Combined Sewage Overflow

When rainwater flows off paved surfaces such as rooftops, parking lots and roadways, it picks up contaminants, including dangerous heavy metals. During heavy rainfall, sewage treatment plants are often overwhelmed by the increased volume, causing untreated sewage to overflow into the city's waterways.

New York City is investing billions of dollars to offset CSO problems and update sewage treatment plants. Until recently, however, these solutions have revolved around expensive construction projects such as large underground holding tanks - "Band-Aid" solutions with no other benefits. Simply by employing the natural qualities of soil and vegetation, green roofs retain 50-75% of rainwater that falls on them, easing the pressure on the CSO system while offering a variety of additional environmental, aesthetic, and social benefits.

In fact, a Toronto study demonstrated that green roofs provide multiple benefits while actually offering a more cost effective solution than other stormwater management techniques. By greening 6% of the city rooftops at the cost of $45.5 million CDN, Toronto could retain as much stormwater (3.6 million m3/yr) as a storage tank costing $60 million CDN -- and green roofs also contribute to heat reduction, energy cost savings, job production, habitat restoration and more!