Large Scale Central

Determining Octagonal roof angles

General Discussion
#1

OK so for the MIK build I need to build a roof for my water tank. I want it to be octagonal. I am making the panels out of 1/4 plywood. I know that to make a flat octagon the peak angle would be 45 degrees and the sides would be cut 90 degrees from horizontal. But as I lift the peak to form a cone (OK I know that not the right term but I don’t know what it would be called) does the peak angle stay 45 like I think it would and what happens to the sides so that the panels will glue up flush to one another? This would depend on how tall I make it right? Is there a formula to figure it out so that I am not cutting by trial and error?

If this isn’t as easy as applying a formula could I con someone into drafting this for me in a CAD program or Sketch up or something? I think once I know the angles I am smart enough to set my compound miter to cut them right.

#2

You know, if I had a piece of paper, a pencil, and a scientific calculator to I could show you with high school trig in about 5 minutes.

Without pictures it’s going to be tough. Perhaps someone has the time to draw it up and post it.

Alternatively, you could give the height of the roof, and the diameter you want and the overhang over the diameter and someone could calc it up for you. (maybe even me (http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-cool.gif))

Greg

#3

I had the same problem, when I built the water tank for my buddy. I got sent a 3D set of plans, I’ll try to find them for you.

#4

The outer rim of the tank is 3 3/4". By eyeballing it a 2" height above the tank rim looks good and a 1/4 over hang looks right. Now I wish I had taken High school trig.

#5

Here is the link to the set of drawings that I used for the working water tank.

http://www.largescalecentral.com/forums/topic/16463/working-water-tank-build?page=5

#6

Thanks Dave

#7

So Dave, I read on further and you put a 10 degree bevel on the panels. Is this a constant or a variable depending on height? Also does the angle at the peak stay 45 degrees? I am having a hard time wraping my brain around what needs to happen as we go 3D.

#8

the height changes the angle for sure.

Devon, cut up 8 triangles, tape them together and see how the height affects it (don’t tape one of the joints)

Greg

#9

So I can see that the angle changes just by forming my hands into a triangle then raising the triangle. I think I could even mock something up for that peak angle, its the bevels on the panel edges that are more worrisome to me so that they are flush fit. I can see that changes as well.

#10

I’d try a few angles with paper first. Or else make a round roof. Cones are easier to figure out!

#11

Hi

for those of you that intend to build an octagon roof, there is a YOU TUBE video on how to do it and how to calculate all of the dimensions. just enter “octagon rafter construction calculator”

there are several others on the subject.

Bill

#12

I would draw it out to scale… A top (plan) view and a side elevation view. Measure the Length of the base of the triangle from the plan view and the height of the triangle by measuring the Length of the sloped portion of the roof peak to eves . Note that the angle of the peak should be less than 45 degrees. I don’t mess with calculators and have more fun drawing to scale!

#13

I had the same problem when I built 2 switch towers and had to put what is called a hip roof on them.

It is a 4 sided roof so building a 8 sided one is even harder.

Have you tried cutting out the form using paper or scrap pieces?

If it doesn’t come out exact you could use caulking or glue to seal up the miscuts then cover it with the corrugated tin or use wood shingles or tar paper.

#14

Here is the part I think you guys are missing. The angle at the peak doesn’t scare me. I can figure that out with paper by trail and error. I will probably do just this to get and idea of the height I want since I eyeballed the 2". But using 1/4 plywood which is what I was intending to use the edges where the eight panels join together need to be beveled so that they are flush with one another. the only reason this would be critical is at the end of the panel because the edge forms the fascia. Now using silicone to fill gaps is not a bad idea all around because I am going to use either foil tape for a tared tin roof or sand paper for a rolled asphalt roof. I suppose if I have to I can make a cedar fascia to cover gaps. I was just hoping there was a way to figure that bevel or some nice soul who understands sketchup or CAD would take pity on me.

I know with some fiddling I can get it.

#15

Devon

When I did a water tank years ago, I first cut them square and then eyeballed the angle and made it a few degrees tighter because I wanted the surface flush, the underside not so much because of viewing angles. I’m guessing your angles are around 15*

Panels 4+" x 1.57" Geometry has the formula for the square of the rise and run divided by something too foggy froggy to remember. Trig is more about sine waves …

Half brain thinking, another cup o joe to go …

John

#16

Devon, take a look at this video. It will either give you the info you are looking for or confuse the he77 out of you. There are two parts.

#17

I guess it would help if I included both parts. Sorry.

#18

Devon

In case you didn’t find the sheathing bevel angle in all those numbers,

here’s an easy way to do it.

You know that the bevel angle would be 22 1/2 degrees if you were building an octagonal box.

you also know that the bevel angle would be 0 degrees if the pieces were laying flat.

so in other words at 90 degrees to the horizontal the bevel is 22 1/2

and at 0 degrees to the horizontal the bevel is 0

so at 45 degrees roof slope the bevel would be half of ninety or 11.15 bevel angle

and at 15 degrees roof slope the bevel would be one sixth of 22 1/2 or 3.75 degree bevel angle

I think you get the idea how this works - - -

Bill

#19

Yes bill that’s exactly what I was looking for.

#20

So a 22.5 degree 1/2 turret with a 24/12 roof pitch which gables into a pair of cross gables at a 10/12 pitch should be beveled at whatever the sandpaper angle says when modeling it then?

Edit: Because this a yes or no answer

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