Thanks Raphael. The herringbone might be a little too busy for some tastes, but I've decided that I like it... if for no other reason than "form follows function" in this case.

Anyone have any more advice to give me on the roof system? I have a semi-tractor-trailer load of Firestone PolyIso insulation in my barn, twenty 20' long 2x10's, and I'm less than a week from starting on my roof using a built up system very similar to the one I described in my first post in this thread.

I'm toying with the idea of running my 2x10's 4ft apart rather than 2ft apart. To keep the 1x4 battens from bowing in the middles, I'll turn a 2x6 flat and run it parallel to the 2x10's. Basically, I'm taking my old design and replacing every other "2x10 on edge" with a "2x6 on flat". This lets me use a little less wood, lets me use the 4x8 polyiso slabs without ripping them to 2' wide, and most importantly will give me 1/2 the thermal breaks that I would otherwise have had. One concern is whether half as many 2x10s (anchored to purlins with Simpson L brackets) will be sufficient to keep the whole roof system from sliding off of the timber frame. (I think it will work, just running some numbers on the weight of the slate and the shear capabilities of the L brackets) Another concern is whether the foam insulation, over time, will not compress where the 2x6 bears on it. Any thoughts on this?

Perhaps I'll build a little section of the roof tomorrow and try to get a feel for how the materials will work together. Here's the same polyiso insulation where I've used it for infill walls...

timberframe infill using PolyIso insulation