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I design brick arches and cast stone using simple geometry and AutoCAD. We cut brick shapes and then prefabricate traditional masonry arches that stand the test of time. I want to share the mathematical formulas of arch building. by Justin Wethington

48" 4J O/S 1/4" joint 3/4 bond

By Justin Wethington, Apr 18 2016 01:50PM

We often get jobs that are either renovations or additions, and the architect and masonry contractor want the new brickwork to match the existing building. Brick arches built decades ago were made without autoCAD and designed with form following function. What we pay close attention to when we need to match an old brick arch are: structure, bond, skew, striking tool, and joint width. Here is picture a bricklayer sent us.

He said he needs 16 jack arches with a 48" M.O.(masonry opening) for an addition to a YMCA on the Eastern shore. The picture he sent gives us enough info to go ahead with the design. It looks like the building is flemish bond but the four course tall jack arch is 3/4 bond but closer to 1/2 bond. This is common in older buildings because bricklayers did not usually build jack arches this height with flemish bond, too many little pieces. They wanted to use the full length of the brick and still achieve 4 course height (12 1/2"). It also looks like the jack arch joint is tighter and slick joint and the rest of the building is grapevine joint and 3/8". We decide to design the jack arch with 1/4" joints, a 70 degree skew, and 3/4 bond.

We were sent oversize handmade brick from the bricklayer to build the jack arches. The length of the handmade brick is 8". When we took the 8" brick and stood it up as a stretcher, then leaned it 70 degrees, we figured the max height of our flemish bond stretcher in the jack arch to be 7". The old YMCA jack arch was probably made from brick longer than 8" because the 3/4 bond looks closer to a 1/2 bond. Since handmade is a long as these brick get nowadays without ordering special slugs, we proceed to match the bond as best we can. If the jack arch is 12 1/2" tall with 1/4" joint it leaves us with a 5 1/4" bat. We then designed the jack arch the same way we always do here at Archway System, with a georgian style, double struck consistent joint.

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