The most important thing to be concerned with while building an octagon deck is to keep it level and square. We recommend to start by setting the prenotched posts at each corner positioned at a 22.5 degree angle. Between each of these posts you will install double flush beams. Make sure each beam is the same length and equidistant to the opposite side.
The perimeter framing will be flexible for you to adjust before adding additional framing. Use temporary supports to stabilize the frame while working. Once the deck perimeter is square and level install joists using angled joist hangers to strengthen the octagon frame. Avoid installing joists to the corner intersections to prevent interfering with the post beam notch connections. Adding decking and bracing will improve the strength and structural stability of the octagon.
If you are planning on installing a gazebo on top of the octagon make sure your footings are sized to support the extra weight of the roof and that the gazebo is properly attached to the frame.