RE: Trailer roof
Dave good suggestion
If it were me going with the roll metal, I'd glue it down, probably with a roofing cement, and just use sheet metal screws to secure it. Roll metal is so thin @ 70mph it going to do some serious vibrating if it not glued over the whole surface area.
I think the biggest issue with Roll metal being used for a roof is going to be expansion/contraction,, when that stuff warms up it's going to grow and wrinkle up big time. so what ever glue you use must stay flexible
good luck