The Wealth of Green Space

Green roofs occasionally need to be weeded and checked for tree seedlings and may need to be watered, depending on the conditions of the season. Some plants do die off—usually 5 percent in the first year—so they should be checked every spring to see what needs to be replanted. “You should not have to mow it or maintain more than pulling a few weeds,” Hegge says.

Green roofs can either be intensive and extensive. Intensive roofs have growing media more than six inches deep. This type of roof is more expensive because it requires more materials and labor. It usually has plants with deeper root systems, and can handle being walked on. Extensive green roofs are lightweight and have a growing media of less than six inches; the plants used have shallower roots. Designers install extensive roofs to get the green roof benefits at a lower cost.

A cross section of a green roof would have many layers: Decking, a roof membrane, insulation, drainage mats, root barriers, and filter fabric are all hidden under the soil. A green roof is a sophisticated system, and each layer serves an important function. The construction and installation of the layers differs depending on the system installed. The layered components require very little maintenance, but can be replaced if a problem occurs.

Leaks shouldn’t be anymore of a problem than with a standard roof. “For the most part, if you do a green roof, it’s almost like a bulletproof membrane,” says Rick Carter, vice president of architecture and engineering firm LHB, Inc., in Minneapolis. “Sure, a leak isn’t impossible, you can always have a leak. But you’re as likely to have a leak on a green roof than on a standard building.”

Best Buildings

All those benefits do not come cheap. Green roofs typically cost two to three times more than a typical roof for design services and materials.

Among the good candidates for green roofs are large buildings, particularly in tight, urban spaces. “People who are seeing the biggest benefits from green roofs are property owners such as municipalities, universities, and others that own a lot of buildings and plan to own them for a long time,” Zoll says.

The Minnesota Green Roofs Council has noted approximately 80 green roofs in the state, most of them in the Twin Cities. Among those projects is the lawn-bowling green on Brit’s Pub in Minneapolis, several buildings on the University of Minnesota and Macalester College campuses, and Minneapolis Fire Station 14 in North Minneapolis.

The Edgewater condominium development, near Lake Calhoun in Minneapolis, chose to install a green roof for sustainability and marketing reasons. “We wanted to do something from the beginning that was very innovative,” says David Graham, founding principal at Elness Swenson Graham Architects, the firm that designed the Edgewater. “We felt that since we’re doing this building that will be around for many years right on the lake, we wanted to do a green roof to make a statement about water quality and filtering storm water.” The roof functions as a backyard    for the Edgewater’s three penthouse owners.

Several new projects are being planned. Of the hundreds of roofs his company installed last year, Hegge estimates that only two of those were green roofs. However, at press time, the company had bids out on 15 green roof projects. “People are becoming more conscious about being environmentally friendly,” Hegge says. “And it’s time for green roofs to be recognized.”