We are in process of creating a conditioned attic in an old uninsulated farmhouse and discovered the slate roof may need to be replaced. (Zone 5, northwest MA). I have access to a lot of 3″ and 2.5″ polyiso.  The slate is over board sheathing so wondering if we can leave the board sheathing on and then attach 5.5 inches of rigid foam insulation on top of the board sheathing and put 2.5 inches of rigid foam inside the rafter bays (I understand 41% of insulation must be above sheathing in this zone).  Also wondering if we run horizontal 2x4s as purlins for metal roof can it be screwed into the board sheathing or does it need to be screwed into the rafters?  Thanks for any feedback.