Japanese roof logic
Posted: 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.
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.