Originally posted by Roger Nair:
The drop-in dovetail removes critical compression fiber in the upper portion of the receiving beam.
In my three bent office frame with 8x8" principle rafters and flush purlins I've used drop in dovetails at the gable ends and tusk tennons w/ diminished haunch in the middle which gives me an unbroken plane of wood in compression on top of the rafter pair that takes the most weight.
This seemed to be a good balance between maximum strength and ease of assembly. The tusks are just barely pegable with a full 8" rafter, I think raising the purlins is the best approach for narrower rafters. I'm not sure how the ridge purlin was going to go in but you might want to think about a king post.
