Pics may help. I'm not sure what you mean by beams cuz around here we call what I think you are describing, rafters.
Around here we go for a minimum R40 and you won't get that with an inch of foam. SIPs placed on top of the rafters is generally how it's done here. You also have to keep in mind Summer cooling. The heat of the sun on the shingles may need to be dissipated for shingle longevity.