Wednesday, April 8, 2009

All Academic II

It's a little bit embarrassing to post up pictures of my first building, though I imagine a lot of builders would feel similarly hesitant about showing their early work. When I look at the little well pump shed now, it seems hopelessly crude and there are a million things I would do differently. However, that said, the purpose of the project was to cement together my nascent understanding of compound roof joinery, and in that regards the project was a success. As I mentioned in the previous post, after struggling with my existing notes, I found that my Japanese layout texts easier to grasp and essentially followed the methods they espoused.

The pyramidal roof - termed hō-gyō (方形) in Japanese - I made entirely in Douglas Fir, and set the common slope at 5/10. Japanese roof layout, whether in traditional measure of sun 寸 and shaku 尺 or in metric, is always done on a base-10 basis, unlike the base-12 used in N. American framing practice. Base-10 is convenient since a calculator is set up to operate on that basis, thus one can move directly from the calculator to the framing square without having to perform any extra conversion.

I connected the wall plates, keta, together at the corners using the most basic and simple joint employed for this connection in Japanese carpentry, a haunched single tenon on one piece, fitted into a single mortise on the other, and wedged. This joint is termed ko-ne-hozo-zashi (小枘差し), which means, roughly paraphrased, "little root tenon assembly":


The corner is reinforced with a diagonal brace, hi-uchi bari (the 'flint' beam) which is let into each face of the keta with a housed sliding dovetail. The rafters are received atop the keta in individual notches, which permit the rafter to remain undiminished in size at the connection. This is one of two primary methods for connecting the rafter to the plate used in Japanese carpentry (though there are several other less common methods too). I fastened the rafter to the plate using a zinc-coated deck screw.

Here's a look at the completed hō-gyō roof frame:


I chose to use an octagonal king piece at the center of the roof to receive the upper ends of the rafters, as this allowed the rafters to have a simple plumb cut. I also chose to cut the rafter and hip tails plumb for some reason, though this is more commonly done in western framing. I prefer the 90˚ end cut for common rafters in most cases.

Once the roof frame was together, I fitted the rest of the rafters, and then applied some sheathing, using job site scraps. Then I moved the roof over as a unit to the waiting shed frame:


After that, I simply popped it on top, and this is how it looked:


Besides a slight mistake I made in rafter spacing (can you see it?), the curious bracing I did on the front, which I thought looked 'neat' at the time, had a purpose of providing an exit for a garden hose connection at the pump. This form of bracing is actually rather poor, structurally speaking, and I certainly wouldn't employ it on any buildings that people were to live in and/or which take any significant roof load.

Though the building is only about 1 meter tall, I couldn't resist taking a noki-shita (under the eave) picture:


Clearly, as seen in the above picture, I had little understanding at the time of the ideal configurations for rafters in relation to perimeter fascia. Gotta start somewhere though.

The braces, which were Yellow Cedar, I then infilled with Yellow Cedar tongue and groove boards, which could be had cheap at the local building supply:


And here are a few final pictures of the building with it's roof partially complete:


Obviously, such thick roof shingles are totally inappropriate for a tiny little building like that, but it was all I had on hand and time was, as it often seems to be, running out. I actually moved out at the end of that month with the landlord agreeing to finish up the shingling. He sold the place a couple of months later. Two years after that, I was in the area and swung by to take a look to see how the little shed was aging - hopefully with some grace. When I got there I was stunned to see that the new owner had painted the entire thing lavender in color. I tightened my jaw, sat right back down in my car, and drove away in silence. Jeez...

Ya can't control what will happen to the things you make for others, all you can do is control how you make it in the first place. This little well pump shed was a great project for me and was the start of a long path of study, still on-going, in Japanese roof carpentry.

3 comments:

  1. 2 questions for you Chris:

    -it seems like the simple M&T at the plate corner (ko-ne-hozo-zashi) leaves a lot more meat (wood) than all of the fancy half laps with sloped tables, and 1/2 slopes and cogs and wedges and through tenons and such. Especially if it's sitting on top of a post.

    -I've never understood the Japanese concept of fascia (kaya-oi??). I may be wrong here, but why is the fascia on top of the rafters in a typical Japanese building?

    I'm definitely missing your obvious mistake, but I'm curious. I like the little building by the way.

    non-anonymous Brad

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  2. Hi Brad,

    thanks for your two questions.

    1) the 'simple' connection, ko-ne-hozo-zashi, leaves a lot more meat on the mortised piece, however the beam with the tenon on the end has been reduced down to just the meat of the tenon, and is thus greatly reduced in strength. If the tenoned beam wants to twist, the tenon alone would be poor at resisting that.

    Also, while the mortised piece is left with a 'nose' (extension beyond the hip rafter) at the side of the intersection which provides a good point upon which to mount a jack rafter, on the other side of the hip there is no keta nose (just the tenon end grain is visible), and thus the jack rafter connection to the hip is completely dependent upon a fastener.

    The 1/2 slope you mention is still present in this simple joint, since that is the cut angle required to seat the hip rafter cross-wise. The lack of a cog joint for the hip itself has a few implications:

    -it is easier to fit the hip rafter, however the strength of the connection will be dependent upon a metal spike or similar fastener though the hip and into the keta joint.
    -if there is shrinkage at the keta, there will be a visible gap where the hip rafter emerges
    -without the housed cog joint, then either the trench cut for the hip rafter into the keta must be quite deep to allow the hip to drop down enough(thus weakening the keta connection), or the hip must be diminished in height with a lap along with a shallow trench cut into the keta, with the result of that that the hip rafter is unduly weakened. This could be remedied somewhat with a tall section hip rafter, but that wouldn't work except with a double roof, or by having all the common rafters similarly tall.
    -a hip rafter which has a partial lap cut out of it and, on the uphill side, a pointed abutment (or squarely-cut one, depending) to push against the inside corner of the keta connection is a stronger connection than having no cog joint on the hip.

    The ko-ne-hozo-zashi joint is not used at wall plate (keta) junctions on larger buildings but is used more typically for the purlin (moya) connections that support the hip rafter at intermediate points within the roof. Since the keta connection is visible to the inside and outside, the lack of a keta nose on one side of the hip, as you get with the simple joint, is viewed as a negative (it looks 'unbalanced') when considering the aesthetic situation.

    The ideal for a joint in which both members are sized the same and bear similar loads is that a joint-cutting should remove a similar amount of material from each half of the joined pieces. The simple joint I used removes something like 80% of one timber and only 20% of the other, so it is not the best design for a highly stressed area. Think about the recently discussed Daibutsu-den in Nara, with the 2000 ton roof and the 20˚ sagging at the hip corners, as an extreme example of the stresses that can be realized at the hip corners.

    The 'best' construction, IMO, setting aside cost considerations, is the twisted lap joint (neji-gumi) with cogged and housed hip. This joint is most effective when done with keta of deep vertical section.

    2)The kaya-oi is but one means of dealing with the termination of the roof at the eave edge. Simply, it ties the ends of the rafters together and provides a means of strengthening the eave edge against vertical loads. The work 'kaya-oi' means 'tile-rest', so it most particularly associates to placing a tiles roof atop. In such cases, the kaya-oi is in fact but the first piece in a build up of several layers at the eave edge. Further, the kaya-oi, which is typically as deep in height as it is wide, serves to bridge up space at the edge of the eave, which allows for ready insertion of a cantilever beam (ha-ne-gi) which is hidden inside the roof envelope. Perhaps the reason you are having trouble understanding the kaya-oi is that you haven't seen it in the context of the double roof system?

    Even given the simplest example, as with the roof for the well pump shed, which has a single layer roof, there should be a nice piece of wood with which to finish the lower exposed edge of the roof covering support material, and something nice to finish the exposed underside of the eave off with. If you simply ran plywood, for example, down to the eave edge, then the edge of the ply would be visible (in absence of a rafter tail covering board) as would the underside of the ply when looking up at the rafters.

    The important thing to remember, I think, is that in Japanese architecture the under-eave space (noki-shita) is considered a primary design element, and not simply something to be 'covered up' with a soffit. There is a lot of elaboration and variation in the eave in Japanese architecture. A Japanese carpenter would likely place a lot of value, in considering his own work along with that of others, as to how cleanly and beautifully the eave is detailed and trimmed. Given the preference for exposed rafter tails, and as often as not no gutters, the detailing of the eave edge is an important concern, notwithstanding aesthetic considerations.

    I could go on at length on both of your questions, but hopefully I will be able to address those issues adequately in posts to come.

    Again, thanks for your questions!

    Cheers,

    Chris

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  3. Brad,

    a couple more points occurred to me since I wrote the above last night:

    in terms of post support, the ko-ne-hozo-zashi connection receives all of the support directly under the mortised piece, while the tenoned piece essentially hangs off its tenon and received no direct post support.

    There are of course various ways to configure that joint so as to gain some post support

    In terms of my mention of an ideal of 50/50 removal of material for the keta to be joined, I should clarify that I was thinking more of the case of two pieces having to half-lap each other, which is the essential basis of the neji-gumi form of connection. However, that is a simplification, since in the case of the hip rafter attaching to the crossed keta, some material is often removed from it and the top of the intersection, thus more wood is removed from each keta than otherwise in a 1/2-lap. That means that the material removed from each keta in achieving the 1/2-lap needs to be less than 50%, since any further removal of wood to allow for the hip rafter trench would weaken the connection too much.

    While it's not a simple 1/3-1/3-1/3 situation, it might be on the order of 40%-40%-20% (keta A - keta B - Hip) material removal from each respective piece.

    Well perhaps it would be a good idea to do a more involved exploration of this topic in a blog entry soon.

    Cheers,

    Chris

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