common purlin to principal rafter joint

[Alexis]

Hello all,

I'm reading "historic american timber joinery" by Jack Sobon and have a question...

In all the modern frame that I saw, the common purlin to principal rafter joint is always either a dovetail or a soffit tenon. On page 42 of his book, jack says that in the past, the typical joint for this was a simple throught thrench in the rafter with a pin to secure it. I'm wondering why I haven't seen this joint in modern timberframing. It seems simple to cut and you can use long continuous purlin that will tie the frame longitunally. Since common purlin are not huge timber, it is relatively easy to find long lenght... The raising part would be very easy because they can be set afterward. I see only advantage with this joint but no modern example and I am thinking that I am missing something...

thanks

Alexis

[Jim_Rogers]

I'm not totally 100% sure but I think there is a code rule about cutting the top surfaces of load bearing members all the way across, as it lowers the strength of the piece.

Years ago, there were no code rules so they could do things like this.

But we have seen that over the years these timbers sometimes fail and the rules were established to try to prevent anyone from getting hurt.

Dovetails have been seen to withdraw some after they shrink; when the timber drys. Tusk tenons have been used for this instead. But as mentioned it is more difficult to raise the frame.

[moonhill]

I frequently commit the all mighty sin of cutting all the way across the top section of a rafter.  It is only an inch deep.  I rest the whole or slightly reduced purlin on top of the (compromised) rafter.   The purlins are in plane above the top of the rafter.   I am mostly looking for assistance during assembly.

Tim