Part 1: Simple radius arch
Points of reference:
- Height: top of arch above floor. All arches in a house will typically share this number. The height will be two inches below the header.
- Rise: distance from height down to where the curve begins. For a doorway that is 36 inches wide, the rise will be 18 inches and the run will be 36 inches.
- Spring line: where the rise hits the outside edge. Where the curve becomes straight.
- Run: width or arch.
How to do it:
- Mark the center of the opening
- Draw a plumb line up to the height Mark.
- Measure down 18 inches from the height Mark. This Mark will be the pivot point for drawing the radius.
- Partially set a drywall screw into the pivot point
- Use a piece of wire bend to the correct length to draw the radius beginning at the top and swinging the pencil left and right.
- Draw a level line at the bottom of the arch — the spring line.
That is how to draw the simplest—and least common—arch.
Why is the easiest arch to draw the least used? Because it is hard to make a radius arch look like the other arches in the room if the widths are not all the same. Different spring lines in the same room look like after-thoughts.
A half-circle arch closes off a substantial amount of walking space, causing headache potential.
Part 2: Drawing a segmented arch:
A segmented arch is one that uses a portion of the radius from a larger circle. Segmented arches allow you to keep height and spring line consistent. In this example, the height will be the same as the radius arch was — two inches below the header.
- Mark height
- Determine where the spring line will be (figure the rise). Myron chooses seven inches.
- Determine what the radius of the segment of the circle will be.
One way is with a construction calculator:
- Radius = 26-3/4 inches.
Another way is to use the formula:
- Measure down from the height 26-3/4 inches
- Partially drive a drywall screw
- Prebend a piece of wire at 26-3/4 in
- Swing a line left and right from the center until the line intersects the spring line.
- This allows you to keep a consistent spring line height with slightly different radii
- This also provides a definite point of transition from curve to straight.
Draw an ellipse:
Step by step:
- Measure to find the center
- Plumb up
- Mark height
- Mark the rise (7 inches again)
- Draw the level spring line
- Get a piece of string with a knot at either end, 36 inches apart (or whatever the run is).
- On an elliptical arch, this is called the major axis.
- Half of 36 is 18 and that is called the minor axis
—Myron Ferguson, aka That Drywall Guy, points out on his website that over 80% of the visible interior of a home is covered with drywall, and "The bitterness of poor quality remains long after the sweetness of low price is forgotten."