How to Build a Brick Arch

 

Tip: If your pillars are capped, remove the caps before building an arch. Chip out the old mortar from underneath using a hammer and joint chisel. With a helper nearby to support the cap, use a pry bar and shims to remove each cap from the pillar.

To determine brick spacing, start by centering a brick at the peak and placing a compass point at one edge. With the compass set to the width of one brick plus Vi", mark the form with the pencil.

Place the compass point on this new mark and make another mark along the curve. Continue making marks along the curve until less than a brick’s width remains.

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Divide this remaining width by the number of compass marks and increase the compass setting by this amount. Using a different color, make final reference marks to either side of the peak. Extend the pencil lines across the curved surface of the form and onto the far edge.

Cut two 2×8 braces V2" shorter than pillar height and prop one against each pillar with 2×4 cross braces. Place shims on top of each 2 x 8 to raise the form so its bottom is even with the tops of the pillars. Rest the plywood form on the braces.

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Mix mortar and trowel a narrow %" layer on top of one pillar. Place one brick, and then rap the top with a trowel handle to settle it. Butter the bottom of each subsequent brick and place it in position.

Place five bricks, and then tack a string to the center point of the form on each side and use the strings to check each brick’s alignment. Take care not to dislodge other bricks as you tap a brick into position.

 

To balance the weight on the form, switch to the other side. Continue alternating until space for one brick remains. Smooth previous joints with a jointing tool as they become firm.

Butter the center, or keystone, brick as accurately as possible and ease it into place. Smooth the remaining joints with a jointing tool.

Lay a bed of mortar over the first course and then lay the second course halfway up each side, maintaining the same mortar joint thickness as in the first layer. Some of the joints will be staggered, adding strength to the arch.

 

Dry-lay several more bricks on one side—using shims as substitutes for mortar joints—to check the amount of space remaining. Remove the shims and lay the final bricks with mortar, and then smooth the joints with a jointing tool.

Leave the form in place for a week, misting occasionally. Carefully remove the braces and form. Tuck-point and smooth the joints on the underside of the arch.

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