by Russell Boniface
Associate Editor

Summary: The largest extensive green roof in Minnesota, a 2.5-acre vegetated green roof on the Target Center in Minneapolis, was recently completed. The roof was a collaboration of the Minneapolis office of Leo A Daly, the architect/engineer of record; Kestrel Design Group, a Minneapolis-based landscape architecture firm; and Inspec, Inc., developer of the roof system design and leak detection system. The project involved project management, problem solving, and teamwork in architecture, landscape architecture, and engineering.

The extensive vegetated green roof on Target Center is the largest in Minnesota, the fifth largest in North America, and the tenth largest in the world. It is also the largest extensive green retrofit in the world. Photo © 2009 Bergerson Photography.

The project called for the design and construction of an extensive vegetated green roof to be built on the main arena of the Target Center, the site of sports and entertainment events. The center is owned by the City of Minneapolis and was opened in 1991.

The Target Center project was a team effort. Leo A Daly provided project management; mechanical, electrical, and plumbing systems; and fire protection services. Kestrel Design Group was responsible for the vegetation, drainage system, membrane protection, and soil media. Inspec developed the roof system design and leak detection system. Stock Roofing installed the green roof, setting a goal of 100 percent recycling of materials.

The green roof will handle almost one inch of rainfall without runoff, capturing an estimated 1 million gallons of storm water normally draining into the Mississippi River per year. The stand-alone, low-weight roof maximizes the amount of green. Photo courtesy of Leo A Daly, Kestrel Design Group, and Inspec, Inc.

The Target Center green roof is the largest in Minnesota, the fifth largest in North America, and the tenth largest in the world. It is also said to be the largest extensive green retrofit in the world. The green roof is 113,000 square feet, or nearly 2.5 acres—the size of almost 3 football fields. It is also the first green roof installed on an arena in the United States.

“The roof was showing quite a bit of wear, so it was time to replace it,” says Frank A. Anderson, AIA, LEED AP, project manager, Leo A Daly, Minneapolis. “The City of Minneapolis owns the facility and has undergone a process of trying to vegetate green roof systems on its buildings.” According to the City of Minneapolis’ green roof policy, a green roof needs to be an option when a city building is being re-roofed. Anderson says a large challenge met by the team was factoring into its design the arena’s high-pressure wind uplift, with the roof being 150 feet above the arena floor. Gary C. Patrick, AIA, executive vice president of Inspec, called it a “real coordinated effort to keep the arena operating while getting the roof on too.”

Green roof benefits
“One of the drivers was the issue of stormwater runoff,” says Peter MacDonagh, director of Design and Science, Kestrel Design Group. “It is important because the city is on the Mississippi River. Minneapolis enacted a stormwater utility, charged per square foot of hard surface. There are no waivers, everyone gets charged, even City of Minneapolis property.”

There are more than 30 species on the roof. A pre-grown vegetation mat consists of sedum, native plants, and succulents. Photo courtesy of Leo A Daly, Kestrel Design Group, and Inspec, Inc.

The roof features a 2.75-inch thick growing zone in the center and a deeper 3.5-inch thick growing zone around the perimeter to maximize storm water retention. The green roof will handle almost one inch of rainfall without runoff, capturing an estimated 1 million gallons of storm water normally draining into the Mississippi River per year.

The green roof also will help mitigate the heat-island effect in downtown Minneapolis by reducing the roof’s temperature. The roof has an R value of 36. “I think there is additional R value that comes from the green roof,” Patrick says. “The workhorse for energy is in the insulation below the roof membrane. It will help keep the temperature down and maintain the roof system based on a cool system above rather than direct sunlight typically on a membrane.” The roof is expected to save almost $300,000 dollars in energy costs annually.

The green roof system
The stand-alone, low-weight roof maximizes the amount of green and exceeds codes. A passive leak detection system is in place called electro field vector mapping, with a screen that pinpoints leaks in the waterproofing membrane. “That was a design challenge,” says Patrick. “We have a green roof and lots of insulation below the membrane below the green roof. There is a lot of soil and plantings. If something happens down the road, you want to be able to get to it.”

There is a 20-year maintenance guarantee on the project for the vegetation—with no more than four square feet of bare soil allowed on the roof at any time. Photo courtesy of Leo A Daly, Kestrel Design Group, and Inspec, Inc.

Above the waterproofing membrane is a drainage layer; a filter layer to prevent roots from migrating into the system; and an irrigation drip system within a growing soil medium layer. The drip irrigation system will provide water to the plants during plant establishment and droughts. “It’s water under low pressure,” says MacDonagh. Piping is spaced every 18 inches. On top is a pre-grown vegeation mat that will grow down into the soil medium layer, with it becoming a single piece. The waterproofing membrane is designed to withstand constant dampness. The green roof also has a paver assembly.

A diversity of vegetation
There are over 30 species on the roof. The pre-grown vegetation mat consists of sedum, native plants, and succulents, which are plants with thick cuticles that absorb water. “When there isn’t much water available, it constricts, like a cactus,” says MacDonagh. “They are from mountain rages and places that are dry, gravelly, and have thin soil profiles and high temperatures. They have high ultraviolet and infrared resistence because they are exposed and stay green throughout the year.”

The green roof incorporates a paver assembly. Photo courtesy of Leo A Daly, Kestrel Design Group, and Inspec, Inc.

Native plantings include those from bedrock prairies along the Minnesota River and Mississippi River. Notes MacDonagh, “If two or three plants die, there are still a large number of plants that can fill in and take their place. Resilience is important.”

The plants can withstand dry and moist consitions and stay low without much biomass during the growing season, adds MacDonagh. “Occasionally tree seedlings grow up there and they get pulled out. That is the main manitenance. There is a 20-year maintenance guarantee on the project for the vegetation—no more than four square feet of bare soil on the roof at any time.” The green roof is also designed to attract Karner blue butterflies, an endangered species.