In regards to green roofs - I built a shed with one on it.
If you'd like to do it a few things to keep in mind:
You don't want too great a slope. Mine was less than 10 degrees. Too much and your soil can slide.
You'll have to have extra-strong side walls below the roof and double up on the support beams (I just nailed two 2x4s together instead of using one)
I'd strongly recommend using EPDM as a liner. Plastic will break down too quickly. Cover the roof with landscape fabric, put the liner on top of that, put down a layer of vermiculite for drainage, cover that with a layer of landscape fabric, then add soil.
Be sure to have a runoff hole/spout to let the water drain.
If you'd like to do it a few things to keep in mind:
You don't want too great a slope. Mine was less than 10 degrees. Too much and your soil can slide.
You'll have to have extra-strong side walls below the roof and double up on the support beams (I just nailed two 2x4s together instead of using one)
I'd strongly recommend using EPDM as a liner. Plastic will break down too quickly. Cover the roof with landscape fabric, put the liner on top of that, put down a layer of vermiculite for drainage, cover that with a layer of landscape fabric, then add soil.
Be sure to have a runoff hole/spout to let the water drain.