The City of Minneapolis replaced a worn-out conventional roof with a vegetated green roof on the City-owned Target Center sports and entertainment events center. The nearly 2.5 acre (almost 113,000 square feet) extensive vegetated green roof is the largest in Minnesota and, at the time of design, the fifth largest in the United States. The roof features a 2.75-inch thick growing zone in the center of the main arena roof structure, with a deeper 3.5-inch thick growing zone around the perimeter where the building has greater structural capacity. The greater depth at the perimeter maximizes stormwater retention and plant vigor. The roof’s vegetated matter is a pregrown mat of sedum and native plants rolled out similar to grass sod; this mat is augmented by plug plantings native to Minnesota’s prairies. The roof features a water-efficient irrigation system. The roof includes an electro field vector mapping leak detection system to pinpoint leaks in the waterproofing membrane.
- The roof is able to handle up to 0.9 inches of rainfall without runoff, capturing an estimated 20 million gallons of stormwater per year that would otherwise flow to the Mississippi River.
- The green roof is designed to attract Karner blue butterflies, an endangered species.
- 100 percent of the existing roof ballast was recycled: 450 tons of existing roof, 590 tons of existing pavers, 140 tons of existing roof membrane, and more than 60 truckloads of existing roof insulation.
- The green roof will last twice as long as a conventional roof, at least 40 years.
- The green roof helps improve air quality in downtown Minneapolis by mitigating nitrous oxides and volatile organic compounds as well as reducing air borne particulates.
- The green roof helps mitigate the heat-island effect in downtown Minneapolis by reducing the roof’s temperature by as much as 80 degrees Fahrenheit.