Here he is securing wire mesh over the gravel in the roof frame. I don't know all the technical details, but basically the gravel is there to provide good drainage. Under the gravel is a sheet of resin, to keep the roof waterproof. There are drain holes drilled straight through the resin and the wood underneath it, in case extra water needs to drain.
On top of the gravel, Dakota put a mixture of cactus soil and broken clay pot. Then, he transplanted a plant from a planter on the deck into the roof soil. It's a sedum-like plant that shouldn't need much water or care, and will fill in and spread out over time.
Here's the completed project. Looks pretty classy, I think. It will also keep the coop cool this summer, when things get scorching hot around here. The chickens have yet to give an opinion on their new roof, but I have a feeling they will like it a lot.