I am a smaller builder in SE Tennessee (zone 4a) who is doing more and more modern, low slope, shed roof style structures. I’m increasingly using a roof assembly with exterior polyiso sheets. I wanted to run my assembly by the great minds here just to make sure I’m on the right track… 

Here’s my latest build, interior to exterior: 
– 2×12 roof rafters w/ R-38 fiberglass 
– OSB sheathing, taped seams for air seal 
– 2” polyiso sheets (re-claimed when possible) 
– tyvek standard house wrap
– 2×4’s lagged into the 2×12 framing underneath perpendicular to slope (we score the 2×4’s with a chainsaw to create a small channel against the Tyvek in case water shows up; we caulk the lag penetrations to ensure the water barrier is truly watertight) 
As an aside, our last build experienced a ferocious thunderstorm with high wind and torrential downpour before we got the metal roof on. We couldn’t find a drop of water inside. 

I know that I’m just outside the 31% ratio for zone 4a according to Table R806.5. But I’m also right on the border between zone 3 & 4, so I’m not super worried about that.

Anything I’m missing? I like this assembly for a few reasons:
– metal roof screws don’t penetrate my WRB 
– the materials are readily available 
– the assembly avoids using spray foam 

Thanks for taking a look!