Planning a green roof as your stormwater management tool in urban areas

Posted in Blog, green roof, Stormwater on January 31, 2017

With increasing percentages of impervious surfaces in cities and with antiquated or combined  sewer infrastructure, urban communities have been forced to come up with new ways to manage stormwater.  Across the U.S., stormwater must be managed to avoid flooding and water pollution. This can happen either by expensive technical means or by green infrastructure. When assessing BMPs (Best Managing Practices) for stormwater retention in urban areas, you should consider solutions that use readily available roof space, are easy to install, work reliably long term, and are simply beautiful.

In recent years, green roofs have gained more and more recognition as an important part of green infrastructure and as an excellent way to manage stormwater in urban areas.

Why? Put simply, green roofs work for stormwater management. Per the General Services Administration, green roofs can lessen stormwater flow by up to 65% and slow down the flow rate by 3 hours. Data from Green Roofs for Healthy Cities show that green roofs can hold 70-90% of precipitation that falls during the summer months, depending on the plants used and the depth of media. During winter, green roofs can hold 25-40% of precipitation.  An added benefit is the value of increased insulation on the roof surface, which can reduce cooling energy requirements in the summer by up to 75%.


Planning for the stormwater management aspects of green roofs is typically handled by civil engineers. Because the use of green roofs as BMP for stormwater retention is relatively new in the U.S., we’d like to summarize a few key things to consider during the green roof planning process. It’s critical to have all the necessary information for the permitting process and to know how you can optimize stormwater retention on your green roof.

Stormwater Retention Levels

During the permitting process, very low default numbers for water retention are sometimes used
because there is no basis for better performance values. However, our rooflite® soil products have proven water retention values that are much better than the defaults frequently used. How do we know this? All rooflite soil products are tested not only based on FLL Guidelines but also on all
relevant ASTM standards. This ensures our green roof media can be used as a proven, consistent, and reliable component of urban infrastructure. This is why our high-quality soil products come with certified test reports showing our better performance.

Increased Stormwater Retention Results

Because civil engineers view green roofs differently than landscape architects, urban planners or
environmentalists, they are usually focused on permit requirements. These requirements can differ
depending on location and sometimes do not reflect the latest stormwater retention information and calculations.

While research scientists are eagerly trying to find ways to realistically assess the real value of green roofs for water retention, there is a broad consensus that the current models are insufficient. It’s
important to be aware of this as you plan for your green roof. At rooflite, our soil systems have optimized properties for both water management and plant health, which always leads to significantly better stormwater retention results than what current theoretical models predict.

Additional Amenities

And the best part of green roofs? Your green roof stormwater BMP will come with an abundance of additional benefits and values. While additional amenities may not be the primary concern for civil
engineers, green roofs provide a host of amenities, including green space, environmental benefits, and preservation of valuable real estate at ground level, to building owners and occupants and the wider community.  Combine all of that together, and you have a unique set of advantages that no other stormwater management tool can provide.

A more natural and resilient environment, a healthier, happier and more productive population, and a much better financial bottom line are all added values that should encourage planners to choose green roofs to manage stormwater in urban areas.