Manage / Absorb Stormwater Runoff
Green roofs are a best practice for onsite water management. Here's why: They can reduce the storm water runoff volume to sewer systems by 50 - 90% and peak storm water runoff flow by 75%-90%.*
By comparison, a typical roof will retain 10% of storm water runoff.
Why should I care? Hundreds of communities in the United States have combined sewer systems. During heavy rainfalls excess water and the water contaminants associated can lead to combined sewer overflows into our watersheds. Green roofs present an opportunity to mitigate this all-too-common problem by absorbing much of the water in the root systems and releasing the rest back into the atmosphere through evapotranspiration. If there is runoff, green roofs slow it so that the sewer system isn't overburdened during peak usage.
Most commercial spaces are required to retain stormwater on site. A green roof eliminates or greatly decreases the space needed and costs associated with dealing with the stormwater.
View a study on storm water retention conducted in the city of Boston which demonstrates a 64% storm water retention rate in a green roof study conducted over a 10 year period.
Ford's Rouge Plant, at over 450,000 square feet, is the largest green roof in the United States and saved over $10 million on the installation of a storm water runoof treatment system.**
* Penn State University Green Roof Research
** Apex was not involved in the Ford green roof installation