Concerning rooftop gardens, they are an ideal solution and you could easily grow with 6-8" of Mel's Mix.  What we usually suggest is that you lay down a sheet of clear 4 mil plastic as a barrier between the roof top and the soil mix. Weight is usually not a problem with the soil if you use Mel's Mix. Next comes your wood border either 1X or 2X lumber and either 6 or 8" deep, then fill that with Mel's Mix and I'm sure you know the formula for that. Top it off with a grid and you're ready to start planting. Roof tops require a few special precautions because of the extreme sunlight and often wind. But its very possible to have a very productive and gorgeous garden on the roof top.

If it's a large project, we recommend SFG beds 4ft. wide by 12 or 16 ft long with the usual 3 ft wide isles between them. We also recommend a thick layer of mulch once the plants are up and growing to help conserve water and keep the soil from getting too hot. Roof top gardens are usually quite an advantage when considering vandalism and its easy to limit access to those that are supposed to be there. Of course the disadvantages are the visual and promotional opportunities as only the flights over the town will see the gardens!