The green roof expected to be approved today for the Target Center in Minneapolis represents a bet by city leaders that the arena will remain viable beyond the city's lease with the Timberwolves.
That's because consultants say it takes 20 years for the extra expense of a longer-lived green roof to become cost-effective; that's when a conventional roof installed now probably would need replacement. That would come in 2029, four years after the Timberwolves' lease expires. The arena already is the seventh oldest in the NBA.
The roof replacement is expected to be approved by the City Council for a cost of $5.3 million, making it the city's most expensive project since it bought the arena. The project includes a green main roof and conventional replacement of the arena's 29 smaller roofs.
Initially consultants concerned about whether a heavier green roof would overload the structure advised against it. But under prodding from two green-roof advocates on the council, Lisa Goodman and Scott Benson, they did more homework.
They recalculated the building's structural capacity, adjusted for a lower snow-load factor and looked into roofs that use shallower, therefore lighter, rooting layers for vegetation.
The recommended roof would feature a growing zone 2.75 inches thick for much of the 115,000-square-foot main roof and 3.5 inches in a 32-foot-wide edge where there's more structural support. Because that's thinner than some heavier green roofs, a drip irrigation system would provide water when rain doesn't.
Supporters say that this depth should handle rainfalls of up to .9 inches without runoff. The 32 species of sedum and prairie plants that do well in thin soil also will reduce rooftop temperatures so less heat will radiate through downtown.
"This is about the most simplistic roof that there is for doing right by the environment," Goodman said. Consultants say they think the roof would represent the largest such thin green roof in Minnesota.
