Green roof gardens on top of buildings serve multiple purposes from insulation to improving air quality plus a chance to grow some of your own food.
Having a quiet place to go to right above your head is a plus.
Of course a project of this type needs a building that can structurally support it.
The Scandinavian Green Roof Association shares a wealth of information on Urban Green Roof Gardens including a Beginners Guide to Building a Green Roof.
Some of the information offered is gathered from a course in 2004 at Malmo University called Green Roofs and the Urban Environment so it will need updating.
Training sessions and seminars are offered in the U.S and Canada by Green Roofs for Healthy Cities as well as practical information like the green roof elements graphic above.
Up on the roof for Green Day # 211
(* Green Roof Elements graphic from Green Roofs for Healthy Cities site)