A green roof (or "living" roof) is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems.
Green roofs serve several purposes for a building, such as recreational space for residents; increased benevolence and decreased stress due to the aesthetically pleasing landscape; longer-lasting roof; increased property values; possible tax breaks; lower electric bills; improved air quality; and reduced greenhouse gas emissions; reduced stormwater runoff; increased urban biodiversity; reduced heat island effect; and building soundproofing.
There are two types of green roof: "intensive" roofs, which are thicker, with a minimum depth of 12.8 cm, and can support a wider variety of plants but are heavier and require more maintenance, and "extensive" roofs, which are shallow, ranging in depth from 2 cm to 12.7 cm, lighter than intensive green roofs, and require minimal maintenance.
If you don't have any walking area on your roof or prefer a low maintenance green roof, go "extensive."
If the rooftop is walkable and you want to enjoy a garden, then a "semi-intensive" garden with some grasses and low shrubs, or an "intensive" garden with a thicker substrate of 50cm permitting many kinds of plant and even trees, can be possible.