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Octagonal roof question

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Forum topic by Belg1960 posted 690 days ago 1740 views 1 time favorited 16 replies Add to Favorites Watch
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Belg1960

790 posts in 1692 days


690 days ago

Topic tags/keywords: question

Good morning, I was hoping I could get some help trying to figure out how to produce a roof like in the following pic.

I found this but I’m not following on how I can figure out what size my own specific pieces would need to be from this. Also for me its a little confusing to follow, need it in PLAIN English. LOL
http://mathcentral.uregina.ca/QQ/database/QQ.09.06/verner1.html
Thanks for any input, Pat

-- ***Pat*** Rookie woodworker looking for an education!!!


16 replies so far

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chrisstef

10653 posts in 1633 days


#1 posted 690 days ago

compound miters … not my specialty. Each end would have to be cut at 20 degrees.(360 / 8 =40, 40 / 2 = 20)Then you have to figure out the taper. Its 8:30 and i havent finished coffee yet … FWIW. Rip both sides with a bevel as well. Id start with some scraps and tinker until my brain melted.

LJ Jim jakosh has helped me with with some funky geometry … drop him a PM im sure hed be more than happy to help.

-- "there aren’t many hand tools as awe-inspiring as the #8 jointer. I mean, it just reeks of cast iron heft and hubris" - Smitty

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Belg1960

790 posts in 1692 days


#2 posted 690 days ago

Chris, thanks for the input. I have been working with some cardboard. I was hoping I could find a computer generated, webpage which just allowed you to enter in the dimensions but the only ones I’m finding are for gazebos and the ones I found say they are not accurate when used for small projects.
I will drop Jim, a pm and ask him if he could help out. Thanks for the lead, Pat

-- ***Pat*** Rookie woodworker looking for an education!!!

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chrisstef

10653 posts in 1633 days


#3 posted 690 days ago

Ive used an octagonal calculator before but it was for a flat piece of plywood. I was thinking you would stack up pieces of, lets say 1×6, but my brain probably wasnt firing yet. Are you going to use plywood pieces?

If so, i assume you could cut your 8 triangles to match and rip a 20 degree bevel along each edge. Attached with some cleats on the inside. Sounds like you will need some sort of jig for the tablesaw.

Again just me thinkin out loud here, far from an expert. I did take geometry twice in high school and not by choice.

-- "there aren’t many hand tools as awe-inspiring as the #8 jointer. I mean, it just reeks of cast iron heft and hubris" - Smitty

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lunn

206 posts in 935 days


#4 posted 690 days ago

octagon = 8 pieces = 22 1/2 degree cuts For the center roof support make a octagon with each side the width of the stock you use for the rafters. Cut the rafters to whatever pitch you want for the roof. Run the rafters down to the coners of the house. That will give you the width, lenghts and angles of the roof pieces. According what pitch you give the roof.

-- What started as a hobbie is now a full time JOB!

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Belg1960

790 posts in 1692 days


#5 posted 690 days ago

Chris, I remember back when I took trig saying to myself what the heck am I ever going to use this for. Well should have paid better attention after all. Came back to bite me u know where!!! I got the angle part just need to figure out what the sizes of each of the pieces will be. Was thinking of using miter saw with stops. Dropped PM as suggested.
Isn’t surprising how coffee make things clearer??

-- ***Pat*** Rookie woodworker looking for an education!!!

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Jim Jakosh

11297 posts in 1732 days


#6 posted 690 days ago

Hi Pat. I got the PM and I’m working on it. What I do when I start something like that is to scale the picture and make a drawing of the part that I plan to reproduce. Then I add in all that I know about it. On this one we know it is octagon so all the angles for the 8 sides around the center at 45 degrees (360/8).
I use the scaled dimensions for all my figuring, and then I multiply all the dimensions ( except the angles) by a scale number to get the to the real finished size.

I scaled the roof at 2.100 wide at the bottom, .650” wide at the top and 2.150 high. So if you wanted the roof to be 8” wide at the bottom, you would multiply all the dimensions I get by 3.809 ( 8/ 2.100).

On this one, I would not cut a compound angle on the long edge of the roof pieces because you will be using a filler on each joint. Let me draw it out and I’ll take a picture of my work with calculations and photo it and post it later…........Jim

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!

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Sylvain

543 posts in 1126 days


#7 posted 690 days ago

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404 - Not Found

2544 posts in 1596 days


#8 posted 690 days ago

http://www.pdxtex.com/canoe/compound.htm

Type in angles and number of sides, this will work it out for you, though still a degree of headscratching involved

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Sylvain

543 posts in 1126 days


#9 posted 690 days ago

deleted

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

View Jim Jakosh's profile

Jim Jakosh

11297 posts in 1732 days


#10 posted 690 days ago

Hi Pat. I’m not a computer jock and don’t know how to load this stuff into a program to spit out the answers so I did it long hand and showed you the steps in case you need to apply it to something else. It uses geometry, trig and algebra in simple form.
Remember that I started with scaled dimensions off the screen and that these are the root numbers to use for the dimensions.

This one shows the dimensions I started with and how to figure the length of one of the top sections. It came to 2.474.

This shows how I figured the width of the Flat at the bottom and the Flat on the top of the roof section
F ( top) is.248 F ( bottom) is 1.186

The shows the shapes and sizes of the 8 roof sections and the 8 sections of the body and how I got them.
The body sections are .976 wide, 1.700 long and have a 67.5 degree angles on the vert. edges.

This just a wrap up showing them again and a table to use to figure your actual cutting sizes. For example if you want the body to be 8” high. Divide 8 by 1.700 which is 4.706. Us that for your scale and just multiply that scale times each of the dimensions on the last page.
That would make the body sections 8 long and 4.593” wide.
The tops sections would be 1.167 wide at the top, 5.581 at the bottom and 11.642 long, The angles would be the same as they are.

Check it out and let me know if it works for you??
Thanks for the challenge!!!!!!!!!!.....................Jim

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!

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rance

4128 posts in 1787 days


#11 posted 690 days ago

Jim, have you tried it in SketchUp? Seems like an easy solution to find out the shape of the sides if you know the height, the roof slope, and the size of the hole at the top.

Edit: Yeah teejk, the actual picture looks like a Hexagon. Even though the title and link refer to Octagons. Maybe just a design change. :)

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

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teejk

1208 posts in 1311 days


#12 posted 690 days ago

is that an Oct or a Hex?

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Jim Jakosh

11297 posts in 1732 days


#13 posted 690 days ago

Rance, I don’t know how to run sketchup! I understand this way better. I’ll bet someone could do it on there but it won’t be me!!
Teejk, It is an octagon.

............Jim

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!

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rance

4128 posts in 1787 days


#14 posted 690 days ago

Jim, for those of you that can do the math, I applaud you. Thanks for stepping in with the solution.

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View Belg1960's profile

Belg1960

790 posts in 1692 days


#15 posted 689 days ago

Guys, just so you know I’m PM’ing Jim with some questions as not to sound like a moron in open forum. LOL I will add further info as I understand better what I’m looking at. Thanks for all the responses, Rance if I could use Sketchup I would be so much better off, I just don’t have the inclination to learn it as I hate the learning curve. Man that must be SOME CURVE cause even drawing a box takes effort.

-- ***Pat*** Rookie woodworker looking for an education!!!

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