Japanese roof logic

Traditional framing and building practices, using wood, stone, straw, clay.
AntoineLaMothe
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Japanese roof logic

Sun Mar 01, 2015 12:33 pm

Hi to all,

I'm currently working on two timber frame projects, one 18'x30' 6/12 gable roof and another, larger, being my house and having a hip roof, also at 6/12.

I've built a few smaller timber frames, and each project so far was moving further towards a japanese way of building and designing. Still, I'm not qualified in any way and can only pretend using japanese techniques in an inspirational manner. There's not that much real traditional here (for now!).

For the 18'x30', wich I will build first (this June), we will be using ''natural'' logs, sawed on two faces as the Koya beams. The client has opted for hempcrete and 2x4 walls, and thus I will have large keta beams on top on the walls to distribute the weight along the walls. It's not ideal, i would have prefered to do a complete timber frame, but that how it is. I want the koya beam to sit firmly ON the keta (jointed but leaving enough wood under), so i will need an additional moya on top on that to support the rafters. There is another moya at half the distance between the wall and ridge.

Now, my understanding of japanese timber frame structures is that it tries to bring all the roof and snow loads down in a vertical direction only, while the western approach is to assume that the rafters will want to work horizontaly, and therefore to add a locking element to prevent that force, like a tying joint or truss system. First of all, is this right? Am I simplifiying too much?

Following this logic, I get to the situation at the ridge, where the rafters meet, and find a problem. I will be working with green wood (you do what you can...!), and so I already know that my roof will settle down of a certain amount, mainly at the center. If I install my rafters one against each other in an ogami awase style of ridge beam, and the ridge lowers, wouldn't my rafters be working against each other exclusively, defeating the purpose of the japanese roof? Should I install them side to side, not connected to each other instead, so they can apply their load downward even when the roof settles?

Hopefully I am clear enough... I can send a basic sketchup of the frame if you are interested.

Please don't hesitate to share your visions and experiences!

Thank you.
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Chris Hall
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Re: Japanese roof logic

Sun Mar 01, 2015 11:32 pm

That's a good question Antoine, and something I have also pondered in the past.

Ogami-awase, 拝み合わせ, means that the rafters lay atop the ridge and meet one another at the roof apex. The ridge itself may be backed or left unbacked.

I think that since you are using green lumber for framing and can expect shrinkage, all the more so on larger sections, the safe course of action is to bring the rafters together at the apex with a gap between them, like 0.5~1.0cm or so. That will allow the ridge section to shrink across the width and the rafters can draw together somewhat without starting to press themselves off the ridge's upper surface. The fasteners holding the rafters to the ridge would preferably be nails, as they have a certain ductility which allows for some movement between the joined parts.
AntoineLaMothe
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Re: Japanese roof logic

Mon Mar 02, 2015 12:28 pm

Thanks Chris,

That's what I'll do. Do japanese carpenters put the rafters side to side in temples or larger constructions? It's a part of the frame we rarely see clearly on pictures found on the net.

By the way, if you or anyone have links to images or info on japanese like frames that were adapted to cold weather in north america, like blogs or articles, I'd be very interested in this kind of info. Everytime I get to work on the plans, I encounter problems linked to the insulation around such frames. In fact, even north american timber frames rarely work smoothly in that aspect, imo.
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Chris Hall
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Re: Japanese roof logic

Mon Mar 02, 2015 6:44 pm

"By the way, if you or anyone have links to images or info on Japanese like frames that were adapted to cold weather in north America, like blogs or articles, I'd be very interested in this kind of info. Every time I get to work on the plans, I encounter problems linked to the insulation around such frames. In fact, even north American timber frames rarely work smoothly in that aspect, imo."
Oh, I have many ideas in that regard.

Looking to Japan is a bit pointless, as Japanese timber architecture is adapted primarily for ventilation, not insulation. Central heating there remains quite uncommon.

Most of the North American solutions revolve around jacketing the frame with SIPs or straw bales, neither of which strikes me as a good solution.
AntoineLaMothe
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Re: Japanese roof logic

Mon Mar 02, 2015 9:41 pm

You're right.

I've worked in a very poor quality timber frame builder 5 years ago and they were using SIP. I mean, they kinda work, in the end... it's just that everything about the system feels so opposite to the minding of building a joined wood structure for a house. As for most techniques that use another 2x6 wall outside of the timber frame... well it's easy to understand why it's not ideal.

The structure I'll be building will be a hempcrete one, and I can't wait to see how it goes. If it works out as well as it is supposed to, I'll be impressed. Right now, the main arguments against it is that you have to fight to get a building permit.

Anyway that's another subject, and it's big and it's not exactly the place!
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Paul Atzenweiler
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Re: Japanese roof logic

Tue Mar 03, 2015 5:41 pm

For cold weather timber framing can you use the "snow flake" truss Chris recently posted? :lol:
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Chris Pyle
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Re: Japanese roof logic

Tue Mar 03, 2015 6:28 pm

Chris Hall wrote:
"By the way, if you or anyone have links to images or info on Japanese like frames that were adapted to cold weather in north America, like blogs or articles, I'd be very interested in this kind of info. Every time I get to work on the plans, I encounter problems linked to the insulation around such frames. In fact, even north American timber frames rarely work smoothly in that aspect, imo."
Oh, I have many ideas in that regard.

Looking to Japan is a bit pointless, as Japanese timber architecture is adapted primarily for ventilation, not insulation. Central heating there remains quite uncommon.

Most of the North American solutions revolve around jacketing the frame with SIPs or straw bales, neither of which strikes me as a good solution.
I'd be very interested in hearing your ideas Chris.
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Chris Hall
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Re: Japanese roof logic

Wed Mar 04, 2015 10:32 pm

Thanks Chris. The two biggest issues with a timber frame wall structure are:

1. Thermal bridging
2. Inadequate resistance to shear loads for modern doors and windows

A common solution involves SIPS. You can build a house entirely out of SIPS - they are not called 'STRUCTURAL insulated Panels' for nothing - so wrapping them around a timber frame is a questionable idea, it seems to me. SIPs are entirely rigid in their resistance to shear loading, while a timber frame is a relatively pliant structure. Not only does the SIP render the timber frame largely decorative, in structural terms, but it's like mixing two completely different systems. In nature do you ever see creatures with both a skeleton and an exoskeleton?

Not to mention the other drawbacks to SIPS in relation to their energy-intense manufacture in usually non-local large factories, use of off-gassing materials, chemicals, the waste disposal issue, the landfill issue after their lifespan is reached. And who knows what their lifespan is anyway?
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Evans
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Re: Japanese roof logic

Wed Mar 04, 2015 11:40 pm

Chris Hall wrote:In nature do you ever see creatures with both a skeleton and an exoskeleton?
Armadillos? Turtles?

I mean, it says plenty that neither of those two animals is what we tend to call "graceful" or "elegant", but still.
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Chris Hall
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Re: Japanese roof logic

Thu Mar 05, 2015 12:10 am

Not the same thing- they are both creatures with skeletons.

A turtle has a skeleton upon which its body tissue are supported, including the protective shell. The shell is not an exoskeleton. The armadillo also has a endoskeleton, and the armored plates are a development from scales, and serve a protective function. The armored plates are not supporting the animals viscera:
1654573-an-armadillo-skeleton-0.jpg
1654573-an-armadillo-skeleton-0.jpg (64.9 KiB) Viewed 5881 times
Animals with exoskeltons - those with the skeleton on the outside of the body - have the soft innards supported by the skeleton (the carapace). Animals such as insects and crustaceans have exoskeletons.

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