Green roofs are a good idea. Here's an abridged list of their benefits: lowering heating and cooling costs; mitigating the urban heat island effect; extending the life of a roof; storm water management; and turning carbon dioxide into good, old oxygen.
And for new buildings in Copenhagen, they're mandatory as of last month.
Copenhagen has become the first Scandanavian city to adopt a policy that requires green roofs for all new buildings with roof slopes of less than 30 degrees. Copenhagen presently has 20,000 square meters (over 215,000 square feet) of flat roofs. It is hoped that as much as 5,000 square meters of new development each year will be covered with vegetation.\n
This is part of Copenhagen's effort to become "the world's first carbon neurtral capital by 2025," and unlike a lot of similar targets and visions, it seems like it's being taken seriously.