What are the dimensions for a roof

I'm assembling my parts and materials list. I'm building a 10' +/- long Ken-Skill like trailer, I'm thinking about the roof. If I use 1/8" Luan to make the curve, how long should the piece be to span from the front, up and over to the hatch hinge? 12' 10' I'd rather not have a seem...I guess the same will go for the aluminum roof.
Any thoughts?
Any thoughts?
