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Target Center’s green roof controls stormwater runoff

July 1st, 2010 / By: / Case Studies, Feature, Landscapes

Green roofs are all about the cross-section. Building one is like making a giant sandwich, with different layers of materials serving different purposes. In most cases, geotextile fabrics play a prominent role in green roofs, beginning with the roof membrane itself. Atop the Target Center in downtown Minneapolis (home of the NBA’s Timberwolves and the WNBA’s Lynx), a new green roof sports a couple of unusual geotextile layers one that evenly distributes water across the entire roof and another that helps combat wind uplift.

The Target Center roof is one of the largest and most complex vegetative roofs in existence. “No one out there has built this system before,” says Frank Anderson, an architect with the Minneapolis office of international architecture firm Leo A Daley. Anderson came to the project when the city of Minneapolis, which owns the Target Center, began to consider replacement of a 20-year-old conventional membrane and ballast roof. The city has an ordinance that requires all roof replacement projects to consider green roofs as an option in a project cost-benefit analysis. Anderson invited roofing specialist Inspec Inc. and landscape architecture firm Kestrel Design Group Inc. (which had recently designed green roofs for the Minneapolis City Hall and Minneapolis Central Library) to join the project, and the three firms came up with a variety of replacement options.

They considered a 40-year timeline because, though typical roofs must be replaced after 20 years, the vegetation on a green roof doubles the life of the lowest (and most critical) layer of the sandwich—the waterproof membrane itself. That increased longevity made a green roof the most cost-effective option, despite nearly double the up-front investment. Minneapolis, which recently implemented dedicated stormwater utility fees, also stood to save around $10,000 annually. The roof keeps about 3.8 million liters of water per year on the roof (it evaporates or is used by the plants) rather than letting it run into the stormwater system and then local waterways.

The design team recognized early on that wind would be a huge issue. The Target Center green roof is 12,000m2 and perhaps a dozen stories in the air. It’s on the western edge of downtown right in the face of prevailing winds. If the roof were not engineered properly, stiff winter wind could scatter the layers of this expensive sandwich all over downtown Minneapolis. The key to beating wind, of course, is to lash down the top layer, but when that layer consists of plants and dirt and rock, that’s easier said than done.

Because of the roof’s size it was likely the team would be custom-growing vegetated mats that could be cut, lifted to the roof and rolled out. They decided to grow those mats on top of a geotextile erosion control fabric. As the 10 species of sedum sprouted in a field near Washington, D.C., their roots got entangled in the mat, essentially locking plants and soil and fabric together. When the mats were ready to be harvested, the grower cut through the erosion control fabric and rolled up the pieces for transport to Minneapolis.

Once on the roof and unrolled, the erosion control mat, embedded within the 25.4mm-thick sedum sod, was tied to the neighboring mat and—this is the critical piece—to a series of concrete paver pathways. This, says Kestrel landscape architect Peter MacDonagh, “allowed us to get the necessary weight on the roof.” The paver pathways are heavy enough to satisfy wind uplift requirements. By tying the vegetated mats to them and to each other, the entire top layer becomes one unified system.

The rest of the cross-section, working downward, consists of about 5cm of expanded shale growing medium with drip irrigation within it, a water retention layer, a Colbond Enkadrain® combination filter membrane and drainage layer, a Sarnafil® roof membrane system and, just above new, more efficient roof insulation, a leak detection system that uses electrical currents to pinpoint water that seeps through the membrane.

It’s the water retention layer, a mineral fleece called “The Green Geotextiles®” made by BOOM Environmental Products, that the designers hope will ensure the long-term quality of the roof. On a green roof this size, it’s not unusual to see “dead zones,” areas where, due to quirks of the roof or issues with drip irrigation, plants aren’t getting enough water. The water retention layer wicks water evenly across the entire roof, making moisture uniformly available.

To test that product and the entire system, the design team built a full-scale mock-up at Kestrel’s office. Anderson remembers when they activated the irrigation for the first time. “Water shot across the retention layer,” he says, “and the growing medium was still dry in places.”

But this isn’t just a technically advanced roof, it’s a pretty one too. Those paver pathways, which also serve as firebreaks, are arranged like the veins of a leaf. Seen from an airplane or a downtown skyscraper, the Target Center’s roof looks as green as it is.

Adam Regn Arvidson, ASLA, is a regular contributor to landscape architecture and design journals. His piece on artist Greg Hull appeared in the Jul/Aug 2009 issue.

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