From afar, the pattern of the pavers that divide the growing zones gives the roof the appearance of a leaf.
Courtesy Steven Bergerson From afar, the pattern of the pavers that divide the growing zones gives the roof the appearance of a leaf.

Since Minneapolis opened the Target Center in 1990, local government has regularly refreshed the arena to maintain its competitive edge against newer sports and performance facilities elsewhere. Recent upgrades under the supervision of architecture, engineering, planning, and interior design firm Leo A Daly include an installation of seating that improves sight lines, as well as acoustical modifications in which 3,000 additional baffles were hung from the arena ceiling.

As the square-block building’s 29 rooftops approached the end of their collective lifespan, however, its overseers faced a critical juncture. The Minneapolis City Council had just passed a resolution ensuring that green roofs should be considered for any municipal construction or roof rehabilitation project. “Combine a vegetated roof with any membrane and you get two to three times the lifespan,” says architect Frank Anderson, a senior associate at Leo A Daly’s Minneapolis office. “The city first had to decide how long the building was going to be viable. It selected a Madison Square Garden approach by continually upgrading an existing building.”

In June 2009, the city of Minneapolis began installing an extensive green roof on the largest of the Target Center’s roof surfaces—the 113,000 square feet covering the main arena structure—and completed the job in early September with a construction budget of $5.2 million. In addition to consultancy leader Leo A Daly, Minneapolis-based landscape architecture studio Kestrel Design Group and the Midwestern engineering/architecture firm Inspec rounded out the design team, with Stock Roofing Co. shepherding execution.

Without disrupting programming in the Target Center, the existing roof was stripped to its truss structure and rebuilt. “We were limited to the weight of the roof we were taking off,” Anderson says, citing an average of 17.4 pounds per square foot. “The roof load, rigging loads, speakers, fire protection, and all the other incidentals that make a successful event center had to be taken into account. Nor could we add structure due to the associated cost.” Almost 100 percent of the detritus—1,180 tons of existing rock, pavers, and roofing membrane, and more than 60 truckloads of existing roof insulation—was recycled into insulation and ballast, notes Inspec vice president Gary Patrick.

The Target Center’s roof, which is the fifth largest green roof in the United States, is divided into nine sedums and features 23 regional species of vegetation.
The Target Center’s roof, which is the fifth largest green roof in the United States, is divided into nine sedums and features 23 regional species of vegetation.

Besides being the first vegetated surface on a sports arena, the Target Center’s is the fifth largest green roof in the United States. A project of this scale has specific challenges. For example, the original roof was designed with greater structural capacity around its perimeter to withstand 160-mile-per-hour wind gusts, but designed for only 90-mile-per-hour winds at its center. As a result, the growing zone in the center of the main arena roof is 2.75 inches thick, but at the edges it measures 3.5 inches thick. Moreover, the Bonn, Germany–based research organization FLL specifies maximum fire-break spacing at 125 linear feet and 15,000 square feet, and Target Center insurer FM Global has adopted these standards to reduce its risk exposure. New pavers were laid between the growing zones to provide these divisions—as designed, they appear like the veins of a leaf.

The growing zone is but one in a series of components that make up the new roof. The roof also includes three 2-inch-thick layers of polyiso board insulation that are fastened to the existing steel deck; between the insulation and its cover board sits an electric field vector mapping (EFVM) leak-detection system; and a Sarnafil reinforced-PVC roofing membrane tops that. Pre-grown vegetation mats sit on a growing medium layer interspersed with irrigation tubing, and this pairing is supported by water retention, filter, and drainage layers.

In addition to the sedums that typically populate extensive green roofs, the vegetation mats include local prairie plants. This combination reflects the mix of flora found in the dolomite bluffs overlooking the Mississippi and Minnesota rivers, says Peter MacDonagh, Kestrel’s director of design and science. “The bluffs are essentially like a fractured roof pavement, with certain species that grow when it’s very wet or very dry,” Since the Target Center’s arena roof approximates both the climate and growing substrate of the bluffs, so do the nine sedums and 23 regional species featured in the vegetation mats.

“At Kestrel we include native plants on all of our green roofs to increase plant species diversity and landscape resilience,” MacDonagh says. “Catastrophic failure, such as Dutch elm disease and emerald ash borer, often happens when relying too heavily on a few species of one genus.” The combination of plants not only fosters this mini-ecosystem’s resilience, but also mitigates nitrous oxides, VOCs, and airborne particulates.

Construction of the analogous landscape was labor-intensive. “When a concert comes, it brings its own semitruck to the event center,” Anderson explains. “All parties had to coordinate the work efforts so that loading and unloading could happen alongside the roof installation.”

Yet the result is worth it, economically and ecologically. The Target Center’s green roof will capture 1 million gallons of stormwater annually, absorbing as much as 0.9 inch of rainfall per weather event. Because approximately half of Minneapolis’ 30 inches of precipitation falls in smaller amounts, the Target Center earns a 50-percent credit on its National Pollutant Discharge Elimination System tax, MacDonagh says. He also notes that the garden atmosphere should not surpass 92 or 93 degrees on 90-degree days, compared with 170 degrees over a black asphalt roof.

Studies suggest that the Target Center’s green crown should last more than 40 years, and the design team calculates payoff by the 21st year of operation. EFVM, which monitors roof breaches via an applied low-voltage electrical charge, will ensure the durability of the installation. Patrick, for whom the project represented INSPEC’s first use of EFVM, expresses enthusiasm for the system and plans to use it again. And Anderson says the the project’s overall success bodes well for further deployment of extensive green roofs in Minnesota. “There has been resistance to a very thin system like this,” he says. However, the Target Center’s roof is thriving despite tough winters, which should be a boon to the deployment of extensive green roofs throughout the Midwest.

Materials and Sources

Pavers: Wausau tile ballast pavers, Wassau Tile,

Roof insulation: Sarnatherm, Sika Sarnafil,

Leak detection system: Aluminum Grounding Screen, Sika Sarnafil

Coverboard: DensDeck, Sika Sarnafil

Roofing membrane: G410 Roof Membrane, Sika Sarnafil

Drainage layer and filter layer: Colbond Enka Drain 3811R, Colbond,

Water retention layer: Green Geotextiles 200N Water Retention Mat, American Engineering Fabrics,

Growing medium layer: Skyland Rooflite Extensive MC, Skyland USA,

Pregrown vegetation mat: Sempergreen, Green Roof Producer,

Vegetation: Bachman’s Nursery,; Prairie Moon Nursery,

Irrigation: Below Flor Flat Dripeline Tubing, KISSS,

Lightning protection: Master Electrical, Independent Protection Co.,

Green Team

Architect: Leo A Daly,

Roof consultant: Inspec,

Landscape architect/biologist: Kestrel Design Group,

Client/owner: City of Minneapolis, Community Planning and Economic Development,

General contractor: Stock Roofing, a Tecta America Company,

Photography: Leo A Daly, Inspec, Kestrel Design Group

David Sokol writes about architecture and design from Beacon, N.Y.