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To make an arched fireplace opening make the top of the arch tangent with the bottom of the rounded Rumford throat.*
Then where the arch drops below the throat at each side** fill those pockets with mortar or plaster and feather into the throat so as to make a smooth, streamlined, compound curved throat out of our simple curved throat.
* The code requires that ...
"the throat is at least 12 inches (305 mm) above the lintel and is at least 1/20 the cross-sectional area of the fireplace opening."
The code refers to the narrowest top of the throat - not the whole throat component. The problem is one of semantics - we call the whole rounded throat component the "throat" but the code means the narrowest part or top of our throat component - the part that is "at least 1/20 the cross-sectional area of the fireplace opening." - Jim
** Called the "voussoir" as John Walkup in North Carolina pointed out.
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Buckley Rumford Fireplaces
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