Comparison of urban green roof performance

Urban drainage systems struggle to cope with downpours, leading to closed roads, flooded houses, and waterways pollution. Green roofs can capture some of the deluge and release it more slowly, helping drainage systems cope. Researchers at Columbia University in New York compared the three most common types of green roof in their city over a one year period, including 243 storms, with rainfall from 0.8 to 180 mm.

A vegetated mat system – plants attached to a special drainage system that can be rolled out like a carpet – retained 36% of the total rainfall on its building. A built-in-place system – constructed and planted on the intended building – retained 47% of the total rainfall on its building. Finally, a modular tray system – trays that restrict runoff, provide air space for drainage, and can be put directly onto the roof’s waterproof membrane – retained 61% of total rainfall. The researchers caution that every green roof will perform differently, even within the same type, depending on things like local weather patterns, vegetation chosen, and quality and type of installation. Still, their results make an encouraging case for the utility of green roofs in cities.