Medium Depth Roof Garden System (growth media depth of 5" to 8") includes plants such as sedums, herbs, grasses and other vegetation, which can grow in this depth of media. Un-irrigated systems can be provided without difficulty; however, drip, mist or spray irrigation systems may be required to support more diverse plant types or for installations in semi-arid climates. The anticipated saturated weight above the membrane assembly is less than 50 pounds per square foot.
There are many practical things to consider for a roof terrace, most importantly how much weight it can take. You can calculate the weight of your planted area in cubic meters of saturated soil as this is the heaviest element.