I tend in a different direction that some (most?) permies people with regards to things like this. People are naturally drawn to trying to find a natural product for the task at hand, in this case roofing. The problem, in my mind, is that the very products that were developed and implemented these days in roofs came about because of the problems with using natural materials. If you want your building to last, you must keep it dry. Even people that draw heavily from natural materials for building use other materials when necessary. The late, great Mike Oehler used EPDM pond liner for roofs, because it worked. I don't know of a natural material that will work nearly as well, so in instances like that, I try to use previously-used materials of modern manufacture. It isn't hard to find used rubber sheeting for free, or nearly so. Many times it will have small tears or whatnot. So what? If you get the material free, use twice as much and overlap it heavily. It has already been manufactured. If I use it after it has been discarded, I save it from the landfill, and no new roofing material was manufactured because of my use. I use what I get locally, so there is no additional shipping or associated fuel or human costs involved. It isn't a natural material, but it is lessening the environmental impact. And you will have a roof that doesn't leak, rather than spending lots of time and effort trying a natural material that may or may not leak. Just one man's opinion.