Modern but “natural” living roof for Midwest climes

Well,  we just started thinking again about the inevitable building of a living roof here in mid Missouri…
Octagon Structure is up ( 4x8 and 2x8 rafters,   Oak, about 12’ spans, no more than 5’ between rafters on outer edge).  We’re going to one way or another build a living roof over top using info we can glean and adapt from “ building green”.  Have a 20x50’  epdm  45 mil membrane on hand. Just wondering if anyone had any practical ideas on how to proceed.
Insulation above or below the epdm? We’re using 2” rigid extruded polystyrene. Could use recycled 4” foam off school roofs too. Think it’s r30 stuff.
Recommendations?
We’re thinking of cutting the epdm into 8 pie wedges ( approximately 12’ long by 10’ wide at widest)  joining them with cleaner and 6” tape,  then polyurethane glue then sealing edges with 100% silicone .  Overkill?  I’d rather overbuild than repair in 15 years.
We’re in a no internet zone of 21st century Missouri so it’s difficult to find info or even get permies on line, but I’ll try to check when I can for replies. We’d love to hear from “ local” folks who have tried ( and succeeded!).
Thanks much. Tom

I would put the 2" extruded polystyrene above the membrane to protect it, since the poly is waterproof. 4" R-30 foam sounds like polyiso which is not waterproof, so I would put that under the membrane. And yes, if you can, use both. There is no such thing as too much roof insulation.

How much slope do you have on the roof deck? 1" per foot is supposed to be the maximum for green roofs without additional structure to hold the topping in place, and you want some positive slope to minimize the possibility of standing saturated soil. Some recycled commercial roof insulation is tapered in thickness, which would help improve the slope if needed.

It sounds like your roof is about 24' "diameter" octagon, so if you have a 20' x 50' piece of EPDM, I would use it in one piece as much as possible rather than making lots of seams. A fat semicircle (with a 7' +- seam cut out to allow a bit of cone shape depending on your exact roof slope) and a wedge or two to fill out the rest of the octagon seems like the best way to go. Just make sure to arrange lap seams to shed water regardless of glue joints, for extra insurance.

Yeah, I wouldn't cut that EPDM if I could at all avoid it.  I like Glen's idea of cutting one wedge out.  Another idea would be to lay it on there whole and fold under triangular wedges at each pie piece intersection.  Then there'd be no seams and it should follow the roof decently.

My buddy does living roof saunas and he covers them with moss.  

Thanks to you both for interesting ideas. I was wondering about the one piece idea but thought that it would be lumpy from the folds, but hey, since it’s under everything it’d be hidden.
As for more info on my project, it’s roughly 24’ diameter octagon, 1.5/12 slope. Round posts average 8-12”  diameter, crazy double center “post”.  Oak framing with oak 1x12 board and batten siding. I’ll try to post some pictures. It’s an added bedroom to our now too small straw bale home.