its a strange one paul, the rest of the shell is built so well but roof is a poor job... had a better look at it today and i think i might just get away with some filler in the lower areas, luckily the solar panel will cover the worst depression... your right about the extra weight of another layer of steel, i didnt think about that... i really didnt want to go down the plywood route if i dont have to.. im just glad the roof isnt rotten, just a bit of surface rust but most is still shiny steel...