More likely than not, the joinery in the roof system of a timber frame is often loaded with tension and so special consideration is needed to make sure everything is held together. In this detail we show a collar tie to rafter connection in two ways.
The first, and often preferred, method used by engineers employs pieces of all thread and extra large washers to hold everything together. The tie beam in this case has a 2 inch locator or stub tenon and has a diminished housing going into the rafter. This joint is strong and you can upsize the diameter of the steel and the washers to accommodate any increasing loads depending on the size of the roof. While not “traditional” joinery, this will ensure your frame stands the test of time.
The second method uses a typical mortise and tenon connection with pegs holding everything together. In this case, a deep tenon and multiple pegs are used to resist the loads the roof puts on it. This connection is used when tension is not that great and the pegs have the ability to withstand the loads over the long haul (think about the frame standing 100+ years).
When in doubt, I would recommend you go with the first option but as always, get an engineer’s feedback before cutting your frame.