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How To Build an Octagon Deck

When designing an octagon deck, maintaining its squareness and levelness is of the utmost importance and should take up the majority of your attention. To begin, we suggest positioning the pre-notched posts at each corner at an angle of 22.5 degrees. This will serve as the beginning point. You are going to put twin flush beams in this space between each of these posts. Make sure that each beam is the same length and that it is the same distance from one side to the other.


You will have some wiggle room to make adjustments to the framing around the perimeter before you add any more framing. While you are working on the frame, you should use temporary supports to stabilize it. After ensuring that the perimeter of the deck is straight and level, the octagon frame can next be strengthened by installing joists using angled joist hangers. If you want to avoid interfering with the post beam notch connections, you should avoid inserting joists near the corner intersections of the building. Strength and structural stability of the octagon can be improved by incorporating decking and bracing into the design.

If you want to put a gazebo on top of the octagon, you need to make sure that the footings you use are strong enough to take the additional weight that will be caused by the roof, and that the gazebo is correctly linked to the octagon's frame.

Materials Needed:

  • 2x8x14’ Pressure Treated Lumber

  • 2x8x8’ Pressure Treated Lumber

  • 4x4x8 Pressure Treated Lumber

  • 1-1/4x6x8 Cedar Decking

  • Cement Mix

  • 10” x 8’ Sonotube

  • 6” x 4’ Sonotube

  • Sonotube Saddles

  • 4” Bolts, Nuts, Washers

  • Screws

  • Joist Hangers

  • Wall Anchors

  • Cabot Deck Coating

  • Wood Glue

Tools Needed:

  • Hammer

  • Clam Shell Shovel or Post Hole Auger

  • Cordless Drill

  • Hand Circular Saw

  • Shovel

  • Wheelbarrow

  • String line

  • Chalk


Steps to Build an Octagon Deck

Chalk out the Deck

This is a cantilevered deck with poured concrete piers measuring 12 feet in diameter. This section assumes that you wish to attach the deck to the home. Find a location 6 feet from the house's edge (plus an allowance for a ledger board), about centered on the entryway. Mark this location as the deck's central support column. Hammer a nail into it. Attach your string to this peg and draw a 6-foot-radius circle and a 4-foot-radius circle using chalk. Mark the locations of each of the eight support posts.


Attach a line to the right edge of the door and hammer a peg at the junction of this line and the 4-foot circle. Carefully rotate the string 45 degrees counterclockwise. Strike the second chalk line and hope that the subtended angle will cover the doorway. If so, drive a peg into the four-foot intersection. Adjust these two lines so that the 45-degree arc is centered on the door frame if it does not. After installing the first two pegs, continue around the circle until all post holes are pegged.


The Centre Post

At the center post, drill a hole to a depth below the frost line. That indicates a minimum of 42 inches in my county. Use a 10-inch length of Sonotubing in this hole. Before pouring the concrete, indicate the height of the pour by marking the perimeter of the Sonotube at a place below the level of your home's entryway that corresponds to the joist width and deck board thickness. At this height, cut the Sonotube and pour concrete to the top. Incorporate a 4-inch carriage bolt in the middle. I employed a 3/4-inch bolt. You may choose to use rebar, but I did not.


Now, drill a hole for each of the eight outside posts using a 6-inch Sonotube below the frost line and approximately 3 inches above the soil surface. Now, refrain from pouring until you've reviewed the next phase, in which you'll embed saddles in the eight posts.


Aligning the Post Saddles


Refer to the illustration. When embedding the saddles into the eight outermost post pours, align the saddles in posts 1 and 3 with the rectangle depicted in the first diagram. With posts 5 and 7 following the same rectangle, go to the subsequent station. These are the odd-numbered posts. Using the second diagram as a guide, the EVEN-numbered posts are poured and the saddles realigned. Align saddles on posts 2 and 8, then posts 4 and 6. The reason becomes evident in the next two phases.


Starting Deck Foundation


Using 3 inch deck or brass screws, secure a pressure-treated 4" x 4" upright to each of the eight saddles once the concrete has hardened. The tops of these uprights are marked and cut to be level with the center post. Screw pressure-treated planks flush with the top of each upright to the uprights to form a rectangle.


Finish Deck Foundation


As in the preceding phase, the ODD numbered posts are framed to form a rectangle that intersects with the rectangle formed by the EVEN numbered posts. Follow the illustration, then attach the pressure-treated lumber and double the rectangles. We now have an eight-sided polygon as the deck's base. Each of the polygon's Points or Vertices will support a deck joist. (It does not matter whether you begin the rectangle with even or odd numbered posts, as long as you end up with an 8-sided polygon.)


Detail of Joists on Centre Post


This is a detail to show the joists mounted on the center post. They should be pushed together, as we proceed.








Some Arithmetic


The deck's circumference is formed by the end joists. This image provides information regarding the length calculation of each end joist. Sincerely, my recommendation is to cut one end of the end joist at 22.5 degrees, then measure twice and cut the second end at the same angle! The true technique here is to maintain a 45-degree subtended angle. This calculation is predicated on a line with zero width; the results are off by at least the joist thickness. How many planks with identical thickness have you ever observed? Consider the calculation just as an approximation.


End Joists All in Place


This is a photograph taken from the kitchen door following the installation of all end joists. Careful inspection reveals that the joist ends on the central post are still free-floating. At this point, you can rotate the deck joists to align the deck with the entryway of the house. When you are satisfied with the orientation, attach each primary joist to the respective vertices using screws. At this stage, I slanted the ground beneath the deck and spread several inches of gravel across it.


Connecting to the House


I utilized two ledger boards to accommodate two vent pipes and to provide a platform for connection beyond the door lintel. Observe that I have put the initial few CROSS joints. There is much still to come. Finally, now is a good opportunity to secure the joists' center post ends.


Clamping the Main Joists to the Center Post


The 4-inch carriage bolt embedded in the center post is obviously too short to reach the top of the joists, so I inverted a second bolt and joined both bolts together, allowing me to clamp the joists. Obviously, I began by attaching a second nut and a huge washer to the second bolt beforehand.



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