Drain sheets or drain core are the most important feature in a green roof system.
As the name suggests a drain sheet allows free water flow underneath a growing medium, and removed from the roof.
Without a drain sheet, the growing medium would sit in puddles on the roof, fully saturated, jeopardizing the plants development and health.
Drain sheets comprised of little cone shape dimples, densely placed next to each other, either roll-formed or by vacuum forming are the most common.
Also available are drains that look like cat hair packed together in between a filter.
Either of these materials would be acceptable, depending on the application. The compression strength of the drain core could be a factor that determines the material used. For roofs you can drive on, compression strength is very important, because of dynamic and static forces that weigh on the drain core. Vacuum produced drain cores are often stronger and can withstand higher weight & pressure than the hair-like or rolled form drain cores.