Sharp-eyed readers may have noted in my previous posts that something was different at the barge board miters. I had found through multiple re-fits of the components, that the dovetail design of the joint, perhaps due to the fact that the dovetails were only 9˚ in slope, simply didn’t pull the two barge boards as tightly together at the miter as I would like. Well, to be more accurate, they held together fine until the upper ridge was installed, where, due to a myriad of parts locking together at once, the miter opened slightly on one end of the gable, no matter what I did to try to adjust it out. I realized as well that rain and sun cycles would likely conspire to only make the situation worse, and that the joinery solution I had devised was simply inadequate.
I was on the verge of beginning all over again and making another set of barge boards (hafū), but I held off and gave the issue more thought. Then I was strongly considering fitting a metal threaded rod in for a while, breaking my design goal of no metal, and using plugged mortises on the backside to conceal the pockets for the nuts. Neither solution was appealing.
Then I noticed, kind of like a veil being pulled back, that I could in fact fit a spline across the miter and peg it. There was actually room for this, and for some reason i had convinced myself earlier that there was no room. Go figure.
The pegs for this spline would land not on the hafū board proper, but could be placed so as to be on the upper portion of these one-piece boards, the portion simulating a tiled ridge. Since I was already pegging the upper ridge, I realized that pegs on that same portion of the hafū, the ’tile-ridge’ portion, would look just fine. Further, they would be placed so as to flank the projecting end of the exposed ridge, thus would be somewhat obscured.
So, the problem was resolved – a crosswise spline, pegged on each side. Here’s half the joint:
This new solution of course renders my first ‘solution’ largely moot, and I will take this lesson forward with me to the next lantern. The good news is that it did not necessitate re-making new barge boards (though I remain tempted!).
Here’s a view of one of the spline mortises:
Though there is a light gap on the bottom of the miter in the photo, that comes from the way I was holding the piece in my hand on the table top – all in all, the miters look nice and tight now when all the parts of the roof are assembled, so ‘mission accomplished’. I’ll post up pictures of the assembled joints soon enough so you can see for yourself.
Now then, back to the making of those pesky intermediate roof ribs. When I last left off, I had fit them to the roof boards. The remaining task was to dovetail their ends so they could lock to the exposed ridge. Here’s a close-up of one of those dovetails:
The next task was to mortise the exposed ridge for the sliding dovetails. I made a MDF jig to work with my router:
I could have housed these joints as I did the connections between the ridge and the hafū , however that would have meant that I would need to fit the ridge simultaneously onto 20 backed rib/hafū top surfaces at the same time, and I thought that would be a pain in the, pardon my Scotch, arse. So, I went with barefaced dovetails, and if the ridge shrinks appreciably across its width, hopefully it will pull the ribs in with itself – otherwise there will be slight gaps at the non-humid time of year (winter season around here).
I’m not so sure, thinking more about it, that this solution would not have also worked fine for the hafū connection to the ridge as well. I think though that a cogged sort of connection between the exposed ridge and the hafū, which is what you get with the housings in this case, would be a stronger joint in terms of locking the hafū in position laterally. If I were to re-design this joint, I think I would still go with the housings, however the internal dovetails, as mentioned above, would be nixed and I would do a simple cog-lap instead. Or, possibly, combine the two ideas, using wider dovetails on the ends of the hafū and no housings. That might be the ‘best’ way to go. That said, narrower dovetails are to be preferred though in general, as they proportionately shrink less across their width than fatter ones, and thus remain mechanically more cohesive over time.
Weighing up the pros and cons of different joinery approaches to a given problem, factoring in the effects of shrinkage/swelling upon both joint strength and aesthetics through seasonal humidity changes and expected loading, along with ease of assembly, de-mountability, and so forth, often is cause for a fair amount of head-scratching.
I’ll eave off on the fitting of ridge to those intermediate ribs, as I have taken the opportunity now, with the lantern housing free of it’s roof, to commence work on the frames for the grilled openings. These frames are dadoed into the posts (hashira), sills (dodai), and plate (keta). I processed with trenches in those parts sans camera. We pick up the fabrication again at the point where I need to trim the ends of the frame pieces to fit against one another. The verticals, since they lie against the posts, which have been ‘backed’ to be flush with the overall prism formed by the lantern head, obey similar geometry. Though I haven’t reached the point in the kō-ko-gen-hō series yet where I explain the geometry specific to splayed-post structures – – it is, to cut to the chase, the same as that on the ends of splayed sawhorse feet and tops, a cut line given by the chūkō angle.
I set up an angled shooting board with a little wedge to use my plane to trim the ends at this compound angle:
After initial trimming of the end, I stand the piece up to see how it look in relation to the post it will fit against, and adjust the trim as necessary until it is parallel with both directions of post splay:
I follow the same structural logic with the lantern grill frames as I do with timber frames – I stand the posts on the ground (which would be on granite plinths), fit the sills between the posts, and place the plates on top of the posts.
The ‘post’ portions of these grill frames are connected to the sills with bridle joints (aka, ‘open mortise and tenon’). Given the mix of shapes and slopes here intersecting at the joints, these are purely hand-cut affairs:
Oh yes, more to come! Go to post 36