It makes good environmental sense because the green roof will cut down energy consumption at the arena -- at a time when arena management is looking at higher evergy bills. It's also a great demonstration for the viability of the green roof in a large civic project: it will be the largest green roof in the state of Minnesota and the only one on an NBA arena.

But the project is not without some issues. First, it will take 20 years for the $5.3-million roof to pay for itself in reduced energy bills. Given how disposable arenas are these days -- and Target Center is already the seventh-oldest arena in the NBA -- investing money in the facility with an expected payback in 20 years is certainly an optimistic act on the part of the City Council. (Especially when you consider the Timberwolves are responsible for the vast majority of revenue at Target Center, and their lease ends four years before the payoff on the green roof.) Second, the payback assumes nothing goes wrong with the installation. Given that previous consultants had recommended passing on a green roof because it could overload the arena's structure (especially when combined with the heavy snowfall occurring in most Minnesota winters), that assumption certainly is optimistic.

So what exactly how does a green roof add to Target Center?

In this case, the green roof will add insulation to the Target Center roof, keeping the facility cooler in summer and warmer in winter. Technically, there are 30 separate roofs at Target Center -- one larger one, 29 smaller ones -- and the large, 115,000-square-foot roof is the one with the green roof installed. The growing zone will vary between 2.75 inches and 3.5 inches depending on how much load the building will support at a given point.

In terms of a green roof, 2.75 inches is fairly thin, so an irrigation system will be installed to keep the green roof green. In addition, the green roof is expected to absorb enough runoff in the city sewer system to save $10,000 annually in stormwater charges.

Prairie grasses and sedum will be planted.