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Growing Green Rooftops in Urban Areas

green roof
The green roof at the EPA Region 8 building in downtown Denver, CO includes six varieties of drought tolerant sedum.

A green roof is not just a roof that is green in color and it is not a roof made of any environmentally-friendly roofing material. Instead, a green roof is a roof topped with plants – a living roof.

Roofs covered with living plants have many benefits in urban areas - from reducing city heat and air pollution to decreasing the rate of storm water runoff. Green roofs also serve as an extra layer of insulation, reducing the amount of heating in the winter and air conditioning in the summer, and thus decreasing energy consumption.

In urban areas, where heat-absorbing concrete and asphalt have replaced native vegetation, summer temperatures can climb higher than in nearby rural areas. This phenomenon, called the “heat island effect, can raise the temperature of urban areas 2 to 10 degrees. Green roofs can help combat the heat island effect. On a hot summer day, the surface of a green roof can be cooler than the air temperature, while a rooftop without plants can be up to 90 degrees Fahrenheit warmer.

Green roofs can slow storm water runoff, which can be a major problem in urban areas. When the natural landscape is replaced with concrete, asphalt, and other impermeable materials, storm water runs off very quickly, causing erosion of the banks of streams and rivers which changes the habitats available for fish and other species. The plants and soil of a green roof release rainwater much more slowly than a flat roof, more like the natural environment. Plus the water that runs off a vegetated roof carries fewer pollutants because the plant roots filer out some of the pollutants naturally.

Green roofs also carry the same benefits of any open space or garden. The plants help decrease air pollutants and, through photosynthesis, decrease levels of the greenhouse gas carbon dioxide. They also provide habitat for birds and other small animals. Plus the rooftop space can become a beautiful space for people to enjoy.

Green roofs have been relatively common in parts of Europe for a long time. Vikings (Norse) planted roofs with grasses in northern Europe more than a thousand years ago. In the last few decades green roofs that utilize more modern technology have been installed in Germany, France and other Western European countries. Now green roofs are becoming increasingly common in American urban areas. Chicago’s City Hall was topped with a green roof in 2001. In 2003, the Multnomah County Building in Portland, OR acquired a meadow of drought-tolerant sedum, grasses, and wildflowers on its roof. A 3000 square foot green roof was added to Atlanta’s City Hall in 2003. And in 2006 the EPA building in Denver, CO installed a 20,000 square foot green roof covered with six varieties of drought-tolerant sedum. Green roofs are sprouting up on some residential buildings as well including apartment buildings and lofts. There are engineering considerations when an existing building is retrofitted with a green roof because of the added weight.

Green roofs usually have some sort of soil or an engineered growing medium in which the plants grow. There also needs to be a waterproof membrane to project the building from leaks. A green roof may be partially covered with plants or entirely covered. Although there are examples of vegetation grown on sloping roofs, most green roofs are on flat roofed buildings.

There are two varieties of green roofs: intensive and extensive. An intensive green roof has the plants and growing medium built into the roof. An extensive green roof has trays or other movable planters on top of the roof. There are advantages and disadvantages to both roof types. An intensive green roof can often have a greater depth of soil or planting medium. This means that it can become home to plants with deep roots and it can also have greater insulating benefits. However more planting medium means that the building needs to be designed to support more weight. On an extensive roof, the plants and growing medium are contained within shallow trays. The trays can be easily changed out if needed, making the system more flexible. However the shallow depth of most tray systems means that only plants with shallow roots will thrive.

For more information:

The above, written by Lisa Gardiner, was designed for Understanding Climate Change Today, an online course for educators. Please visit NCAR Online Education for more information about this and other courses.

Last modified April 9, 2008 by Lisa Gardiner.

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