The Roman arch is probably one of the most efficient uses of architecture even today. The engineering of the arch is one of weight distribution, allowing the structure to withstand tremendous pressure, relying on its very design for its strength. Even though we don't need to rely on the Roman arch as much as they did in Rome hundreds of years ago, there's still plenty examples of Roman arch designs in use today. In modern homes the addition of a Roman arch to the exterior can make it look attractive. Using salvage bricks with plywood backing, you can get that look for your own home.
Lay the plywood out flat on the ground. Measure 24 inches across the top and make a mark. Measure up 36 inches on both sides and make marks. Start by laying one brick on each corner of the bottom of the plywood standing on its edge, the end of the brick flush with the outside of the plywood. Measuring across from brick to brick, it should be approximately 32 inches. Continue stacking the bricks horizontally along the edge of the plywood until you reach the mark at 36 inches, approximately 16 bricks. This is the point where the arch begins to angle inward.
On approximately the 16th brick, or 36 inches, place a ¼ inch shim between the 16 and 17th bricks, on the outside edge only, with the inside edge of the bricks touching. This gives the 17th brick a slight angle inward. Place another wedge on the outside of the 17th brick, sandwich that wedge with another brick, then another wedge, and so on. After several bricks the arch pattern will begin to emerge. Because of the irregularity of salvage bricks, you may have to use a combination of shims to get the arch symmetrical. When the piers of the arch have angled together within 6 inches of each other, stop. Adjust the arch if you need to to get it centred, using the mark at 24 inches for reference.
Observe the space between the bricks on top. This is where the keystone will join the two sides of the arch together. Draw out a rough diagram of the keystone space. Cut it out and fit it into the space; if it's not perfect, cut and trim until it's close. Take this template to a home improvement store and purchase a keystone as close to the template as possible from the masonry department. The keystone is forgiving and as long as it's close, you can adjust the sides of the arch to fit it all together.
Drop the keystone into the designated space on your arch and gently nudge and adjust the arch until all the joints touch, are tight and look even. Add or remove shims where needed to get the arch symmetrical. Mix the mortar compound with water in a wheelbarrow and trowel it into the cracks all along the edges. Pull out the wedges and finish filling all the cracks with mortar. Wait 24 hours, then stand the arch up and move it into its location, plywood and all. Then slide the plywood out.
Once you begin laying out the bricks on the plywood, the design will become apparent. Don't be afraid to experiment with shims and bricks. Look at pictures of ancient arches, and you will notice how exceptions have been made to get arches to work.