A roof is an unused space just waiting for a creative landscape design, and planting it can have a mitigating effect on the environment of the building, but keep in mind the huge weight and damage wet soil can bring to the structure. No matter how sturdy the roof, plant material options will be limited, because of the limited root space on a roof. Large trees, even if they are located above structural columns in the building, need more room for root growth than a rooftop garden can provide. Use plant material suited to roof gardens—Sedums, Junipers, Herbs, Daylilies, vines, and ornamental grasses.
If you don’t know the weight and load your garden design will put on a roof, you could be creating a very dangerous, elevated Eden. Hire a structural engineer and check building codes. Don’t play with the idea of growing plants on a roof without careful load and drainage calculations. Use special structural soil manufactured to be light-weight. Provide an over-designed underground drainage system to accommodate future plant growth.
A roof garden must be able to withstand extreme weather and earthquake activity. If it fails under stress, all the floor below it can be damaged. A leaking roof can destroy an entire building. Rooftops that carry gardens must be able to carry wet soil and drain properly. Retrofitting an older building is difficult to do successfully, so the best rooftop garden for an older building is a garden of large planters. New buildings can be designed to accommodate a roof garden, and offer better opportunities to create a safe, successful structure.