"Green roofs are becoming common in Chicago, as well as Atlanta, Portland, and other United States cities, where their use is encouraged by regulations to combat the urban heat island effect. In the case of Chicago, the city has passed codes offering incentives to builders who put green roofs on their buildings. The Chicago City Hall green roof is one of the earliest and most well-known examples of green roofs in the United States; it was planted as an experiment to determine the effects a green roof would have on the microclimate of the roof. Following this and other studies, it has now been estimated that if all the roofs in a major city were "greened," urban temperatures could be reduced by as much as 7 degrees Celsius."
http://en.wikipedia.org/wiki/Green_roof

So, what is better. Solar panels or a green roof?
Would Walmart be more "green" to have a green roof as compared to solar panels?

Or say you wanted a "green" parking structure- 2 or 3 stories of parking and roof which
was green roof or had solar panels?