Target Center green roof: Is it worth it?

Green roofs, while pricey, are usually a good thing. By planting atop a building's roof, the structure can produce less runoff, cool itself and even contribute to the cooling of a densely populated area such as a downtown. 

But the green roof on the Target Center, expected to be approved by the City Council for a total of $5.3 million, has us scratching our heads. The project would be the city's most expensive since it bought the Target Center and will take 20 years to actually become cost effective, the Star Tribune says

What's the irony in that? In 2029, the Timberwolves's lease at the Target Center will have expired four years earlier.

In other words, we are making big bets that the Target Center will still be a viable arena for Minnesota basketball, concerts and other large events. It's already the seventh oldest in the NBA and we know how much sports teams like to build new places to play. 

More from the Strib:
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.
So is it worth the city's money to green wash the Target Center roof? Is the environmental impact worth the investment?