I am trying to properly lay out an arched doorway and can not seem to get this... what I do know is as follows.. the two side walls are 8'9" from the floor to the bottom point of the arch...from a center point between these two vertical lines taking a vertical measurement from the horizontal midpoint is 15 1/2"... I know the two sides are 11'-11" apart... what I do not know is how to determine the radius that would generate this arc.

What I usually do is tack a panel of flat material (plywood or cardboard) over the space that the arch will be needed and temporarily bridge (with a straight piece of wood) the space between vertical lines that defines the arch's edge, at a point approximately halfway from the existing bottom of the opening and the existing top of the opening... level with the floor. Then I provide a pin (nail) at the middle of the vertical board to which I tie a piece of string. The string should be as long as the distance from the pin to the top opening is. Next... I tie a pencil to the other end of the string and, with the string pulled taunt... I trace the arc from one side of the space (across the top) to a point opposite where I started from. This provides a template to be used for the actual construction of the arch. You can adjust the height of you "pin" to increase or decrease the circumference of the circle (hence the radius of the curve).

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Originally Posted by RGabel

I am trying to properly lay out an arched doorway and can not seem to get this...what I do know is as follows..the two side walls are 8'9" from the floor to the bottom point of the arch...from a center point between these two vertical lines taking a vertical measurement from the horizontal midpoint is 15 1/2"....I know the two sides are 11'-11" apart...what I do not know is how to determine the radius that would generate this arc.

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You may not be able to lay out a true circle for this arc. You may need to use a little trial and error at this point. Obviously you are building an arc so you are going to use plywood? I will use plywood in my explanation. Lay two 4'/8' sheets of plywood side by side to create an area that is 4' by 16'. Cut one piece of plywood 3' 11" long or cut both peices 5' 11 1/2" if you prefer to center. This will give you an overall length equal to your opening 11' 11". Measure down the 4' sides 15 1/2" if you have some small nails drive them in at about 3/8" above these points this is the thickness of shingle mold. That would put the nails at about 15 1/8" from the top. If you have a piece of material that is flimsy, 'not rigid' a wood like shingle mold you can use this to make the arc. Make sure your plywood is tight in the center. Place the shingle mold at the length of 15 1/2" from the top and at the center pull it to the top of the two pieces of plywood and there is your arc. You may need to place another small nailat the center of the plywood if you are doing this by yourself.

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Originally Posted by Kstar4u

What I usually do is tack a panel of flat material (plywood or cardboard) over the space that the arch will be needed and temporarily bridge (with a straight piece of wood) the space between vertical lines that defines the arch's edge, at a point approximately halfway from the existing bottom of the opening and the existing top of the opening... level with the floor. Then I provide a pin (nail) at the middle of the vertical board to which I tie a piece of string. The string should be as long as the distance from the pin to the top opening is. Next... I tie a pencil to the other end of the string and, with the string pulled taunt...I trace the arc from one side of the space (across the top) to a point opposite where I started from. this provides a template to be used for the actual construction of the arch. You can adjust the height of you "pin" to increase or decrease the circumference of the circle (hence the radius of the curve).

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Thank you for the response but what I am really attempting to do is find the actual formula
That would provide the answer every time this condition arises. I can use the
Plywood method and the string to layout a template but with as many arcs as I have to deal with, it will take forever. My on site carpenter has an autocad that can generate
Exactly what is required but I wanted this information for my own use. Thank you again for taking the time to assist... Richard

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Originally Posted by jsnbrd

you may not be able to lay out a true circle for this arc. You may need to use a little trial and error at this point. obviously you are building an arc so you are going to use plywood? I will use plywood in my explaination. lay two 4'/8' sheets of plywood side by side to create an area that is 4' by 16'. Cut one peice of plywood 3' 11" long or cut both peices 5' 11 1/2" if you prefer to center. This will give you an overall length equal to your opening 11' 11". Measure down the 4' sides 15 1/2" if you have some small nails drive them in at about 3/8" above these points this is the thickness of shingle mold. That would put the nails at about 15 1/8" from the top. If you have a peice of material that is flimsy, 'not rigid' a wood like shingle mold you can use this to make the arc. Make sure your plywood is tight in the center. place the shingle mold at the length of 15 1/2" from the top and at the center pull it to the top of the two peices of plywood and there is your arc. You may need to place another small nailat the center of the plywood if you are doing this by yourself.

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Thanks for your response but after working with this information, it does not appear to work as required. What I was really seeking was the formula to calculate the layout as I have a number of openings requiring an arc. My carpenter has an autocad program that will give us the porecise layout but there has to be a mathematical solution that can be used on other sizes. I am given all the interior radi but the major curves for the exterior were not provided. Thank you for your assistance... Richard

Gabel -- get yourself a jobber 5 calculator -- it will handle all these conversions for you. Mine is at work right now so I can not give you the radius at this time but I will try to remember to bring it home with me tomorrow. -- Savage [just noticed is 6 now]
Jobber 6 - Best Construction Calculator in the world..PERIOD! Jobber 6 does it all. Save time and $!

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Originally Posted by TheSavage

Gabel -- get your self a jobber 5 calculator -- it will handle all these conversions for you. Mine is at work right now so I can not give you the radius at this time but I will try to remember to bring it home with me tomorrow. -- Savage [just noticed is 6 now]
Jobber 6 - Best Construction Calculator in the world..PERIOD! Jobber 6 does it all. Save time and $!

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Thanks for the response... will try to locate a Jobber 6... I am traveling right now and 900 miles from home... what type store usually handles this calculator??

No stores that I know of -- you can get it though the link I posted -- Ive had mine about 5 years and use it all the time.
Look over this demo.
JOBBER 6 - The Ultimate Builder's Construction Calculator. The top of the line performance in adding dimensions, metric conversion, imperial conversion, calculating stairs roof truss design cubic feet concrete circles triangles area roof rake constru
-- Savage

your radius is 172 and 11/16 inches. [ [ r = (m² + ¼c²)/2m]]
now how did the post above not answer your question? Your question was where can I get it and I answered same.

Here I was sitting here with a old book and a pencil doing your work and you give me a bad rating -- sweet really sweet-

Have a good day -- Savage
note that = 14 feet 4 11/16 inches --