I struggle to see much 'green' about green roofs. What you are doing is going to great expense to make a roof look like a meadow. Which it's not - it's a roof. You have to beef up the rafters because of the extra weight. You have to tank the roof with a hi-tech (and not very green) waterproof layer laid over sheathing ply - so you basically have something like a felt roof at this point - and then you have to go to great lengths adding edging materials to ensure that your organic matter doesn't slide off and become a large mound of slurry. Oh, and it all costs a fortune.
What is the point?