Here’s a way to lay out arches that need to have specific heights at the ends or center, even though the spans vary. First, snap a long chalkline on the floor (see drawing). Lay your material at one end, square to the line. After determining the span, end height and center height, put the end of tape measure #1 at the center height and stretch it out next to the chalkline. Hook tape measure #2 on a nail located at the end height and stretch it out at an angle so that it intersects tape #1. Now move tape #2 until the numbers on both tapes match at the chalkline. Put a nail in the chalkline at this point. This is our pivot point. Hook one of your tapes over it and use it as a giant compass to trace your arch. On huge arches you can make one half and use it as a pattern.
—Spencer Thompson, Santa Monica, CA
Edited and illustrated by Charles Miller
From Fine Homebuilding #76
