How to Build a Block Arch

Outlining the Blocks and Centering Form

• 1

  Mark the center point of one of the psf boards by measuring the two sides of the board and finding the mid point. Connect the midpoints of the two 4- foot sides and two 8-foot sides to find the center of the board.

• 2

  Anchor the base point (part of the compass that does not write) of your drawing compass on this center point and draw a circle with a diameter of two feet (radius of one foot) by swinging the drawing arm of the compass. You can measure the exact radius of the circle by marking a point that is one foot away from the center point.

• 3

  Anchor the base point at the mid point of the psf board again and draw a circle whose diameter is 2 feet and 8 inches (radius of 1 foot and 4 inches). You will now have two circles.

• 4

  Draw a line that cuts both circles exactly in half by referencing two midpoints of the two sides of the psf board indicated in Step 1. Either connect the two midpoints of the two 4-foot sides, or connect the two midpoints of the two 8- foot sides.

• 5

  Mark points that are 4 inches apart with your marker along the outermost circle. The first point should be at the same point as the line that was drawn in Step 4. These can be measured by using your drawing compass.

• 6

  Draw lines from the points along the outermost circle to the midpoint of the circle using your marker and straight ruler. It is imperative that these lines are exactly straight so that the arch will function properly.

Cutting

• 7

  Cut both circles and the entire board exactly in half along the line that you have already drawn using your hot wire cutter. Place one half of the board away from you as you will not be using this for the rest of the project. However, you can use it to make another arch of the exact same size and proportions as the one you are about to construct because it has the same lines drawn on it.

• 8

  Cut along the outside of the outermost circle using your hot wire cutter. You should now be left with a large half circle with the lines you have drawn on it. The excess board can be discarded or saved for other projects.

• 9

  Cut along the outside of the innermost semicircle using your hot wire cutter. Be careful and take your time as this cut is crucial to the structural efficiency of the arch. You should now be left with a thin arch that is divided into seven pieces by the lines you have drawn on them, and the semicircle that you just cut out. This semicircle will be the centering form of your arch.

• 10

  Cut along the lines of the thin arch that divide it into separate pieces using your wire cutter. These separate pieces that you have just cut will be the blocks of the arch. Make sure to keep track of the order of the blocks, as this will make assembling the final product much easier.

Assembling the Arch

• 11

  Stand the flat side of the psf semicircle up on a flat surface with plenty of room to work. A good place for assembling this is a work bench that is at least 6 feet long.

• 12

  Reassemble the blocks in the exact same order that you cut them, placing them along the standing psf semi circle. Start with the two outermost blocks and work your way up until you place the final block at the top of the arch. This block must be added last because it acts as a wedge and is called the keystone.

• 13

  Place the two bricks on either side of the arch blocks that touch the workbench (the outermost blocks are the only ones that should be touching the workbench). The bricks should be just touching the edge of the blocks, not pushing or wedging them in any way. The bricks will provide support for the thrust created by the arch.

• 14

  Remove the semicircle very carefully from under the blocks. Try to nudge the semicircle out very carefully by tapping it. The blocks now form a block arch without any adhesive between them. The blocks can be reassembled into this form over and over again by using the semicircle as a centerform and the two bricks as thrust support.