First I swept off the asphalt shingle roof. Then I added carpet padding and set it up so that the water would drain properly and won't pool in the back. Then I added a layer of black plastic.
Next I put down a layer of fresh mulch comprised of tree and leaf matter as well as mosses. Then for the final level I added a layer of straw that has been decomposing with horse manure. As you can see, I saved some of the grass that was already growing out of that.
At the Cob Cottage Company across town, they have built similar roofs that have lasted over 15 years. This is already double the traditional asphalt shingle system which makes me happy, not to mention I will have a new wildflower garden every year out my window!!