Green roofs reduce and delay stormwater runoff, helping to prevent combined sewer overflow (CSO) events. CSOs occur when stormwater discharge from heavy precipitation floods sewer systems and sends a combination of stormwater and raw sewage into the watershed. Extensive green roofs in particular can retain an average of 75% annual rainfall and delay runoff by 3 to 4 hours (Weiler, 2009). Green roofs also filter pollutants from rainwater, roofing materials, and atmospheric deposition.
Improved Air Quality
Green roofs and walls absorb airborne particles and carbon dioxide while releasing oxygen, effectively filtering and cleaning our atmosphere. They also reduce the ambient summer temperature around a building, allowing it to draw less energy for air conditioning and to lower CO2 emissions (Coutts et al., 2015).
Urban Heat Island Reduction
Impervious surfaces in urban centers absorb and retain solar radiation, leading to higher average air temperatures in cities compared to suburban and rural areas. Urban heat island effect has negative repercussions on human health, surrounding watersheds, and energy consumption. Green roofs reduce urban heat island effect by dramatically lowering rooftop temperatures through shading and evapotranspiration, keeping surrounding air temperatures cool. (Coutts et al., 2015).
Redevelop Migratory Corridors and Ecosystems
Green roofs replace ecosystems that have been displaced by city infrastructure. High-rise build spaces can create habitat for essential pollinators, migratory birds, native plant species, and more. The non-profit organization CICEANA in Mexico City uses a roof to cultivate and protect 25 different species of endangered native plants (Earth Pledge, 2005).
The average food product in the U.S. travels 1,500 miles before reaching a consumer, increasing food costs and CO2 emissions (CUESA). Food roofs in urban communities bring high-quality fruits, vegetables, and herbs to densely populated areas. They promote healthy eating, create jobs, and allow for a more sustainable, seasonal food system.