Green Roof Hydrology

In dense urban areas and other constrained sites where stormwater infiltration is infeasible, green roofs can perform important and valuable water management functions by retaining rainfall, detaining runoff, and increasing evapotranspiration. And although these qualitative stormwater benefits are commonly understood and accepted, there is little available guidance for designing the quantitative hydraulic performance of green roofs during individual storm events. As a result, design professionals are reluctant to claim stormwater management benefits when designing new projects that include green roofs, and regulatory officials are reluctant to allow green roofs to satisfy stormwater permit requirements.

Fortunately, the state-of-the-practice is shifting towards a view of Nature-Based Solutions—including green roofs—as infrastructure and a practical and affordable way to provide public benefit and repair damaged ecosystems. Green roofs can work in conjunction with, and sometimes in place of, single-purpose grey infrastructure for economic, environmental, and social reasons related to resilience, repairing the connection between humans and nature, and the potential for buildings to provide community benefits beyond the structure itself.