Gardening on the roof requires much more preparation than gardening on terra firma.
First, it must be determined whether the roof can support the weight of the soil, the plants and the water. It may need to be retrofitted. Barring that, gardeners can place planters around the perimeter, which is generally its strongest part.
The containers can be almost anything: ready-made planters; boxes made of reclaimed wood, old milk cartons, children’s wading pools. A screen at the bottom holds in a lightweight substance, like packing peanuts for bulk, topped with a barrier fabric so the soil can’t go through. Potting soil, mixed with ingredients to lighten it, is put on top.
When gardens are planted directly on the roof, a waterproof membrane is laid down first, followed by insulation and a root barrier. (A guide to roof gardening is available at baylocalize.org.) -NY Times
An extreme example of a roof garden, in Vancouver.]]