Hi Mr. Rookie;

The metal installation specs may specify 2' spacing for supports.

The width of the boards matters for wide spacings between purlins or rafters, the wider the better. If you are trying to span 4' with 3/4" x 6" boards it will be weak. Even so, it may work, unless someone in the future wants a different type of roofing than metal.

I have heard it is good to put felt paper under metal roofs in the event of a leak or condensation forming on the metal, if this is what you ment by a vapor barrier.

Can you add some purlins to make the spacing 24" or use 1"x 12" and wider boards?

Jim

The closer you look the more you see.
"Heavy timber framing is not a lost art" Fred Hodgson, 1909