# How to scribe a segmental arch.



## zccarpenter (Nov 4, 2007)

I'm framing a segmental arch across a 16' garage opening. I know how to scribe/calculate normal arches by finding the radius but the radius to this arch will be over 15 to 20' (i.e. take up too much room to layout). The opening is 16', the sides come down 8" on each side to 0" in the middle. What's the best way to scribe this without snapping a bunch of lines and rotating on a string?


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## A W Smith (Oct 14, 2007)

zccarpenter said:


> I'm framing a segmental arch across a 16' garage opening. I know how to scribe/calculate normal arches by finding the radius but the radius to this arch will be over 15 to 20' (i.e. take up too much room to layout). The opening is 16', the sides come down 8" on each side to 0" in the middle. What's the best way to scribe this without snapping a bunch of lines and rotating on a string?


why cant you lay it out on the driveway, ground or deck with a steel tape and onto a 2x12? I did this for a 20 foot arched garden bridge over a creek last year.


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## curapa (Oct 8, 2007)

Find the center of your opening, measure up to your desired height, take a piece of flexible trim ( 1/4" round or similar) while holding the two lower ends at the desired locations bent the trim up to the center mark. Have someone scribe the trim. :thumbsup:

Get a firm but bendable piece of trim to avoid a wavy scribe mark.


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## Trimwerx (May 24, 2007)

^it can take a little experimenting to come up with a piece that'll bend just right for that procedure, but it is a really easy way to do it.


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## neolitic (Apr 20, 2006)

If you're using azek on the job, it has the perfect properties for that trick--including the 18' length.


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## wallmaxx (Jun 18, 2007)

I like to calculate everything I can. In your case draw out a grid reference on plywood and play connect the dots. Sorry for the W I D E image. I needed the size to scale the dimensions to keep them readable.










Best of luck to you.


v/r
Mike


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## karma_carpentry (Aug 7, 2005)

That is great, WallMaxx. What kind of calculations did you use? Did it involve Sines or Cosines?


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## Bone Saw (Feb 13, 2006)

auto cad, I was about to post the identicle dwg last night, but the post count of op made me think twice


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## wallmaxx (Jun 18, 2007)

Bone Saw:thumbup: ACad. 

I use a scientific calc in RPN. The eqn is :

S SQ R SQ 4 * + 8 R * / 

then convert to feet and draw it.


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## zccarpenter (Nov 4, 2007)

Thanks for all the useful tips. My size ended up changing to 12" down on either side to 1.5" in the center. I found an equation (Rise Squared + Span/2 Squared)/2xRise. My Radius ended up being 37'. Also, I found a nifty little way in my Layout book. However, there is stuff all in the way in the garage, the driveway is dirt and has a dumpster in the middle, so there is no real room to make a pivot point and scribe (except maybe on the golf course outside). So, tommorrow I think I will try Wallmaxx's method (thanks, by the way). And as for using flex trim and scribing, I may be trying that also.


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