Here is an exploded view of a pair of purlin joints where they meet the rafter.
An experienced timber framing engineer should review each frame design for the exact design of this joint.
Loads, spans, type of wood, and possibly other factors will have to be considered when designing this joint.
My examples are only examples of two different types. One cut has a curved cut on the bottom, and one is just reduced.
Only an engineer can give the exact sizes of these joints.
Do you ever use a dovetail joint to lock the purlins onto the rafter?
Mike,
It certainly is a great option but I find a good screw like a GRK RSS works well to hold things in place. It is certainly easier not to cut the dovetail as well.
Brice