Whoever said call a commercial roofer is totally correct. Put 4" or so up there and cover with a rubberized system and you're done forever. They don't ballast that type of roof anymore either and it will not blow off.
Installing foam under the roof I'd be wary of myself. If you ever had a fire in your house the foam melts and produces gases that you'll be quite dead from. If you go under the roof with foam you'd almost have to frame down below the beams and cover the foam with drywall.
Also shingling any "flat" (less than 2/12 ) roof is not permitted it that it'll leak.
You could cover said roof with sips panels
What Are SIPs | SIPs and then ruberize over it.