Bay window hip roof, wall cleat backing angle

Traditional framing and building practices, using wood, stone, straw, clay.
Ian Anderson
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Bay window hip roof, wall cleat backing angle

Sat Jun 04, 2016 8:56 am

Hi everyone,

I am currently building a small hip roof for a bay window. It is a standard 45 degree window with about a 20" projection. I have build one hip roof like this before without trouble, so I decided to give this one a bit of a flare. I played around with different slopes and curves, and ended up settling on a 12/12 pitch, with the radius of curvature on the common rafters set at phi times the rise. I had a hard time finding info on the traditional curves on these roofs, so just went with what looked good.

My first head scratcher came when I had to transfer the common curve to the hips and wall cleats. I know I could have just built it and scribed the curves, but I wanted to understand the geometry. After some research I found this method. http://sbebuilders.blogspot.com/2015/07 ... fters.html This worked beautifully, so now I was left with just cutting the backing angles. I calculated them as I would with straight rafters, but the wall cleat did not come out as anticipated. It appears that the backing angle changes along the span of the cleat. In the end, I just cut it at the calculated angle, then tuned it up with a spokeshave. It is rough, but then again it is rough framing. I would love to do this sometime with an exposed rafter system where it must look good. Does anyone have experience with this?
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Chris Hall
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Re: Bay window hip roof, wall cleat backing angle

Sat Jun 04, 2016 10:19 am

The backing cut on the hip corresponds to the slope of the common. With a curved common, the slope varies, so the backing cut must vary as well. Were the hip rafter horizontal, there would be no backing cut, and were the hip rafter vertical, the backing cut would be 90˚.

The Japanese method is straightforward enough, however if you do not have a background in Japanese straight hip rafter work, I think that trying to explain their method is too onerous a task, at least for the time I have available right now.

The French work Encyclopédie des Métiers, in their La menuiserie Series, Volume 3, contains extensive material on curved hip work. Here's a planche detailing their method:
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That should solve it for you.

If you want a larger format version of the picture, please email me and I'll send it to you.
Ian Anderson
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Posts: 2
Joined: Mon Nov 24, 2014 11:19 pm

Re: Bay window hip roof, wall cleat backing angle

Sat Jun 04, 2016 1:45 pm

Thank you Chris, It looks like it is time for me to get your last volume on hip roofs anyway.

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