It’s not often that a roof serves butterflies and basketball players, but the roof at the Target Center in Minneapolis, US, does both. That’s because this green roof not only provides shelter to the National Basketball Association’s Minnesota Timberwolves and their fans, but it also features vegetation designed to attract the endangered Karner Blue Butterfly.

Capturing storm water

But attracting butterflies wasn’t the main reason the city of Minneapolis went with a green roof when it replaced the existing EDPM roof on the sports arena. “We had a problem in the city with sewer overflow problems, and every drop of water that went on the old roof went onto the sidewalk, into the sewer system and then into the Mississippi River,” said Minneapolis City Council member Lisa Goodman. The new green roof is designed to capture more than 2 cm of rain before run-off occurs, and is expected to prevent an estimated 3.8 million liters of storm water.

Largest pioneer on a sports arena

At 10,000 m², the Target Center’s vegetated roof is the largest in Minnesota and the first green roof installed on a sports arena in North America. The new roof was not to weigh more than the previous ballasted roof and the existing loading limits. A vegetated system over a lightweight waterproofing system met that challenge. The Sika Sarnafil waterproofing PVC membrane was chosen for the green portion of the roof. The membrane handles the effects of root growth, is resistant to bacteria and handles varying hydrostatic pressures as well as preventing moisture wicking at seam edges.

Tight installation schedule

When a roof has 29 different areas and is in the middle of a bustling downtown, there are bound to be installation challenges, and this roof was no exception. One major challenge was installing the roof at the same time that the new Minnesota Twins baseball stadium was being constructed next door.

Freight and materials had to be scheduled carefully because of the congested area, and 25 people worked day and night, with much of the material haulage taking place at night.

In addition, the roof had to be installed around the schedule of the Target Center, which remained operational during the installation and held 65 separate events during construction.

Happy basketball players and butterflies

Throughout the design and installation process, a Sika Sarnafil representative was always available to make sure the quality of construction was up to their specification. The vegetation on the roof consisted of a pre-grown mat of sedums and plants that are native to Minnesota. Lupines were also added to attract the Karner butterflies in order to enhance plant diversity and ecological resistance. This roof also reduces energy costs, since it doesn’t get nearly as hot as the old rock ballast roof.

Other cities could also benefit from making their big arena roofs more sustainable. That’s something both butterflies and basketball players can applaud.

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