The architects I work with love to design arches–sometimes even serpentine soffits that roll like ceiling-mounted ocean waves–but they don't spend much time on layout, leaving us to figure them out on site. Fortunately, not every arch we build requires the math department at Columbia University to plot.
The biggest arc I ever swung was for a 100-foot-long soffit we built in a New York City restaurant. At 6 a.m. we blocked the sidewalk in front of Madison Square Garden, pinned the end of a steel tape at the arc's center point, then scribed it onto plywood–265 feet away from the center point.
That was a long way from the first arches I ever built, and it showed how experience and a few tricks can add up to an easier job of swinging arcs for radius work.
We use two types of arches the most: a segmented arch (labeled this because it is a segment of a full circle), and an ellipse, which is basically a squashed arch (picture an oval or an egg cut in half). Segmented arch layout is straightforward, but ellipses are trickier because they're technically a circle with two diameters at right angles to one another.
While some architects use computer-aided design (CAD) programs to lay out ellipses for us–and print the layout with a huge printer called a plotter–we're often called to lay out a graceful arch on the spot. Part of the challenge is that the "right" arch is determined as much by eye as by geometry. Getting the curve to be graceful–not too high and not too squat–is tough.
Framing. The framing is basic and should be obvious from the plans. Just be sure that the header is high enough to clear the arch's peak. If the header is well above the arch's peak, you can bring the framing closer to the arch by inserting diagonal bracing as backers for sheet goods.
Segmented Arches. The key to laying out a segmented arch is to use a radius that's equal to the width of the opening.
Here's how we'd lay out a 4-foot-wide opening:
- On a sheet of MDF or hardboard, we snap out the framed opening.
- From the proposed maximum height of the opening, we pull 4 feet down to the center of the opening. (See "Swinging a Segmented Arch.")
- Using that mark as the pivot point, we use a trammel and pencil to swing the arc on the plywood.
- This creates a shallow arc that does not encroach on shoulder width for hallways, passageways, etc. The formula works for different openings (e.g., use a 6-foot radius for a 6-foot opening).
- For a 3-foot-wide opening, snap a center line at 24 inches down the length of a 4x8-foot sheet of 3/4-inch MDF (CDX is too hard to draw on). Call this the Y axis. (See "Swinging an Ellipse.")
- Snap a line across the sheet, 20 inches down from the top, forming the X axis.
- From the intersection of these lines, place a mark 8 inches out on each axis. (If the marks were compass headings, the top of the sheet would be north and you'd have south at the bottom and east and west sides.)
- Next, make marks at 0, 11-1/2, and 21 inches on your trammel. Zero and 11-1/2 will be the pivot points you'll place over the 8-inch marks on your layout lines on the MDF; 21 is where you drill a hole and insert a pencil.
- Place the 0 mark on the trammel over the north and south 8-inch marks on the Y axis, and then swing those arcs using a pencil in the 21-inch hole. Place the 11-1/2-inch mark over the east and west points on the X axis and swing those arcs. The result is an ellipse.
- Once the ellipse is marked, cut carefully along the X axis and the ellipse itself to form your pattern. The X axis is the bottom of the ellipse–and represents your level line.
- Mark and cut out the Sheetrock or plywood.
- Hang on to the templates for future use.
Elliptical Arches. The best way I've found to lay out an ellipse is to swing four intersecting arcs (i.e. curved lines).
You may not get the exact ellipse you're looking for at first, so be prepared to spend a little time. Remember that geometry you took in high school? Here it is in the real world.
First frame the opening, then hang either Sheetrock or plywood, covering the opening's entire upper portion on both sides. Reinforce any joints in the sheet material temporarily with plywood scraps leaving what amounts to a blank canvas on which you can draw any radius, arc, ellipse, or other shape.
I make level and center line reference points on the installed sheet goods so I know exactly where to apply the template, then transfer the layout to the opposite side. A rotary laser is good for transferring the level line from the front of one sheet to the back of the next–so the soffit of the opening is dead level. It's critical that the layout be exactly the same on both sides so that the arch is symmetrical and square. What I really like about this method is that the architect can see the arch before I cut it out, if need be. This provides an inexpensive way to make changes on the fly.
Cut along the marks. Insert blocking or flexible steel track between the cut-out sheets. Finish the arch bottom by covering it with thin or flexible sheet material, like 1/4-inch luan, wiggle-board, flexible Sheetrock, or scored and wetted-down Sheetrock. Finish as required.
–Erik Elwell owns Thompson Construction, a high-end commercial and residential remodeling firm in New York City.