Not-So-Secret Green Roof Gardens

Green roofs are a natural cure to problems created by blacktop jungles.

| July/August 2002

  • Basic components of a green roof include: 1) Biodegradable wind blanket; 2) Soil; 3) Drainage layer; 4) Insulation (optional); 5) Roof barrier
    Illustration by Gayle Ford
  • Perennial Sedum varieties make up most of the maintenance-free vegetative cover on the roof of the Fencing Academy of Philadelphia. Fescue grass, sedge, allium, burnet, and dianthus provide accents. Landscape Design: David Ben Yaacov, RA. Roofscape: Roofscapes.
  • Chicago City Hall’s living roof is part of the city’s Urban Heat Island Initiative project. The landscape design is based on a formal garden and includes a drip irrigation system fed partially by water collected from the adjacent penthouse roof. Roofscapes contractor: Church Landscape. Landscape architect: Conservation Design Forum. Architect: McDonough + Partners.

Choosing the most appropriate environmentally responsible roofing material for your home may not be easy, but the benefits speak for themselves.

Conventional roofing materials tend to be failure-prone, and most find their way to a landfill within twenty years. Asphalt roofs encourage excessive storm water runoff that washes sediment and other contaminates into our water supply. In addition, roofs, buildings, and roads absorb tremendous amounts of solar energy and reradiate for hours afterward, creating a high-temperature phenomenon called the urban heat island effect, which alters weather patterns and speeds the chemical reaction that creates ozone, the primary component of smog.

Vegetative coverings, known as green or living roofs, can naturally address several major environmental issues—energy/resource conservation, storm water management, climate change, and smog—without taking up even an extra square foot of space.

In Europe, green roofs have been successfully used on homes, office buildings, and parking garages for more than thirty years (more than 10 million square feet of green roofs are installed in Germany each year). Now, several U.S. cities including Seattle, Chicago, and Philadelphia are evaluating the costs and performance of green roof systems for large buildings, and local grassroots groups such as the Northwest EcoBuilding Guild are also exploring ways to bring this technology to homeowners.

The benefits of going green

Green roofs do a lot more than just look good: The plants’ leaves catch dust, the roots filter water, the soil provides noise and heat insulation, and the entire roof provides a habitat for birds and insects. And that’s just the beginning. In urban areas, 75 percent of rainwater becomes surface runoff, and cities spend millions on sewer systems and treatment plants to address these periodic floods. Older cities that use combined systems for handling sewage and storm water cannot handle heavy rains; consequently, contaminates frequently overflow into nearby rivers. Green roofs offer an immediate ­solution. The soil and plant mats retain 75 percent of rainfall, and, even at full saturation, the soil and roots trap sediment from the runoff before releasing it. The retained moisture is released gradually, slowly introducing humidity that helps cool nearby areas.

Nonvegetated surfaces absorb incoming solar radiation and reradiate it as heat. During a typical ninety-five-degree day, for example, blacktopped surfaces can reach 175 degrees or higher. Temperatures in big cities average about 10 degrees higher than in neighboring areas. But computer models performed at the Lawrence Berkeley Laboratory in Berkeley, California, suggest that increasing green space by 5 percent in a city such as Los Angeles would lower summer temperatures by four degrees, resulting in 10 percent less smog and an annual energy savings of $175 million. “Cooling a city could reduce smog more than almost every other pollution-fighting measure,” says Hashem Akbari, a researcher at the lab.



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