Spot the mistake

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Chris Hall
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Re: Spot the mistake

Sun Jun 21, 2015 12:09 pm

Mathieu wrote:A couple of days ago when looking at his blog I noticed immediately that there is a pocket for a jack rafter tenon at the keta level in the hip rafter. Either this pocket should not be there, one too many or I have no idea how he will attach the jack to the keta AND into the hip.
I guess this anomaly is not what we are looking for? But I am still pondering about this one too.
We have a winner!

Yes, the mortise for the jack rafter tenon directly on the keta centerline should not be there.

Here's another photo to show the area more clearly, problem mortise indicated with a red circle:
a0322824_20280320.jpg
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The jack rafter aligns to the centerline of the keta. There are three solutions to framing that:
  • 1) cut a shallow pocket on the end the keta nose and attach the plumb-cut rafter there
    2) cut a trench along the sloped nose of the keta and a mortise pocket in the hip. The rafter passes intact through and up the keta end and into the hip
    3) combine pocket and hip rafter mortise, but only trench the sloping nose of the keta about half-way deep. The jack rafter is similarly cut.
Of these, (1) is the simplest and weakest solution, (2) is stronger in terms of laterally supporting the rafter, however the cut out can lead to the keta rafter trench sides spreading open over time and creating gaps, and (3) is the perfect compromise, retaining good strength while not disturbing the keta nose too much but most difficult to cut. It also requires fitting in two directions instead of being driven uphill.

In the case of the above-shown framing at the hip, the presence of pillow blocks below means the keta-bana is much longer than it normally would be. This may have thrown the person off doing the layout, though I find that surprising since they use CAD for their layout. If the keta-bana had a half-depth trench in it for the jack, that would be one thing, however the way it is framed the jack is clearly mean to terminate in a notch on the nose. so, the mortise on the hip aligned to the keta centerline is superfluous.

Possibly they intend to fill in with a little piece of wood there to present the simulation of the jack continuing up into the hip, but if that is the plan I see no necessity for the mortise in the hip.

I think that laying out and cutting that mortise in the hip would be a very easy mistake to make. Luckily, when the decorative eave ceiling boards are all in place, it would be very difficult to spot from the ground.
Mathieu
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Re: Spot the mistake

Sun Jun 21, 2015 12:26 pm

One more thing I look forward to learn more about is the jack rafter tenon.
From the picture below it seems like he tenon will be diminished in width. First I thought this would be to accommodate a chamfer but that would have been aesthetically unusual for this type of construction, besides it is only on one side. Chris, any idea why they improve the joint like this? I noticed it already on a gensun for this building but can't retrieve the picture readily.
10985044_648316498637053_2534033162242457849_n copy.jpg
10985044_648316498637053_2534033162242457849_n copy.jpg (98.19 KiB) Viewed 5630 times
Last edited by Mathieu on Sun Jun 21, 2015 1:14 pm, edited 2 times in total.
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Mathieu
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Re: Spot the mistake

Sun Jun 21, 2015 12:37 pm

Here is another picture where we can see the jack tenon mortice above the kiyoi. The tenon is diminished on the back side, the side towards the building. Still can't find the gensun picture.
17725_635999869868716_4806899904778432629_n copy.jpg
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Mathieu
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Re: Spot the mistake

Sun Jun 21, 2015 12:52 pm

Here is the one picture I mentioned earlier.
a0322824_23072636.jpg
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Chris Hall
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Re: Spot the mistake

Sun Jun 21, 2015 12:57 pm

Well, the tenon form, typically, is to be 1/3 section height, and the have a square side abutment on the uphill side:
jack rafter tenon 2.jpg
jack rafter tenon 2.jpg (54.13 KiB) Viewed 5629 times
If you don't put the squared abutment uphill, the mortise is rather difficult to cut cleanly and if loaded in compression may split out as a result of the acute internal angle.

The cheeks of the tenon are also tapered - the upper side. Thus the mortise is tapered as well:
jack rafter tenon trim angle is half slope.jpg
jack rafter tenon trim angle is half slope.jpg (69.01 KiB) Viewed 5629 times
If you didn't taper the mortise, again you would have some difficult cut out ahead trying to produce a clean acute internal angle. With the tenon upper surface tapering, the mortise upper wall is a simple square cut to the surface, and the lower wall cut is parallel to the slope of the rafter.

All of this was covered in detail in the study project, as well as in TAJCD Volume V.
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Chris Hall
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Re: Spot the mistake

Sun Jun 21, 2015 1:01 pm

Mathieu,

wondering how you have been connecting the jack rafters to the hip rafter up to this point in time?
Mathieu
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Re: Spot the mistake

Sun Jun 21, 2015 1:08 pm

I cut the jack tenons exactly in the way you describe.
However on the pictures I posted It seems like the cut, square to the hip surface is moved inwards. Usually it starts flush with side of the jack and here it seems like there is a shoulder both above and behind the jack's tenon. Maybe I am misinterpreting the drawings and picture and the line shown with a distance f is just the bottom corner edge of the mortice.
Last edited by Mathieu on Sun Jun 21, 2015 1:30 pm, edited 1 time in total.
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Mathieu
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Re: Spot the mistake

Sun Jun 21, 2015 1:13 pm

a0322824_23072636.jpg
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Chris Hall
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Re: Spot the mistake

Sun Jun 21, 2015 1:53 pm

Yes, I see what you are referring to specifically.

What they are doing there is shouldering the uphill 90˚ abutment inwards from the rafter face, presumably to conceal the joint line, which is a bit hard to hide unless the cut out and fit of that abutment is perfect.

Here's what the tenon would look like:
jack rafter tenon diminished.jpg
jack rafter tenon diminished.jpg (206.07 KiB) Viewed 5625 times
And the mortise:
jack mortise.jpg
jack mortise.jpg (183.07 KiB) Viewed 5625 times
I chose 3mm for distance 'f'. The lower portion of the gensun-zu you posted shows the layout of the tenon on the jack rafter. No surprise that the distance squared inward from the face miter line is 1/√2 of 'f'', which would be 2.121mm in the case where 'f' (a distance measured along the miter or the surface of the hip rafter) was 3mm.

The shouldering of the tenon upper abutment is a high class refinement. Steal the knowledge when you find it...
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Brian
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Re: Spot the mistake

Sun Jun 21, 2015 3:05 pm

My only take-away is that there is a carpentry blog that I dont know about and you guys havent linked to :P

Please link to the blog :)

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