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).
Framing an Ellipse
The posts are 6x6s and the beams are 4x10s - all pressure-treated Douglas fir (Figure 2).
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.
When I installed the 2x8 joists, I let the ends run wild (Figure 4).
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.)
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).
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).