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Author Topic: timber framing a octagonal 8:12 roof  (Read 4221 times)

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Offline plbnyn

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timber framing a octagonal 8:12 roof
« on: March 10, 2008, 10:43:39 PM »
I would like to know how to cut the framing for 4x10's 20' long from a center pole 24" Dia 18' tall to a straw bale 9' tall load bearing wall.

Books, pictures, a visit ( I live in Chico Ca and work in Pleasanton) any help would be great.

Andrew
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Offline Radar67

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Re: timber framing a octagonal 8:12 roof
« Reply #1 on: March 10, 2008, 10:53:30 PM »
Are your walls 5 feet from the center of the pole, or 40 feet? How many rafters do you plan to have?

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Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #2 on: March 10, 2008, 11:09:34 PM »
rafters are 20' long on an inscribed 30' diameter circle. Walls are 26'6" from center pole. I planned on having 8 full rafters from the center pole and about 5' from the tip I will place a perpendicular 4x10 place a 16' rafter and at 8' from the top two more perpendicular 4x10's with that at the interior wall with a two more rafters hitting the 20' rafters at the wall and extending to the over hang of 2'6". Decking Full dimension 2x8 t&g engamann spruce, and raised metal edge roofing.

I saw the sketchup program and will figure out how to master that next week.

Love the feed back.

Andrew
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Offline Radar67

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Re: timber framing a octagonal 8:12 roof
« Reply #3 on: March 10, 2008, 11:43:46 PM »
I may be picturing this wrong, but 20 foot rafters are not going to be long enough to reach a wall that is 26' 6" from the center of the pole.
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Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #4 on: March 11, 2008, 12:57:09 AM »
sorry the rafters are 20' and the diameter of the octagon is 30' (octagon is inscribed in a 30' diameter circle) the diameter of the octagon is 13'3".

Andrew
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Offline Thehardway

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Re: timber framing a octagonal 8:12 roof
« Reply #5 on: March 11, 2008, 09:00:01 AM »
Audel's Carpentry guide has a section on framing octagons.
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Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #6 on: March 11, 2008, 10:41:35 PM »
Thank you I was perusing that book earlier and had to leave the library before I read through it. I will go back.

Andrew
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Offline Jim Haslip

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Re: timber framing a octagonal 8:12 roof
« Reply #7 on: March 15, 2008, 12:48:32 AM »
Is this the design you were looking for? (roughly)

http://www.jlhaslip.com/sm/oct8.pdf

Based on these quick drawings, you could likely get a 10/12 pitch out of the 20 footers, depending on how you handle the top end. Maybe a little more. The left hand side shows an 8/12 pitch, the right hand is about a 10.5/12, but that is measuring a full 20 feet on the rafter length o the post centre.
You might be okay with a 10/12 after deducting for the end-checks and the thickness of the centre post, again, depending on the wood, and the style of connection would influence it, too.

You might want to check the rafter foot connection as well.  What did you plan for a connection there? Spikes? or an English Tying Joint with a tenon coming from the post top? or a birds mouth style? Overhang?

Hope this helps...

Offline beenthere

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Re: timber framing a octagonal 8:12 roof
« Reply #8 on: March 15, 2008, 12:56:36 AM »
Have you seen the pics and thread on Arky's octagon buildings?

http://www.forestryforum.com/board/index.php/topic,26121.msg408425.html#msg408425
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Offline swampfox

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Re: timber framing a octagonal 8:12 roof
« Reply #9 on: March 15, 2008, 02:19:27 AM »
How are you going to join the top?  Are your posts going to have five sides so the faces are square to the plates and whatever braces?

What is the pitch of your common rafter?
Therefore what is the pitch of your hip?
Will your common rafter end at a purlin?
What is going on up top?

We just figured out one of these.  Octagonal Roof:; 18/12, one common rafter that ends in a purlin in between each 45 degrees. hips can extend to whatever leaving a oculus, or meet them all together.  What do you want to do where they all meet?

Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #10 on: March 16, 2008, 01:47:02 AM »
Swampfox,

I was going to use a Kingspost beveled mortise (right name?) for the center post (ponderosa Pine 24 at base and 18" diameter at top). Also strap iron across all 8 full rafters, with lag bolts and then a 1-2' bolt through the strap iron and into the post. The purlins will be at 5 and 10' from the center post with rafters and jack rafters at 30" spacing. Decking will be t&g full dimension Engalmann Spruce with raised metal edge roof. I am having trouble with the load bearing straw bale walls and have had to go to an engineer.

The pitch will be 8:12, snow load 55 psf, dead load 40 psf, wind 100 mph. That is all that I can think of right now.

I will have to look up oculus, and I am looking at a cheeked mortise joint for the jack rafters, 8 common rafters will end at a purlin 14-16' long, and then 16,10' rafters will meet at purlins.

I will up load a rough drawing of the rafter layout tomorrow in the early evening. I would like to get some feed back on that. I will not have a hip. (unless I am not understanding the terminology).

I will ask some questions and read up on rafter terms.

Andrew 
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Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #11 on: March 16, 2008, 01:50:03 AM »
Beenthere,

Yes I have  seen ARky's octagon, I need the open space over the floor, plans are to have wood shop and it will be difficult to move boards over 8' long inside that structure. Although I am still looking at all of the options.

Thank you,

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Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #12 on: March 16, 2008, 01:56:07 AM »
Jim Haslip,

YES, that is my current design, I am looking for the joinery for the top plate and the rafters. I have been talking with an engineer and there is concern with load bearing straw bale walls. I was going to use a modified kings post (24" base and 18" top: 16' Ponderosa Pine center post) I was looking for the options between the rafters and the top plate. I was planning on using a box beam with a 6x6 or 8x8 DF as a top plate, cinched to the walls to pre-compress them. I will up load a picture of the rough drawing for the rafters tomorrow.

Thank you,
Andrew Logan

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Offline plbnyn

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Re: timber framing a octagonal 8:12 roof
« Reply #13 on: March 18, 2008, 11:26:32 PM »
swampfox,

 Ihave read up on rafter terminology, Iwill have hip rafters 8 of them, with 16 jack rafters. I am still trying to get a camera to up load the pics
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