Mark, you asked, "roger, in your first post, you say that the "purlin could be severly weakened" Did you mean the principal rafter?"

There would be two basic schemes for the spline, first the flat of the spline square to shaft of the rafter and second flat parallel to the shaft of the rafter through the neutral center. The former would be unthinkable due to damage to the rafter and the latter safe to the rafter but would create a weak purlin if the mortise is open side to side.

Now here is a thread after my own heart, coming from the heart of common purlin land, and designing them into frames more often than not. It also speaks to some things which don’t see enough discussion, but maybe should

I’ll try to touch on each point as it came up,

Tim - I’m with Roger on splineing the purlins, the added complexity in cutting and raising sees little payback, Yes, the wood you remove is down in the neutral axis, and after you raise such systems a few times you can reach an acceptable level of efficiency, but IMO it’s just a good idea carried too far.

Roger - spoke to some of your points above, and I likewise do avoid DIDTP, and love them raised

Ron - If traditional joinery is your aim, (and I think it’s an admirable one, wheel ain’t broke and all that ) I can say without equivocation, there are no historical examples of dovetailed common purlins, but no shortage of CP’s, far more often than not they are continuous for multiple bays, a common situation being a five bay building, the four bays at either end share continuous two bay purlins, that purlin often having extending relish which projects into the center bay, and at the gable which goes out to help form the rake. The purlin in the center bay, is often staggered and also has projecting relish and shares a dap common with the adjacent purlin, stacked side by side. A common varient, but only one of numerous variations.

Roger - I agree and seldom severe much of this upper grain , but think it easy enough to size the rafters accordingly, and such historical rafters seem to have held up well, at least in experience, even in those whose center support has fallen away I have not found failures due to these daps.

Gabel - I have long raised purlins, so does a framer I work with locally who I see as the best designer I know. I’ve gone high enough to need the “wedge” you mention but more often go around half their depth, And skip it. As often as not, these purlins are continuous, (yup) cog & clasp into the gable principals, and override the rest (essentially two housings) we often finish the purlin onsite just prior to installation (two quick cuts to an existent “mortise” and swing with a spreader, say six picks instead of eighteen, and complete lateral unity, without the need to complete anything “set it and forget it “ as Ronco man would say. An added bonus is moving the principal inside the plane of the roof, puts more relish behind it on the tie beam, a significant advantage when you are concentrating all the thrust to them though the commons, Removing all thrust from the plates is (I think) a huge part of the long success of the English Tieing Joint here in Northern New England.

Tim - This discussion got a little tangential and probably morphed into far more than you wanted, but it’s all wood shavings for the fire.

Be your best

Roger and Will.
Your detailed and insightful responses are greatly appreciated, and I thank you.

Will, at the risk of sounding stupid, what are "daps"? Thanks, Ron

Ron - There are no stupid questions, and if memory serves this ones been asked before. I think Mark asked me the same thing in an earlier thread.

This is a term borrowed from bridgewrighting, in common usage here in my area, though it seems to be seeing wider acceptance, and was seen in Joseph Conwill’s fine articles on Burr Arch and Paddleford Truss bridges in Timber Framing.

Dap - the simplest of timber joints, literally a plowing though from one face to its opposite, a notch.