# Roofing a Geodesic Dome house



## carl moody (Dec 28, 2010)

I have been asked to roof a geodesic dome house. Does anyone have a simple way or formula to measure the number of squares on the house? Carl


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## katoman (Apr 26, 2009)

Here you go - http://en.wikipedia.org/wiki/Dome_(mathematics)


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## jmiller (May 14, 2010)

I'm not a calculus expert, but I don't think there is a formula for measuring surface area of a geodesic dome (a geodesic dome is not really a dome, but a polyhedron, so the link above is not much help).

From what I could tell after a google search, you have to know the surface area of the individual planes, and sum them. I.e. figure out how many shingles it will take to cover each triangle, multiply by number of triangles that size (may be that all of them are the same size, I can't remember), and then throw in some ungodly waste factor because it's all hips.

I don't know how the ones we did were estimated either, but I could ask if you're still around.


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## Mr Latone (Jan 8, 2011)

The surface area could be approximated by the surface area of a sphere minus an estimated percentage that is truncated at the bottom. Try and figure out you drip edge, there is a challenge. But seriously I would imagine there would also be a pretty high waste factor on the roof .


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## jmiller (May 14, 2010)

Mr Latone said:


> But seriously I would imagine there would also be a pretty high waste factor on the roof .


Was the rest of your post not serious?


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## jmiller (May 14, 2010)

Mr Latone said:


> Try and figure out you drip edge, there is a challenge.


No. Drip edge == circumference.


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## tinner666 (Nov 3, 2004)

Not to mention, DE is worthless on a dome.


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## jmiller (May 14, 2010)

Yeah, I dont remember _any_ drip edge on the job. See. It'll be easy.


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## Synome (Jan 9, 2011)

What I would do is tell them straight up that it is going to be a job where the materials are just what they are. The surface area of a sphere should be greater than the dome so that gives you a max, but then you have to add waste so the sphere may be closer to right. Just one of those things where they have to understand that the cost is very hard to set beforehand, and if wasn't sure I would either turn it down or try to talk them into letting me charge them by the hour.


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## Roofman20033 (Jan 29, 2008)

I would use a waste factor of at least 20%. If you have never done a dome home, you are bound to make a few mistakes and use up material in a hurry!
I have know 2 different roofers that have done dome homes and they both told me to run away!!


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## jmiller (May 14, 2010)

Run away? I'd love to have a dome job right about now :shifty:

We used a ton of coil stock too. Since there was no coping we flashed our hip shingles, and flashing a hip shingle takes a long pc of 'coping'.


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## Mr Latone (Jan 8, 2011)

jmiller said:


> Was the rest of your post not serious?


Each of the polygons making up the dome has interfaces with the adjoining polygons. Rake edge, drip edge, some kind of flashing, hip/ridge caps, nothing at all. I was reflecting on the one or two I have seen locally and in I could not recall what was used.

The rest of my post was serious.

Tough crowd


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## JHC (Jun 4, 2010)

Call your HS geometry teacher.


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## jmiller (May 14, 2010)

Tough crowd? Maybe, but you can't post gobbleygook on most forums and not get called out. 

You said: "The surface area could be approximated by the surface area of a sphere..." 

And you were absolutely right. Then again we can approximate the surface area of a cube with a sphere too, depending on how accurate we want our approximation to be. Or, we can measure the area of one face of the cube and multiply by six, and have more than an approximation.

"...minus an estimated percentage that is truncated at the bottom". 

Most I've seen aren't a truncated sphere, but elongated slightly at ground level, like a short squat bullet. A lot of the ground level vertical walls will be broken up with windows, doors, siding, etc- so add another guestimate to the approximation and estimated percentage.

The drip edge thing was just the third strike, and what I decided to comment on. I see your point now though, and yes that is a difficult detail to figure out. Ours has lasted a couple seasons leak free (knock wood), but I'm not sure we'd do it the same.

Anyway, you _were_ right about the waste factor.


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## jmiller (May 14, 2010)

JHC said:


> Call your HS geometry teacher.


He'd have to call his calculus professor for a refresher.


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## Mr Latone (Jan 8, 2011)

jmiller said:


> Tough crowd? Maybe, but you can't post gobbleygook on most forums and not get called out.
> 
> .......
> Anyway, you _were_ right about the waste factor.


I get your points, but it was not IMO gobbleygook. It is a closer approximation to a sphere than a cube. Truncating a base and allowing for windows and doors and a sort of apron the comes down to the ground would certainly be part of the take off.. I would not expect the OP to bid the job directly off a few internet posts. My suggestions may or may not have helped the OP, but it would be an approach I would likely take.

I am not a smart-ass or a wise-guy. You will find most of my posts have substance and will be useful to the topic.

Further
Levity is is used with mixed results on the web. Aside from sticking in a winking smiley or a LOL here and there, one's intent can easily be misconstrued.


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## jmiller (May 14, 2010)

Mr Latone said:


> I am not a smart-ass or a wise-guy. You will find most of my posts have substance and will be useful to the topic.


I know. I enjoyed your post about ice in the gutters so much that I didn't care you hadn't done an intro. You obviously know what you're talking about and I'm sure there is a lot I could learn from you (esp re: metal). 

Let's agree to disagree on the dome, but for the record I have done one. When you actually start roofing it, you realize each triangle took x shingles, and that you could fairly easily have estimated almost the exact number of bundles, even though the waste is ungodly. That mathematics agrees with this real world method is complete coincidence.




> Further
> Levity is is used with mixed results on the web. Aside from sticking in a winking smiley or a LOL here and there, one's intent can easily be misconstrued.


I could definitely use some work on my levity. Apologies and cheers.


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## Mr Latone (Jan 8, 2011)

jmiller said:


> I know. I enjoyed your post about ice in the gutters so much that I didn't care you hadn't done an intro. You obviously know what you're talking about and I'm sure there is a lot I could learn from you (esp re: metal).
> 
> Let's agree to disagree on the dome, but for the record I have done one. When you actually start roofing it, you realize each triangle took x shingles, and that you could fairly easily have estimated almost the exact number of bundles, even though the waste is ungodly. That mathematics agrees with this real world method is complete coincidence.
> 
> ...


It's all good and I prefer to begin with mutual respect. Later, when there is a serious disagreement we can have a real pissin' match :laughing:

One of domes I have seen locally IIRC had ***xx facets.


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## tinner666 (Nov 3, 2004)

jmiller said:


> Run away? I'd love to have a dome job right about now :shifty:
> 
> We used a ton of coil stock too. Since there was no coping we flashed our hip shingles, and flashing a hip shingle takes a long pc of 'coping'.


A tip for your next one. The manufacturer recommends dutch-lapping the hips. Do the flat bottmed ones first passing the hips 3", do the intersecting ones coming back over 3" again. This gives a 6" dutch lap.


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## MrRoofer (Sep 21, 2008)

The area of each triangle x however many triangles + waste = area?

Probably a lot tighter than using the sphere method, although I would probably use the sphere method :shifty:.

Unless it was my own of course...


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## jmiller (May 14, 2010)

MrRoofer said:


> The area of each triangle x however many triangles + waste = area?
> 
> Probably a lot tighter than using the sphere method, although I would probably use the sphere method :shifty:.
> 
> Unless it was my own of course...


That's funny :shifty:

You wouldn't need to calculate waste IMO. The waste will be a bonus with that method, if you can use scraps. So you could essentially count the number of shingles per triangle and subtract one for cut-off use. I wouldn't bid on the guys doing that though... Say the up facing triangles in Tin's photo are going to take 14 shingles, and the downs are going to take 12. Walk around the dome and count every facet, and multiply 13shingles times 112 facets. If we wanted a square footage, yeah you'd measure each unique facet and count them to use as your multiplier. With shingles though you have the overlap mentioned above, so sf won't be as accurate and you'll need the waste factor multiplier as well. Sometimes the shingle is just an inch short and you end up burning a whole extra one.


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## MrRoofer (Sep 21, 2008)

Bad wording on my part, the intent was to include the overlap, waste and field area of each triangle all in one. Then multiply that x how many triangles. Getting a reasonably accurate number of shingles using that method would be relatively simple I would think. The reason I would bid it extra heavy is because of the extra ugly labour factor of 112 starts . The extra points on the extra material hopefully will cover the aggravation.

All of this equates to a reasonably expensive roof, which at least in my case having _not done one_ before, is a better estimate than trying to be tight and losing my shirt in the labour process.

But again, if it was my own, things would be different (which it wouldn't be, my furniture is all square :laughing

I confess, I tend to over equate waste, especially in a day and age of relatively no waste shingles (laminates). Old habits die hard I guess.

I think any method above would work fine, however counting shingles is probably not reallistically an option if it was a new building.

Those pics are super handy, I didn't even have to think about that overlap, nor did I before. I spose at the end of the day I would actually prefer to see this type of roof in metal...

Smilies rule, they are levitous. :cheesygri.


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## MrRoofer (Sep 21, 2008)

Just to add something a bit off topic, one thing that has always bothered me about the appearance of a closed valley, is the uneven lump where the under layer is, as a result of the shingles being followed through with the thought of providing enough lap, leaving somewhat of a random lumpy pattern.

What I see as a solution is to chalk a line and cut the underlap in a straight line, as they are doing in the overlapped parts in the photos of this dome.

KnowhatImean?


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## Mr Latone (Jan 8, 2011)

MrRoofer said:


> .....
> KnowhatImean?


I know exactly what you mean. I have always been a fan of open metal valleys for this very reason. Cleaner looking lines, especially on heavier shingles.

As far as the labor involved in the dome, I agree that the composition shingle is not the ideal choice. Doesn't seem to justify the effort.

Further reflecting on my previous posts, and after viewing tinner's excellent photos, I would think an actual count would not be too difficult.

But the OP seems to have lost interest and so until one of us comes across another dome in need, it's purely mental exercise.


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