Some recent changes in my life mean that I should have more time to dedicate to the frame that I'm planning on cutting. Which means that I'm toying with the idea of switching from the bolted post-and-beam frame (Walter Segal style) to a more traditional timber frame, which is what I was originally hoping for.
The basic design of the frame is now imposed due to the planning restrictions and a consequence is that we have a post with four beams joining it at the same height (not an ideal situation). Two of the beams are relatively lowly loaded, but it strikes me that a nice solution is to adopt a splined approach which can also then be used elsewhere in the building. Spline-reinforced joints will help eliminate the scarf joints that were planned, and should make assembly a tiny bit easier (for various reasons that I shan't go into here). But this poses more questions:
1) The frame is in Douglas fir and the splines would be either oak or cherry depending (largely) on availability and cost. If we're treating the spline as a sort of third-party tenon ("free tenon" is the traditional term I believe), then should we size them as hardwood or softwood? Consensus would (if I've read it correctly) put a Douglas Fir tenon at around 2" while an oak tenon would be more around 1 1/2".
2) Should the wood used for the splines be already dry? Or rather, is there an issue with using green oak for the splines? I imagine that their reduced size (compared to a full size timber) means that sourcing dry wood will be easier, but green oak will be even easier to source.
3) When using splines to join four beams to a post, each spline is anchored to a pair of beams and a slight draw-boring should provide enough tension to hold everything in place. If there are only three beams meeting at the post, one of the splines is now unpaired and requires pegging in the post to resist tension. Given that the pegs would need to enter on the faces that are then going to receive timbers, this obviously poses a problem. My solution would be to have the splines overlap slightly, and to have the spline which is supporting two beams lodge into a notch on the spline which is supporting one, locking it into place (I'll try and produce a diagram for this as I appreciate that it may be a little hard to follow). Replacing this spline with a normal tenon doesn't resolve the issue as the tenon would still need pegging. The only other solution that I can imagine involves pegging the spline from one of the parallel faces and then cutting the pegs off flush to the surface to allow the incoming beam to mate correctly. I imagine this being relatively fiddly, but perhaps no more than any splined joint. None of the descriptions and illustrations that I've seen so far have made this clear despite having seen pictures of splined three-way joints. For the record, there isn't room for the spline to pass right through the post and be pegged externally. That space is needed for a wall!
Thanks in advance for any help that you can offer. I'll try and provide illustrations shortly.
Jon Senior
