Below 2 typical roof garden sections. These are only to give an idea. Of course the designs change depending on the needs in the project.
The upper with conventional design. For drainage 2 nonwoven geotextile (filter fabric) layer and gravel in between were used on top of the waterproofing layer. (mineral drainage layer) Heavy and cost more to take the gravel to the roof. Bigger load on the concrete.
The below is with a geocomposite drainage layer, lightweight and easy to use. The drainage layer is only between 0.5- 1 kg depending on the material. ıt is factory made and tested material. The access water from the vegetation and soil comes to drainage layer and carried to channels.