In Egypt, soil-less agriculture is used to grow plants on the roofs of buildings.  No soil is placed directly on the roof itself, thus eliminating the need for an insulating layer; instead, plants are grown on wooden tables.  Vegetables and fruit are the most popular candidates, providing a fresh, healthy source of food that is free from pesticides.
A more advanced method, (aquaponics), being used experimentally in Egypt, is farming fish next to plants in a closed cycle.  This allows the plants to benefit from the ammonia excreted by the fish, helping the plants to grow better and at the same time eliminating the need for changing the water for the fish, because the plants help to keep it clean by absorbing the ammonia.  The fish also get some nutrients from the roots of the plants.