My first question is what's going over the rafters?
If it's sips that one thing to consider.
If it's boards or planks that's another.
You could consider the rafter spacing to fall where it works out right for each... or for the intended decking materials.
I sold a beginner timber framer some boards for his barn. He spaced his purlins at 30" apart, on a principal rafter purlin roof system.
This meant that if he bought 8' lumber he'd have to cut it back to 90", not much waste. 10' lumber worked right on 120", but 12' lumber had to be cut back to 120" wasting 2' of boards, 14' lumber wasted 18" of lumber cutting back to 150". And with 16' lumber being cut back to 180" he wasted 16" of lumber on each board.
If he had keep the spacing to something that divides by 4' then most of all the lumber I had on hand could have worked out for him, except the 10' and 14' lengths. If he had kept it at a 2' spacing every length would have worked. I know 2' is very close spacing for purlins so it probably wouldn't have worked.
About sips, usually if you're using common rafters then the sips would run from gable to gable, and if you space them out to 30" then you'll be paying for a 8' panel that they'll cut back to 90". Ten foot panels would work.
You may want to discuss the rafter spacing with your panel supplier if you're going with panels.
They might have a recommendation that will help you reduce costs and waste.
Hope this helps...
Jim Rogers
