A little background:


I'm planning the build of a timber-frame house in the "American" style (long beam) in Normandy, France. I have a local sawmill with CNC machine who are prepared to cut the frame for me (I'd love to do it myself, and may still, but time is a big issue). However I'm battling slightly against the local style of timber framing. Houses here are mostly blockwork with timber roof structures and the local carpentry style is very specific. The carpenter is prepared to push himself a little, but he's already outside his comfort zone and we still haven't met eye-to-eye on the roof. The bents are pretty simple, but do include an assymetric roof which has a height restriction on the underside making a simple tie-beam impossible.

What we are trying to achieve is actually a principle purlin frame since the rafters will be I-joists packed with insulation, but the narrowness (4.5m - 15ft) of the house precludes a centre (or in this case off-centre) post. The carpenters solution is a king post which the rafters meet, the triangle being formed by a pair of rectangular section timbers clamped to either side (Known in french as an assemblage moisé). In a normal building, the king post would be a square timber, the principal rafters and "sandwiching" beams would be rectangular and the king post would be notched so that it was sitting on the paired beams. There are various reasons why this is not an ideal solution for us, but lets assume that it's an aesthetic that doesn't work for me.

Possible solutions.
1) Change the design. This simply isn't an option. A change to this would involve raising the height of the house and would involve a new planning application, not to mention leaving us with more unused space overhead.
2) A half dovetail on each end of the collar tie. The problem with this is supporting the king post. It would allow it to be removed altogether, but then the ridgebeam would have to be joined to the rafter mortise and tenon which is going to be an exciting joint!
3) A pair of collar ties with half dovetails (in rectangular section) replacing the paired beams and metal connectors with traditional joinery. I would however be concerned that the rafters would have to lose a lot of material for that joint.
4) Replace the collar ties with a single beam with a spline pasing through the collar tie and rafter. This spline would then provide the tension resistance that this joint needs. (Obviously this drawing is just to indicate the general idea. Neither the spline nor the peg holes are to scale.) This is currently my preferred idea.
5) Take the king post down to the level of the first floor girt so that the ridgebeam is supported by this girt. This would add a not-insignificant load to this beam and would play havoc with the floor joists (the house layout depends on the joists running perpendicular to the bents and not parallel to them.

So over to you. Any thoughts? Any suggestions? Any questions?

Jon