Rafters (or purlins) normal to the roof slope vs. vertical?

[Charred]

In older construction, I've seen where rafters/purlins were ran across the roof slope and laid normal to the roof sheathing. When I've came across this situation on projects, I've always held them vertical which requires the 2x member to ripped on the top & bottom matching the slope and supports.

I have a project now where the client wants the rafters to be laid normal the roof slope for appearance since there will not be a ceiling (see attached section). The roof has a 3:12 pitch. In my mind, the rafters would need to be designed for biaxial bending unless blocking was installed on tight increments. I've had this discussion a while back with some colleagues and a few of them did not understand why or how there would be a need to check bending in the weak axis. My thoughts are that if you take a 12:12 roof pitch, you would have equal design forces in the strong and weak directions. I know the roof diaphragm will provide some plate action at the top - I don't see how this will help with the lower portion of the member.

I can't seem to find any references or literature on this other than how loads are applied - does anybody know of anything or have any input/rules of thumb?

 

[XR250]

This is how I look at it (might not be correct).
Seems if the diaphragm is designed properly, you will only have a normal force acting on the rafter.
I would add some blocking anyway.