predict green roof retention as demonstrated by a validation exercise. Further monitoring of the green roof may reveal stronger relationships between rainfall characteristics and green roof retention.

Beyond questions on performance, there is a kind of chicken and egg problem where they are not used much (in the U.S., at least) because they are expensive and they are expensive because they are not used much. That is true of many emerging technologies. Of course, this “emerging” technology has been used in Europe for centuries, not to mention it is also popular with hobbits.