I sure wouldn't want to try it if I had to purchase the materials. If you cut your own support timbers and can find a scrap of rubber pond liner the right size free or cheap, maybe. I am attracted to the idea, because a bunch of chicken coops scattered around with conventional roofs forms a sort of visual litter in my woods. I am constrained by the location of water and electricity, and the green roof would be a nice way to blend a coop into the landscape. In our woods I might try to cultivate moss instead of herbaceous growth.
