MotorSeven,
Interesting challenge to frame out an arch. I've never done this, so it's something new to me that I've never even thought about. How are you going to finish of the exterior and interior walls?
The spacing and alignment of your lumber makes it look like you are not planing on using any traditional 4x8 sheeting.
Mike has a good point about your headers. Even though they are not load bearing for the roof, they are still supporting the weight of the wall above, plus spanning a sizable distance. How far are the rough openings?
Wood will start to sag fairlly quickly when placed over a span if it's undersized. When this happens, doors stop opening properly and siding begins to crack.
Is that a turkey in there? I'm sure it isn't, but it sure does look like one. hahaha
Eddie