The city says the go-ahead, expected today, signals an investment that outlasts the Timberwolves' tie to the arena.
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.
The new roof is expected to last at least 40 years and perhaps beyond, according to a consultant analysis. It will have a 20-year warranty and maintenance agreement plus a system that uses low-voltage electrical current to pinpoint leaks.
A life-cycle cost analysis found that the green roof is slightly cheaper once it hits 20 years. But the basketball lease expires in 2025, about when the arena reaches the average age when pro arenas are demolished, according to one report.
"This particular improvement could be viewed as expressing our desire to maintain the viability of the building for a longer period," said city finance director Pat Born.
The Timberwolves make up a larger share of the building's annual attendance than they do the share of annual events, according to AEG, the building's manager. But they're still the building's key tenant.
The city has been on a schedule to pay off bonds that financed the 1995 arena purchase by the 2025 end of the lease. But it has been making plans to shorten the schedule by five years with the help of a portion of property taxes tied to tax-increment districts that they have been allowed to keep.
The shorter payoff will free other revenue to pay capital and operating needs of the facility. This includes the city's lease obligation to the Timberwolves to maintain a facility comparable to other league arenas, Born said.
"We have a number of areas in this arena that could be improved on," including amenities such as concessions, bathrooms and entries, said Timberwolves CEO Rob Moor.
Born said if the Timberwolves left the arena at the end of the lease, Target Center could still remain viable for "quite some time."
But looking out that far, Goodman said she doesn't want the city to get pressured into the pro facilities equivalent of an arms race. She said the building can stand the test of time with periodic capital improvements. Steve Brandt • 612-673-4438