Stone and brick arches have traditionally been employed for structural support purposes but are used now more as decorative elements for windows, doors and indoor openings, because there are cheaper and easier methods of spanning gaps. Building an arch that will not collapse is a two-part process involving building a temporary support for the arch and placing the stones or bricks so they can be locked with a keystone, with the stress transferred to the side pillars and the ground. Multiple arches can be built to form an outdoor kiln or oven.
Build the Support
Measure the width of the opening at the highest point before the arch begins. This is called the springing line and will form the base of the wooden arch support form.
Transfer the measurement of the opening to the bottom edge of a plywood sheet.
Find and mark the exact center of the springing base line on the bottom edge of the plywood sheet. This is called the striking point. Tie a piece of string to a pencil and place the point of the pencil on one end of the springing base line. Keeping the string taut, place the other end on the center point and scribe a half circle from one end to the other end of the base line.
Draw a vertical line perpendicular to the springing base line from the center point, using an L-square and a straight edge. Mark the point where the perpendicular line bisects the half-circle arc. This is the position for the keystone, the most important stone or brick, which locks the arch into place. Starting with the keystone, use the arch's half circle to count how many bricks or stones will be needed to fill either side.
Cut out the half circle using a power jigsaw. Use the cutout shape as a template to cut a second similar shape. These will form the two sides of the wood support form.
Nail a piece of lumber on edge to the base of the half circle. Use 2-by-4 lumber if the bricks or stones will be placed soldier style, or upright, along the arch and 2-by-8 if the bricks will be placed sleeping, or horizontally, with the narrow end facing out. Nail similar pieces of lumber along the perpendicular line and two other pieces of lumber on either side of the perpendicular line to form a triangle. Nail the other half-circle plywood to the lumber.
Cut a strip of flexible hardboard and nail it to the curved edge of the plywood.
Building the Arch
Build two pillars on either side of the opening to a height that ends at the point where the arch begins. The pillars should be wide and deep enough to support the arch. Build the pillars wider rather than narrower.
Cut a horizontal length of lumber the width of the opening. This piece will go under the base of the form. Cut two lengths of lumber to go against the side pillars with the horizontal lumber resting on top of them. Lift and place the half-circle plywood form on top of this support frame. The frame will support the form while the bricks or stones of the arch are placed.
Place mortar like a wedge on top of one pillar with the narrow edge facing toward the inside of the arch. Place the first brick or stone on top of the mortar. The wedge shape of the mortar should align the brick so an imaginary line through the center of the brick would run right to the center striking point on the base of the arch form. Repeat on the other pillar.
Butter another brick in the same wedge shape and place it on top of the first brick. Repeat on the other side. Build the arch on both sides at the same time to keep it in balance. Continue until you reach the point where the keystone is to be placed. All the bricks or stones should be aligned with the center point.
Butter both edges of the keystone and wedge it into the space at the top of the arch. This will lock the bricks or stones of the arch into place and transfer the stress tension onto the pillars instead of the wood form. The keystone can be a bigger size for decorative purposes or it can be just one of the bricks or stones used to build the arch. A wedge-shaped stone like an upside-down triangle with the lower point lopped off works best as a keystone.
Allow the mortar to set for a day or two. At this time you can "point" the mortar, pressing and evening the lines of mortar, to give the arch definition between the bricks or stones. After the mortar has set, remove the support frame and the wood half-circle form.