Building an Elliptical Deck

When I was first approached by a couple who wanted an elliptical deck built off the back of their house, I was intrigued. I had built my fair share of decks over the years, but never one without any straight lines. We sketched out the basic dimensions and decided to add a smaller oval two steps down off the front for a viewing area (their backyard is a highway for wild turkey and deer). It took me about five weeks to complete the project, working alone. (See also Day's End, November 2010.)

From Plans to Footing Layout

The homeowners wanted the upper deck to extend 12 feet from the back door. They also didn't want the house to perfectly bisect the deck along the long axis of the ellipse; rather, they wanted the ellipse's curve to reach its apex and start to return before terminating into the house. Guided by these preferences, I drew up the design on paper.

Using my scale drawing and an architect's scale, I figured out the width of the deck every 2 feet along the ledger (on a computer, most design programs can do the same thing). On the site, I measured the appropriate width out from the house every 2 feet, marked the locations on the ground, and then connected the dots with marking paint to get the rough perimeter of the deck.

Once I knew where the perimeter would be, I laid out the beam locations on the ground, allowing for a maximum joist cantilever of 2 feet. Working from the beam locations, I found the post and footing locations (Figure 1).

Figure 1. The footings were located to provide a maximum two-foot joist overhang.

Framing an Ellipse

The posts are 6x6s and the beams are 4x10s - all pressure-treated Douglas fir (Figure 2).

Figure 2. Notched posts supporting beams are a familiar detail throughout the country, but the solid 4x10 beams used here aren't often seen in eastern states. The posts and beams differ in color because they were treated with different preservatives.

The color difference visible in the photo is due to the treatment chemicals - 6-by and larger material is still treated with CCA (green) in Oregon, whereas 4-by and smaller wood gets the newer ACQ (brown) treatment. To keep water off, I flash the tops of beams and posts with a combination of flashing tape and aluminum L-flashing (Figure 3). Because treated lumber is corrosive, particularly in our damp northwest climate, all hardware gets sprayed with a layer of Plasti-Dip where it directly contacts the lumber.

Figure 3. A combination of peel-and-stick membrane and aluminum flashing keeps water out of vulnerable areas.

When I installed the 2x8 joists, I let the ends run wild (Figure 4).

Figure 4. The joists were laid out on 16-inch centers. Their lengths were allowed to run wild; later they would be cut to fit the perimeter of the ellipse.

I laid out the ellipse with the pins-and-string, or gardener's ellipse, method (see illustrations), drawing its perimeter on the tops of the joists. Using a level, I drew plumb lines down from the perimeter marks, on both sides of each joist. Because of the curve, every joist had to be cut at a different bevel, so I set my circular saw accordingly. (The offcuts became blocking for the joist bays, reducing waste.)

Screws anchor the ends of the cable at the ellipse's foci.

Laying Out an Ellipse

If you know the lengths of its major axis and minor axis, an ellipse can be laid out using a string and a pencil. This method involves no math, though it revolves around a geometric construction. The instructions I found online at mathopenref.com/constellipse1.html suggested using string for the layout, but my string was too stretchy and allowed the pencil to wander off the ellipse's path. Instead, I substituted 1/8-inch steel cable. - M.M.

1. Lay out the minor axis so it bisects and is square to the major axis. Measure to find half the length of the major axis.

2. From the apex of the minor axis, mark the two foci on the major axis with arcs whose radii equal the length from (1).

3. Put loops in the ends of a cable so its length equals that of the major axis.

4. Anchor the cable ends with nails or screws at the two foci.

5. Starting at one end of the major axis, use a pencil or marker with the cable to lay out the ellipse.

To minimize the chance of rot taking hold, I paint all cut ends with a copper-napthenate preservative. This is particularly important with treated lumber in the West, as the Douglas fir most of our framing lumber is milled from doesn't accept pressure treatment as well as the southern pine used in the East.

The curve of the ellipse at the ends of the major axis extends past the end joists. Framing to support the decking and railing at these points was assembled from a couple of 2x10s that I blocked, screwed, and glued to the side of the outermost joist (Figure 5).

Figure 5. An assembly of blocking and 2x10s on the flat creates the substructure to support the decking at the ends of the ellipse's major axis.

I laid out the assemblies when I laid out the rest of the ellipse - by temporarily screwing the 2x10s to the outer joists, I was able to mark the upper pieces, which then served as templates for the lower pieces. Blocking between the ends of the joists completed the framing (Figure 6).