I'm in the process of removing all of my garden, and starting afresh. I live in a roof-appartment. My garden is built around the roof, it is about 60 cm height & wide (~2 feet). It is some 30 meters long (~90 feet). I'm looking for tips about what to fill it with. I'm clearing away earth made of fine and coarse lava rock material, which was on top of biological barrier cloth on top of coarse stones (used for water drainage). I understand there are new materials that can be used for such roof-top container gardens. Pointers please ? Danny.