Obviously, green roof technology is a bit more complicated than just throwing a bunch of dirt and seeds on your roof. Though systems vary, most have some similar elements. Before it's planted, the surface must be covered with an elaborate, cakelike assembly. The bottom layer is usually made of plywood, steel or concrete, sometimes topped with rigid foam insulation (the pink layer). Then, a rubberized liquid asphalt is laid down, followed by a polypropylene root barrier, a drainage layer, and a filter mesh fabric that will keep the lightweight growing medium in place. This "soil" consists of compost mixed with expanded shale, slate, or fragments of volcanic rock. An optional wind blanket can help hold baby plants in place. Finally, your drought-tolerant perennials, ideally 2 to 6 inches tall, can be planted.