Hi. We have tried both living wall and living roof with mixed success. You need a good strong structure as if you have sufficient compost and drainage you will be placing a lot of weight on the roof. You will need a minimum thickness of 8-10 cm of drainage and planting material (quite a bit more if you want to use meadow grass) and if built as modular panels (which is a good idea) this weighs about 60kg per square metre dry and obviously more when planted and wet. You really don't want a very steep slope either. For a wildflower / meadow roof that we did with galvanised panels I calculated a peak loading of 200kg per square metre in addition to the weight of the roof itself and wind pressure forces of course. This was not intended to be walked on by the way. For a sedum roof, with a lot less compost but quite a lot of grit, I think it was about half that at peak loading.
Also in my experience we needed a trickle or mist spray watering system to deal with die back in drought periods and you will need access for areas that require re-planting (hence modular is helpful). Nutrients are drained fast and we felt we needed to add them much more often than we expected to maintain the effect. In future I would personally only do this with a structure that is designed to have a visible living roof - ie not a shed.
I would definitely not rely on BUTYL, RPM or whatever as my sole waterproofer for the roof. I would make modular trays to go over whatever I used as my waterproof roof covering. If you gutter to a barrel and install a small high lift switchable pump, you can use that as your watering system when necessary. Personally I would mist spray as it is easier to add nutrients.
We have a very large garden now and I have to say gardening on the ground is a damn site easier!
