Fair enough, shingles + tarpaper (or polymer) seem to work as a pretty good waterproof layer, supported by plywood. I guess it seems to me that the polymer/tar

Fair enough, shingles + tarpaper (or polymer) seem to work as a pretty good waterproof layer, supported by plywood. I guess it seems to me that the polymer/tarpaper layer is more for keeping moisture away from the wood than keeping water out. The shingles seem to be for water.... so if the support layer doesnt need the protection, and instead offers an impervious barrier, why not use this plastic sheet in an overlapping manner (think roof slats for lincoln logs). It essentially becomes shingles + board all rolled up into one. Edit: it also seems that metal roofing does not follow the same set of constraings that shingle roofs do, which suggests to me that this plastic roofing could be very viable.