How to Build an Octagonal-Shaped Deck

The Base

• Whether you're building an uncovered deck or a large gazebo structure, you'll need to prepare the ground and the base of your structure to support its weight, plus the weight of several adults. If you can orient your gazebo on top of a concrete platform, you can simplify your task and focus on building sufficiently strong posts to support the deck. Typically, 4x4 posts of treated lumber are appropriate as support beams that can be set into concrete piers. Only use treated lumber for any wooden elements that touch the ground; otherwise, you risk rot problems.

Deck Framing

• Moving upward from the support posts, the next major element of your deck is the floor framing. To create the octagonal shape, you'll need eight pieces of dimensional lumber, ideally 2x6 boards, all cut to the same length. Each of the pieces needs to be cut at either end so they fit together at a 22.5 degree angle. The simplest way to construct joists for the floor frame puts them all parallel, spaced evenly and with their tops flush with the upper edges of the octagonal frame. Alternately, you can adopt a more complex orientation, in which the joists run outward from the center of the deck to the octagon's outer corners, like the spokes of a wheel. You can use either galvanized nails or deck screws to assemble the frame.

Floorboards

• Once you have constructed the base and the framing for your deck, it's time to build the floorboards. Whereas the framing can be assembled on the ground and then affixed to the base, it's simpler to mount the flooring directly onto the framing, one board at a time. You can select from two basic designs of flooring, depending on the type of framing you use. The simpler method is to orient all of the boards in one direction then cut their ends on a diagonal or straight across, as necessary, to fit the octagonal shape. If you use this method, orient the boards perpendicular to the joists. Alternately, you can use spoke-style framing by installing shorter floorboards that run parallel to each of the octagon's sides. This method breaks the overall design down into eight triangular-shaped sections.

Working with an Octagon

• When working with an octagon, you need to use the Pythagorean Theorem to relate the overall size to the length of an individual side. According to the theorem, the length of the longest side of a right triangle, squared, is equal to the sum of the two other sides, each squared. In other words, if you break down the octagon into smaller right triangles, you can add the appropriate lengths to find the overall size. In simplest terms, the distance between two opposite and parallel sides of an octagon is equal to the length of one side plus twice the length of a single side divided by the square root of two.