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Green Roofs—from Babylon to Blue Earth County

Legend tells us that, around 600 B.C., near present-day Baghdad, King Nebuchadnezzar II built the Hanging Gardens of Babylon, one of the ancient wonders of the world. Although called “hanging,” it is likely that the gardens were actually irrigated rooftop gardens.

A millennium and a half later, in the 9th century, Erik the Red’s Norse followers built grassy sod “green roofed” houses in Iceland and Greenland, because there were no trees available. Another millennium later, early American prairie dwellings were built in a similar style, for the same reason.

These early green roofed houses had a down side; a Nebraska historical society publication reports that “muslin or canvas was often tacked to the ceiling to catch dirt, mice, bugs, and other vermin which might enter through the ceiling.”

Modern green roofs

Fortunately, you don’t have to rough it with mice and bugs if you want a green roof in the 21st century. So what is a modern green roof?

A modern green roof is typically installed over an existing roof on a flat-roofed commercial building. It consists of a waterproof roofing membrane, an insulation barrier, root barrier, moisture retention layer, drainage material, a geotextile filter, a light soil substitute and plants. Green roofs are most noted for their ability to manage stormwater. Green roofs cut runoff by 40 percent or more and remaining runoff is delayed, reducing both volume and velocity. Rooftop plants are natural water filters, too.

There are other reasons to use green roofs. Field experiments in Canada have shown that a 6-inch deep green roof reduces heat gain by 95 percent and heat loss by 26 percent, which greatly reduces building heating and cooling costs. Sound insulation is also improved; a building with a roof less than five inches deep can reduce sound by 40 decibels.

Green roofs can also play a role in reducing a city’s urban heat island effect and in filtering out air pollution.

Intensive green roofs generally have soil a foot deep or more and hold more weight, including people, trees and shrubs. In downtown Minneapolis, Brit’s Pub offers rooftop lawn bowling and rents out its green roof for weddings.

Extensive green roofs have soil depth up to four or five inches and generally use low-maintenance ground-covertype plants. The 4,000 square foot roof garden atop Minneapolis’ Phillips Eco-Enterprise Center was designed to be low maintenance. While new plants need initial watering, Corrie Zoll of the Green Institute says, “Now that plants are two years old, I don’t expect to water them at all this summer.” The largest green roof in Minnesota is on the new Minneapolis Central Public Library, which includes three extensive roofs covering more than 19,000 square feet.

According to designer William McDonough, Ford Motor Company has saved as much as $5 million in stormwater costs by installing a 10acre green roof on its Rouge Center in Dearborn, Michigan. One Vancouver hotel grows herbs, flowers and vegetables on its roof, saving more than $30,000 annually in food costs.

Germany, where one in eight flat roofs is green, leads the world in green roof development. During the past 30 years, as Germany became increasingly urbanized, municipalities developed both stormwater fees and financial incentives for the creation of green roofs. A similar trend is developing in the United States, as municipalities must comply with state and federal stormwater regulations.

Minneapolis has joined the state of Maryland as well as Portland, Chicago, New York, Seattle, Atlanta, Boston and other cities that offer tax credits and other incentives for green roof projects. Chicago leads the way, with more than 200 green roofs covering more than 2.5 million square feet on buildings including City Hall (where beehives produce honey sold by the city), Target, McDonalds and soon, Wal-Mart. Green roof acreage increased 80 percent in the U.S. between 2004 and 2005.

Green roof research takes off

Kestrel Design Group Inc. recently won an Award of Excellence from Green Roofs for Healthy Cities for a demonstration roof, which compares traditional European green roof plants with plants native to the Minnesota bedrock bluff prairie. Hiawatha light rail riders can view the roof, which can also be seen via live web cam on the Green Institute's Web site.

In Northfield, Carleton College students are testing 78 varieties of native prairie plants on an experimental green roof atop the Olin Science Building. The new Ramsey-Washington Metro Watershed District office in Little Canada is reducing its impact on nearby Gervais Creek with a green roof, rain garden and porous parking lot.

The future of green roofs in Minnesota

Kestrel Design’s Peter MacDonagh is optimistic about the future. “Minnesota is ideal place for green roofs. We have relatively low annual precipitation featuring small rain events, cloud bursts that wet and dry up quickly.” A green roof can absorb Minnesota’s typical 3/4 to 1 1/4 inch rain event and keep that water out of storm drains.

MacDonagh predicts most use in urban areas, but does not dismiss green roof technology for rural settings. In Switzerland, egg production has increased in hen houses that have the cooling features of green roofs. According to Blue Earth County Administrator Dennis McCoy, the county is committed to building its proposed justice center to meet Leadership in Energy and Environmental Design standards, with the potential of a green roof as the design work proceeds.

Upcoming Minnesota green roof projects include Minneapolis City Hall, Lund’s grocery store in Minneapolis and several condominium projects in the Twin Cities.

Green Roofs for Healthy Cities plans a trade show in Minneapolis next year, May 2-4, 2007. Additional information is available on the following Web sites:

• Green Roofs for Healthy Cities
• MPCA's Green Buildlings