A solution I have seen a photo of and liked, but never used looked like this:

Prin rafters moved inside roof plane, purlins (in this case prin purlins with common rafters on top, but could work as well for common purlins) interrupted at each prin rafter sitting in a small notch about 1" or so into the rafter and 2" or so down the face of the rafter. The purlin was kept from rolling with a shaped sprocket spiked or screwed to the principal rafter. The purlins were joined across the top of the rafter like Mark was saying with a spline (a blind top spline in this case). Wind braces in the plane of the purlin running from the purlin down to the prin rafter (sitting on top, I assume lagged).

This was from an English timber frame company.

It seemed to me to be a nice design because it has the ease of raising of interrupted purlins and a lot of the strength of continuous purlins. Also, minimal notching of the principal rafter, especially compared to the dreaded dovetails. Much easier to assemble than tenoned purlins.

Anybody seen or done anything like this over here?

cheers,
GH