A green roof generally means a carpeting of plants on a roof, which improves the energy performance of buildings, and reduces storm water runoff. Though many urban dwellers have flowerpots or container gardens on their roofs, these aren't providing the energy savings that green roofs do. In order to start a green roof garden there are some things that must be taken into consideration:

Safety: First and foremost, the garden should be safe. This isn't just about protecting those on the roof from falling off. Since roofs are generally windy places, we also need to protect the passersby from falling tools. So install fencing and securely tie down all equipment. Consult the local department of buildings for any necessary permits and regulations about fence height.

Weight: Wet soil is heavy. So too is a foot of wet snow on top of that soil. So it is necessary to figure out the sustainable weight load of the roof (consult a structural engineer), as too much load in the wrong places can cause structural damage. In most cases, a planting medium instead of soil has to be used.

Containers: A container that is lightweight, but allows drainage is essential. For maximum energy benefits, it should sit flat against the roof. A modular planting system designed for green roofs can be used. So that the plant roots don't burrow into the roof membrane. Some containers like the recycled-plastic trays with a fabric weed block, wading pools, feed sacks and even recycled tires can be used.

Watering: It is to be done daily. This problem can be solved by running a garden hose up the side of the building, and attaching it to drip lines and a timer system.

Plants: Finally, what can be grown up there? Part of it depends on the available weight load.