Plywood on roofing now?

[humble pi]

Does anybody know why the county is requiring plywood sheathing on roofs now? Is it a structural thing? I read the code and can’t find it anywhere. I figured I’d ask here before calling the county directly. Thanks in advance.

Plywood is not strictly required.  
We are building a small house with 2x6 TNG planks for the roof deck.

What's the advantage of these planks over plywood?  I guess it will be your finished ceiling inside, so this is being done for aesthetic purposes.

Yes, it will be our ceiling, we like the look.  
Friends have something similar.  Shed roof, 3 in 12 pitch, made of 2x6 TNG, over 4x10 rafters @ 48"OC.  
The tallest wall has a row of long, narrow awning windows way up high to catch the trade-winds & let the heat out.  
The inside feels solid, symmetrical, old-fashioned, bright & bigger than it really is.  

If you plan to cover the rafters, there is no advantage to a plank roof.  More materials, and more labor.