Steve:

couple things to consider: The first drawing Mark sent has common rafters. You indicated purlins were your wish early on. Mark's second model shows major rafters, more of a bent system that's conducive to common purlins.

I guess it's deciding between the two... whether you'll have a bent system in the roof or not (which would make common purlins possible).

Was playing with sketchup tonight and found a horse at the 3D warehouse, so of course had to build a barn for it... here's hoping this post has some degree of ontopicness.