I am building a tiny house on wheels (24’x8′) and plan to have an unvented roof.  The plan is to use frame the rafters with 2×6’s then 1/2 icnch cdx plywood followed by 4 inches of foam board insulation (r20), 1/2 inch cdx plywood, synthetic underlayment and then metal roofing.  Because of the width restrictions, I was only planning to have about 1 inch over hang.  Any advice on the order of particular details would be greatly appreciated.  Does having little to no overhang affect this?  What would the recommended fasteners be for the foam board?

Thanks