Mark ,
Andrew is right , my bow roof shed has been standing for 4 years now , same plastic on the roof ( poly tarp ) as was on it when i built it , it is showing signs of being repalaced before this winter but stil going strong . I would strongly suggest that you put a florr in yours as i did , my floor is 3/4 plywood over 2x6 stringers or joists 16 inches on center , makes for an awesome platform to work on and keeps alot of cold/moisture out . i actually store the boat in there when not in use , the trailer fits in nicely with the 12 foot wide opening . A word of caution , these sheds can be made 2 feet long or 102 feet long just make sure you can get a poly top that will cover the length and width you need otherwise you will need a couple to cover the frame .good luck you will love it as i do mine .
Dave M