I have a garage with a galvanized tin roof. I'd like to extend the back of it so I would have a covered work/carport area. I have some 21' galvalume that I can put on it and the garage is 24' wide. I would like to have poles only at the corners of the roof, if possible. What is the best way to support a roof like this. I'm thinking 2X6's would bend/warp over a 20' span. Extra poles across the 24' span is no problem but I would like to keep the 21' sides open. Any thoughts? Thanks, Mark