2. Consult the Pros
Hire an engineer, architect, or contractor to make sure your roof is structurally sound.
3. Pay Attention to Pitch
Flat roofs are best, but a moderate slope -- less than 30 percent -- is workable.
4. Know Before You Grow
Decide what kind of garden you want, factoring in light conditions and soil depth.
5. Establish a Water Source
Have access to water -- such as drip irrigation, hose and spigot, rain barrel -- so you won't have to lug buckets up, down, and all around.
6. Plan for Drainage
A sublayer of gravel, sand, pebbles, or lava stone will keep excess water from accumulating, but make sure your roof can handle the weight. Consider installing a specially designed complete green roof membrane, root barrier, insulation, and drainage mats or a series of channels to keep water flowing.