Roof Edge Detail for Green Roof

[Rover]

I'm planning to build a small shed which will be my cabin.  I like the idea of a green roof but want to keep it simple.  The roof area will be 12' x 16' and the decking will be 2"x6".  I'm thinking of building it flat, although I could slope it in one direction if needed.  A roofing contractor aquaintance can supply me with whatever large sheet membrane I need, but not the labour to fit/weld/torch the memebrane.  So I have to keep the details simple.
A) I was going to lay the membrane over the roof decking, wrap it around the edges and nail it to the underside edge of the decking.  This might look a little odd looking at the roof edge.  Then just use 3-4" of plain dirt from the land (was farm land) to grow whatever is native in the area.
B) But I'm not sure how to hold the edge of the dirt.  I was thinking of 4"x4" cedar all around the edge with holes drilled through it horizontally and notches taken out of the bottom to let water pass thru.  Or get geotextile fabric and make long tubes of it filled with gravel and lay that along the edge.

Does anyone have any others ideas for B)?
Does A) make sense?

Of course, I could just slope the decking a bit, forget about the green roof and install full length metal roofing panels.  Simplier and quicker.

Thanks

[glenn kangiser]

Sounds like fun, Rover.  Don't forget that dirt is heavy so beef up your framing and joists to support it --and any snow load.

Figure around 100 lbs per foot for the dirt or 30 to 40 lb psf on your project rough guess at 3 to 4 inches.

I would just s EPDM pond liner - available at Home Depot.

Go as high as you want on the side boards but especially on the top, leave the fascia boards sticking below the joists also to provide a drip edge to make the water drip off and not run down your roof joists bottoms and inside the house - thanks to Mike Oehler for that one.

I think your geotex fabic should work with the gravel but fold it and extend the two edges forward under the dirt to keep it anchored to the roof.  I would suggest around a 2/12 pitch to provide drainage rather than a pool so the plants won't drown and you only have to deal with drainage on the bottom.

You might want to put straw down as a bottom layer also to protect the membrane and tie things together.

You will get some muddy water runoff so some overhang and maybe well supported gutter could help.

The underground complex actually drains right off on to the ground as it slopes down from the roof -- no muddy water over windows that way.

[Rover]

Thanks Glenn,
I figured 20 lbs/sf for the dirt and the snow load in the area is 60 so yes, I've been thinking about that.  Maybe I'll have to use your weights to be safe.  I may do a separate posting on that after I do more research & calcs.

Good idea on the geotextile fabric.  It would have been a pain to try to make a sock of it.  I think I'll run that over the whole roof membrane to protect it. 

I won't have any fascia or soffit.  I'll be cantilevering the rafters (I think that is what they are called) out beyond the wall top plates by 2'.  And they will just have the 2"x6" roof boards on top.  I'm not adding any other fascia or soffit material.  Thanks for the idea on the drip edge.  I'll try an alternate detail instead of wrapping the membrane around.  Maybe some Ice & Water Shield in conjunction with the metal drip edge and that way secure the roof membrane, drip edge and ice & water into the side of the 2x6 roof boards.

I'll keep detailing in my head.

[glenn kangiser]

Think like rain, Rover.  If it gets on the end of a down sloping board it will run down the boom of it and inside.  Your mission is to figure out a way to stop it without a fascia board.