The Wealth of Green Space

The Minneapolis Central Library in downtown Minneapolis is home to three green roofs, covering approximately 18,560 square feet, and is currently running under its projected utilities budget. And while Walter Gegner, the library’s division director of library operations, can’t attribute those savings to the green roofs directly, they do contribute to the building’s overall energy-efficient design.

Green roofs also trap storm water, a major issue for heavily paved urban areas. Heavy rains quickly run off the many smooth, paved surfaces in cities and quickly fill the gutters. Flooding can occur if the system hits capacity and the storm water has no where to go. Indeed, many municipalities will give buildings a credit on their utility bill for installing features that help manage storm water.

Nearly 90 percent of the rain in Minnesota falls at less than one inch at a time. Trapping one inch of rainfall requires only four inches of growing media (such as soil or compost), so a green roof can handle the average local rainfall and eliminate the runoff that would normally be produced by the rooftop it covers. “A green roof takes up to 90 percent of that rainwater offline so that’s not going into a pipe,” MacDonagh says.

For heavy downpours, green roofs are engineered with sophisticated drainage systems to handle even the biggest drenchings. What might have been storm runoff is put to use, evaporating via the plants and cooling the air.

Managing water pollution is also a benefit of green roofs. Elizabeth Ryan, president of Kestrel Design Group, notes that water trapped in green roofs and transpired doesn’t mix with litter and pollutants on the ground and then run into the river like storm water. “If we can utilize a majority of the rain and not send it down to, eventually, the Mississippi, we can cleanse our water and make for a healthier environment,” she says.

Green roofs not only manage water well, they can also increase the life span of a roof.“We typically experience the roofs lasting 15 to 20 percent longer [than conventional commercial-building roofs]. They stay cooler in the summer and warmer in the winter. The UV light doesn’t destroy the roof because of the plants that absorb it,” says Steve Hegge, project manager for Berwald Roofing, a North St. Paul roofing contractor that has been installing green roofs for 20 years.

Hegge uses the example of a hot asphalt driveway. When it’s first laid, it’s that nice, smooth black. But five years later, it’s gray, as the sun has leeched out all the oils. “The sun does the same thing to a roof,” Hegge says. Exposure to heat, cold, and UV radiation is what makes a roof fail.

Plant Matter

The plants on a green roof are chosen for their ability to survive in extreme temperatures and to retain water. To plant the 4,000-square-foot green roof on the Phillips Eco-Enterprise Center in south Minneapolis, designers looked to Minnesota river bluffs for inspiration. “These bluffs have conditions very typical to rooftops in Minnesota,” says Corey Brinkema, executive director of The Green Institute, a nonprofit housed in the center that works towards sustainable business and building practices. The bluffs are “covered in limestone rock with very shallow, porous soil, and they’re very susceptible to drought, rain, extreme temperatures, and lots of wind.”

The river bluff research led designers to choose varieties of prairie grass. Each green roof is different, but generally, varieties of succulents and sedums are used, as well as prairie smoke and spiderwort.