Here's a thought:
Use principal rafters (common purlins) implemented with a king-post truss. Hold the tie beam of the king-post-truss all the way to the bottom of the truss, so that the rafter feet sit on the tie beam. Tie beam (which sits on the ICFs) could be a few inches longer than the span of the rafter feet, leaving room for your insulation. Not sure if the point load would be too great for the ICFs in this design though. However, using a truss design (like this) that imparts no spreading forces to the tops of the ICF walls would be a very good thing.
I've contemplated ICF walls (later faced with stone) with timber trusses for my garage (when my house is finished!), and that's the design I keep coming back to. I would be leery of common rafters on (unbuttressed) ICF walls unless I used collar ties and held them very close to the bottom of the rafters.
Just a thought!
