framing roof with purlins

[dug]

Still trying to finalize my roof framing and need some advice.

I would like to use purlins instead of sheathing but can't find much info on this method. Among some of the benefits I could imagine that installing the metal would be much easier this way. Sheathing, on the other hand, would add more structural integrity than just using the metal roofing for this purpose. Any other drawbacks to using purlins? I realize that laying the roofing felt would be more difficult.

Opinions?

Also- using online rafter calculator I find that the longest foot cut at birds mouth can only be 4 in. (using 2 by 10 rafters). I need it to be flush with inside wall to finish cathedral
ceiling (5 1/2 in. cut) which seems is what everyone else has done. What gives?

I am building in the southwest, we get about 16 in. rain per year and the air is very dry, if this makes a difference.