The codebook doesn't alter sheathing, rafters or purlins based on pitch. The horizontal clear span from plumb to plumb would be the way the NDS measures the span. Roughly then, 4' centers on an 8/12 with 4" wide purlins is going to give a clear horizontal span of about 40".
Your building department can give you the snow load for your area, I'm guessing it will be 30psf. Add 10psf for dead load so 40psf total load.
White oak base design values for #2 are;
Fb-850
E-.9
Fv-220
I would bump up the Fb by 15% for short term loading(snow), 15% for repetitive member use, and you could take 15% more for flat use but I don't think I would. I'd call the adjusted Fb-1125psi
Assume a 6" wide board spanning the 40"=1.66 square feet X40psf=67lbs load
I looked at it as a simple beam in flatways bending;
http://www.windyhilllogworks.com/Calcs/beamcalc.htmI entered;
Load-67lbs
span-40"
width-6"
depth-1"
Fb-1125psi
E-.9
Fv-220psi
Click "enter"
the fail in deflection shows it would make for a bouncy floor but a passable roof board.
BUT,
Let's look at those purlins..
40square feet of roof bearing on a purlin X 40lbs per square foot= 1600lbs load bearing on each 4x6 purlin. I also changed the span in the calc to 140" then entered this info assuming the same species. It's too small, it might squeek in at a 6x6.
