Awesome build! I really appreciate you documenting the whole thing. I hope to do something similar in the future (smaller though).
How did you deal with getting the roofing rubber? I have been thinking about the possibility of using billboard tarps. "Tough, Tear Resistant, 15-17 mil thick, Waterproof, 13-16 oz per sq. yd, Anti-UV, Vinyl, Anti-Mildew, 3-Ply Material"