Great sketch by Chris.

I have a similar configuration on a cabin. I have 1 1/2" thick T&G running perpendicular to the ridge, (no rafter support- just post and beam).

I hope to add 2" thick rigid insulation directly to the top of a shingled roof and then add metal roofing. (there is no attic)

I thought I could glue the insulation down to the shingles and then screw the metal roof over that using screws long enough to drive into the T&G about 1". However, the concern is that the insualtion will compress slightly and prevent a good fastening situation.

Instead, if I screw down 1x4" boards on top of the insulation to hold it down, I can use the 1x4"s and screw the metal roofing directly to them. This gives me 3/4" gap between the metal roofing and the insulation. The insulation acts as the vapor barrier.

Are there any reasons that this will not work? Thanks.