roof garden 
noun
A garden on the flat roof of a building.Obviously people are being driven towards energy efficiency in buildings, and the roof garden would be one way of ensuring that you save energy....
- Designed by John Fleming Architects, all units have balconies and en suites, and each apartment block will have its own roof garden and landscaped courtyard areas, with a new market green at the front.
- For those who want to garden now, starting a roof garden can cost as little as $30.