Bureau of Watershed Protection
What is a Green Roof

Atlanta City Hall Greenroof

A Greenroof is simply a roof that is covered with vegetation growing in a thin layer of engineered soil or specialized growing medium. These are also referred to as ecoroofs or living roofs.

While Greenroofs are a relatively recent innovation in North America, they have been in use in Europe, especially Germany, for the last several decades. The roots of greenroofs go back even further: Scandinavian houses with living sod roofs were built over 1,000 years ago.

Stormwater Absorption and Water Quality
The most significant benefit of greenroofs over traditional roofs it that they absorb and filter stormwater. A typical greenroof can retain 50%-70% of all water that falls on it, thus reducing the volume of stormwater runoff and protecting the watershed. Greenroofs also help clean urban stormwater. Even before it hits the ground, the rain in Atlanta has picked up pollution from the air. The vegetation and soil on the greenroof filter out most of these pollutants before they reach the ground.

Urban Heat Island Mitigation
As our cities have grown, we have replaced natural vegetation with roads, parking lots and buildings. These surfaces absorb, rather than reflect, the sun’s heat and cause urban temperatures to be higher than those in the countryside outside the urban core. It is similar to the phenomena that can make an asphalt parking lot too hot to touch in the summer, while the lawn beside it is cool enough to sit on. Raised urban temperatures negatively affect air quality, energy use, local weather patterns, and ultimately, our health. The vegetation on the greenroof counters this heating effect by reflecting the sun’s rays and transpiring cooling water vapor.

Air Quality
Like all vegetation, the plants on the greenroof absorb carbon dioxide (CO2) and release oxygen (O2), creating healthier air and helping to reduce greenhouse gasses in the atmosphere

Conservation and Energy Savings
Greenroofs protect roof surfaces from the punishing effects of the sun and weather and thus double or triple the life of the roof. Because greenroofs have a cooler surface and natural insulating properties, they help moderate building temperatures, reducing energy use for heating and cooling.

Habitat and beautification
Greenroofs can provide limited habitat for some birds and insects, and some greenroofs are accessible to the public, providing natural beauty and much needed greenspace for urban dwellers.

Designing and Building a Greenroof
Atlanta City Hall Greenroof

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