Water Wednesday: Living Roofs Reduce Energy Use, Stormwater Runoff

By Angela Sena

We’ve come a long way since the days of noisy, hot tin roofs. Today we have shingles, tile, and even roofs that live and grow while helping the environment. These green roofs really earn their distinction during the summer months, and are becoming increasingly popular in the U.S. They can be as simple as a 2-inch covering of hardy groundcover, or as complex as a fully accessible park complete with trees.

A green roof, or living roof, is a roof of a building (usually commercial) that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers, such as a root barrier and drainage and irrigation systems.

Green roofs provide lasting benefits that include:

Municipal buildings, like this Seattle public library, are used to show green roof benefits.

Municipal buildings, like this Seattle public library, are used to show green roof benefits.

  • Reduced energy use by absorbing heat and acting as building insulators.
  • Reduced air pollution and greenhouse gas emissions by lowering cooling demand and providing pollutant-reducing vegetation.
  • Improved human health and comfort by lowering heat stress indoors, and helping to mitigate the heat island effect outdoors in urban areas.
  • Enhanced stormwater management and water quality by reducing urban stormwater runoff and filtering pollutants from rainfall.
  • Improved quality of life by providing aesthetic value and habitat for many species.

Types of green roofs include:

  • Extensive Roof – shallow, lighter roof requiring minimal maintenance
  • Semi-Intensive Roof – roof with moderate cost and maintenance that supports some smaller vegetation
  • Intensive Roof – thicker, heavier roof that supports a wider variety of plants and requires more maintenance
  Extensive Semi-Intensive Intensive
Plant Options Sedum, moss, grass Sedum, moss, grass, herbs, flowers, shrubs Sedum, moss, grass, large shrubs, trees
Soil Depth 2-5 inches 5-8 inches 8-30 inches
Dry Weight 10-25 lbs. 25-40 lbs. 40-100 lbs.
System Types Tray, built-up Tray, built-up Built-up
Maintenance Minimal Occasional/routine Routine
Initial Cost Low Medium High

 

Green roofs support diverse vegetation.

Green roofs support diverse vegetation.

Rooftop areas as a percent of total impervious area range from 30-35 percent in suburban areas to 70-75 percent in downtown business districts, and can be as high as 80 percent in some warehouse and industrial areas. Even partial mitigation of these areas can reduce runoff volume by up to 50 percent.

In summer, green rooftops retain 70-100 percent of the precipitation; in winter, they retain 40-50 percent. Green rooftops can reduce the total annual runoff by 50-60 percent, which is significant in urban areas. This also helps in areas with aging infrastructure and challenging stormwater issues, similar to Kansas City.

The initial cost of a green roof can be 30 percent greater than a conventional roof, but the long-term savings on maintenance and energy costs can offset the initial cost by increasing the roof’s lifespan by 50 percent. Transforming a conventional roof into a green one can actually prolong its life by up to 20 years.

Some notable examples of green roofs in the Kansas City area include the Kansas City Central Library, Polar Bear Passage at the Kansas City Zoo, Richard Bolling Federal Building, Kansas City Power & Light District Parking Garage, and the Kauffman Center for the Performing Arts, which is designed as a 4.4-acre open space park and is one of the largest green roofs in the U.S.

Tell us about some of your favorite green roofs here in the Heartland or around the country. (Make sure you share a link!)

About the Author: Angela Sena serves as an Environmental Protection Specialist with EPA Region 7’s Water, Wetlands, and Pesticides Division. She has a degree in environmental science and management, and is a native of New Mexico and an avid outdoorswoman.

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