Every pole barn I've ever built or seen had 8' spacing on the rafters with 2x4 purlins between them. Seen every pole barn/shop but that way from Minnesota to new Mexico. How much snow do you get in Missouri?!?!
I'm guessing you are building it similar to a house and sheeting it with OSB on the roof? In that case I have no idea. Given the cost of building materials though I'd be looking to save wherever I could and pencil it out. Maybe check with a local building engineer?
