HydroBlox has been tested for this application. The conclusion ~ HydroBlox meets and exceeds every other material for green roofs.
HydroPlanks have been university tested with a result of no finer product on the market.
Why are HydroBlox better?
Plants can grow directly on HydroBlox without soil. The result is a much easier and less expensive installation. Without soil, the weight on the roof is greatly reduced. The logistics and expense of moving soil are eliminated.
As an alternative, sod can be placed directly on the HydroPlank for an instant green roof!