How to Lay Out Arcs in Construction

Place a mark with a carpenter's pencil at the starting location of the arc. Make a second pencil mark at the finish point of the arc.

Secure one eight penny nail at each marked location by driving it in the material with a hammer. Leave the head of each nail half an inch above the surface of the material.

Tie a loop on one end of a string line. Hook the loop on one installed nail. Pull the line to the other installed nail. Mark the location on the string line with a permanent marker where it crosses the second nail. Tie a second loop at the marked area on the string line.

Slide the carpenter's pencil in the loop you tied at the distance between the two nails. Hold the pencil upright with the pencil lead resting on the material. Slide the pencil across the material to make a long arc. Move the loop sitting on the first 8d nail to the second 8d nail. Make a second arc with the carpenter's pencil. Drive a third 8d nail in the material at the point the two draw arcs intersect.

Move the loop hooked on the second 8d nail to the third 8d nail sitting at the intersection of the two drawn arcs. Pull the line to align the pencil with the first installed 8d nail. Move the pencil across the material -- with the string tight -- to the second installed 8d nail to complete the layout of an arc.