Rooftop gardens offer energy savings by providing better insulation, reducing heat affect on building, and cutting back on storm water runoff. But they also offer educational space (for school kids growing their veggies for school cafeteria to social service projects teaching people how to garden and cook), personal gardening, community gathering (shared rooftop gardens for an apartment building), and healthy living in a city. Rooftop gardens also offer longer growing season and less pests. Many cities and/or states - Chicago, San Francisco, and New York State, etc – are encouraging rooftop gardens and offering tax incentives. Read this NYT article (6/16/09) for more information and check with your state to see if you can get a tax break or other incentive to start a green roof on your library.