Green roof slope?

I cant find a forum for rooves, so I've plumped for the natural building forum . . .

I keep finding loads of stuff saying 1-2% slope, but that's so low that it's actually called flat roofing when it's not got stuff growing on it. I've seen sedum planted rooves around here that have a distinctly noticeable slope to them though, and then there's pictures on the web of traditional sod roofed houses which are very steep, but are made with birch bark or somesuch as the waterproof layer which I'm pretty sure provides more resistance to slipping than EPDM. I don't need my roof to be that steep (and I don't have any birch bark or equivalent either anyway), but a slope of about 1 in 2 would be really good for my particular project.

Is such a slope (1 in 2) practical with EPDM as the waterproof layer? (Or roofing felt as a possible alternative.) I am thinking of overlaying it with sand/gravel 'ballast' topped off with significantly sand enriched topsoil, and then either turf or maybe a thin layer of unadulterated topsoil and seeds or plug plants. (Our topsoil is so clay rich that it cracks after a few dry days in summer - not good for a roof without a waterproof layer, but maybe ok with one?)

TIA for any input/suggestions.

Well, since I posted this, search has been kinder to me, and also a few links to really useful posts have appeared like magic, so I have found the answers to my questions above.

So what did you find out?
I am interested.

Please share those threads with us.

Sorry, read, closed, and moved on . . .

Basically what I would call 1 in 2 slope is typically referrred to as 6:12 on here, and there are plenty of references to green rooves with that slope, so I presume it works well.

As for how much 'growing medium' is needed,the only specific thread I can remember was something like " 50 year old roof", but I can't find it any more.
That roof had nothing put on top, but nature had deposited a lot of pine needles, or maybe that was the one that was covered with moss . . . anyways, I've seen from that (nothing) to two feet of soil and everything in between recommended, and considering that it's so dry this year that even stuff in the ground is dying, there is no point putting anything on top more than is needed for protecting the EPDM and for insulation, so I have completely changed my plan and will be going for a couple of inches of sand or pebbles (if I can find suitable smooth ones) on a virtually flat roof.
What ever will survive up there will eventually find its own way there. I'll still try some stuff, (sedums and grasses, but at the end of the day it's whatever nature permits.