Green Roofs Improve
Air and Water Quality

A "green roof" may be no more than a few plant-filled containers, such as these self-watering planters.

In most urban areas, more than 75 percent of the land is covered by buildings, sidewalks and parking lots. All that pavement has turned many cities into hazy heat islands that channel millions of gallons of polluted water into rivers and lakes.

Urban residents across the world are starting to look up, and turn a plentiful resource into a solution: green roofs. A green roof may be no more than a few plant-filled containers, or it may be completely covered with several inches of soil (on top of a waterproof barrier) and a meadow or collection of succulents.

Green roofs have many benefits. They help moderate temperatures, improve air quality, reduce storm-water runoff and create habitat for birds and butterflies. They can create a green refuge wihin a sea of concrete.

Container and rooftop gardening is an exciting trend in the United States. Now more elaborate green roofs, called "extensive green roofs," are catching on too. In Chicago, a citywide program is helping businesses and homeowners to plant gardens on their roofs through tax incentives and technical help. Portland, Seattle and the state of Maryland also offer tax incentives for creating green roofs.

Here are three ways that turning your roof into a garden can help improve the environment:

1. Lower temperatures and better air quality in the summer
A "heat island" is created when a city's asphalt, buildings and rooftops absorb heat from the sun and then release the energy at night. That can make a city 6 to 8 degrees hotter than in the surrounding rural areas. In fact, when air temperatures reach 95 degrees F or higher during the summer, roof surface temperatures can reach 175 degrees F.

When temperatures are hotter, people use more air conditioning, electric plants must work harder and thus more pollution is created. Pollution, in turn, reacts with heat and sunlight to form smog.

Plants cool the air by transforming heat and soil moisture into humidity through evapotranspiration. So, if you increase the number of green plants in a city, the temperature will actually fall. According to an EPA computer simulation, increasing greenspace in Los Angeles by 5 percent could lower summer temperatures by 4 degrees. Those lower temperatures would decrease smog by 10 percent and save millions in energy costs.

2. Insulation in the winter
Green roofs lower temperatures in the summer, and insulate in the winter. On average, extensive green roofs provide 25% more insulation than a regular roof. Heat loss due to wind can be reduced by 50%.

3. Reduce stormwater runoff
When rain falls on a forest or meadow, the water gradually percolates down through the soil. About 30% of it reaches shallow aquifers that feed plants; another 30 percent percolates into deeper aquifers and approximately 40 percent is almost immediately returned into the atmosphere through evaporation and plant transpiration. There is virtually no surface runoff!

Compare that to what happens in a city. Only 5 percent fills groundwater aquifers and just 15 percent evaporates into the air. A whopping 75 percent of the rainwater becomes surface runoff. Communities build stormwater collection systems and direct that runoff—untreated— into rivers and lakes. There's a direct link between runoff from paved surfaces and a decline in water quality for nearby streams, rivers and lakes.

Green roofs can help significantly. On average, a roof that's covered with soil and plants retains 75% of the rain that falls on it. Just 25% becomes runoff. The soil also traps sediments, leaves and particles, essentially treating the water before it even reaches the sewer system.

Visit Greenroofs and learn more about rooftop gardening.

For more articles on improving the world through gardening, see the Garden Activist archive page.