How to Build an Octagonal Deck




About: This picture is how my daughter painted me; old and grumpy. She and the other kids think this picture is funny. My wife agrees

This is a 12 foot Deck I built in 2005. It has gone through 6 Canadian winters and one restain of the deck surface. This instructable will cover each step with pictures and the Bill of Materials. The costs shown are from 2005, with the total about $1500 Canadian, not counting tax, the cost of the tent, or maintenance. 

Step 1: Materials Required

Tools required are:
Hammer, 4 ft Level
Clam shell shovel or post hole auger
Powered hand drill (I used a 1/2" hammer drill.)
Hand Circular saw (mine is a 7-1/4".
Showel and Wheelbarrow
String and chalk

Your skill set is that of a man or woman handy Person
(ie fearless and willing to make mistakes, Measure twice-cut once person)

Step 2: Chalk Out the Deck

This is a 12 ft diameter cantelevered deck, with poured concrete piers. what follows assumes you will want to attach the deck to the house.  Find a spot 6 feet from the edge of your house (plus an allowance for a ledger board) more or less centred on the house entrance. Mark this spot as the central support column of the deck. Drive a peg. Attach your string to this peg and, draw a chalk circle of radius 6 feet and a second circle of 4 feet. Start marking the position of each of the 8 support post. String a line to the right edge of your door and driving a peg at the intersection of this line and the 4 foot circle. Carefully counterclockwise rotate the string 45 degrees. Strike this second chalk line and hope the subtended angle covers the door entrance. If it does, drive a peg at the 4 foot intersect. If it doesn't, fiddle with these two lines so that the45 degree arc is centred on the door frame. After these first 2 pegs are in, progess around the circle until all post holes are pegged. 

Step 3: The Centre Post

At the centre post peg, dig a hole to below frost level. In my county that means a minimum of 42 inches. Use a length of 10 inch Sona tube in this hole. Before pouring the concrete, mark the height of the pour by marking the periphery of the Sona tube at a point a joist width and a deck plank thickness, below the level of your house doorway. Cut the Sona tube at this height, mix the concrete and pour to the top. Imbed a  4 inch carriage bolt in the centre. I used a 3/4 inch bolt. I didn't use rebar, you may wish to.
Now dig a hole for each of the 8 outer posts using a 6 inch Sona tube, below frost level and  about 3 inches above ground.
Now don't pour until you review the next step, when you will imbed saddles in the 8 posts.

Step 4: Aligning the Post Saddles

Refer to the diagram in step 2. When imbedding the saddles in the outer 8 post pours, imbed the saddles in post 1 and post 3 to align with the rectangle shown in Step 2. Continue next with post 5 and 7 following the same rectangle.  I call these the ODD numbered posts. The EVEN numbered posts are poured and the saddles aligned again using step 3 as a guide. Align post 2 and post 8 saddles, followed by post 4 and post 6. Why becomes clear in the next two steps.

Step 5: Starting Deck Foundation

With the concrete firm, use 3 inch deck or brass screws to fasten a pressure treated 4"x4" upright in each of the 8 saddles. These uprights are marked and cut so that the top of each is level with the centre post. Screw pressure treated planks to the uprights to form a rectangle flush with the top of each upright.

Step 6: Finish Deck Foundation

As in Step 5 the ODD numbered posts are framed to create a rectangle that intersects the rectangle of the EVEN posts. Follow the image, screw the pressure treated lumber in place and then double up the rectangles. What we have now is an 8 pointed polygon which is the deck foundation. Each Point or Vertex of the polygon will support a deck joist. (If you start the rectangle with the ODD numbered posts, it doesn't matter, as long as you end up with the 8 pointed polygon.)

Step 7: Deck Joist

There are 8 main deck joists, each crossing a vertices and resting on the centre post, giving us a 2 foot cantilever to the deck. Each joist is 6 feet long and sits at angle of 45 degrees from adjacent vertices.
Note that the joists pushed together at the centre post and as shown in the next step, four of the joists has been cut at a 45 degree angle to assure a tight fit. The joists will eventually be held down with the bolt in the centre post.

Step 8: Detail of Joists on Centre Post

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

Step 9: Some Arithmetic

The circumference of the deck is formed by the end joists. This picture gives some detail about the calculation of the length of each end joist. Frankly, my advise is to cut one end of the end joist at 22.5 degrees; measure twice, then cut the second end at 22.5 degrees! The real secret here  is to keep the subtended angle at 45 degrees. This calculation is based on a zero width line; at the very least the results are out by the joist thickness. How many planks have you ever seen that have exactly the same thickness? Take the calculation as an approximation only. 

Step 10: End Joists All in Place

This is a picture from the kitchen door, after all the end joists have been installed. Look carefully and you see the joist ends on the centre post are still floating free. At this point you can rotate the deck joisting to adjust the deck orientation to the house entrance. When are satisfied that the orientation is O.K. screw each main joist to the correspong vertice. At this point I sloped the ground under the deck and covered with several incehes of gravel.

Step 11: Connecting to the House

I used 2 ledger boards here to accomodate two vent pipes and to assure that I had a attachment surface beyond the entrance lintel.
Notice I have installed the first  couple of CROSS joisting. There are a lot to come.
Last, this is a good time to bolt down the centre post end of the joists

Step 12: Clamping the Main Joists to the Centre Post

The 4 inch Carriage bolt imbedded in the centre post is clearly too short to extend to the top of the joists, so I up-ended a second bolt and connected both bolts together, giving me a way to clamp the joists down. Of course I started by screwing a second nut and a large diameter washer to that second bolt, before hand. This is a temporary solution, which we will alter when finishing the decking. 

Step 13: The Cross Joists

The Cross joists are perpendicular to the end joists are are set at 16 inch centres. Notice that the joists are clamped at the centre. This matrix of Joists, End Joists and Cross joints is extremely strong. Can you see the last one I was about to install, which ends the task of installing the joists for the deck? all joists are secured with deck screws.

Step 14: Installing the Cedar Deck

Start at the outer edge of the deck and work inwardly to the centre. The first row of decking overhangs the end joists by 1-1/2 inches.
Each piece of deck planking is fastened between adjacent main joists. Use deck screws, alternately driving the screws either to front edge or bottom edge of the plank in a zig zag pattern. DO NOT screw down the ends of the planking at this time, keep the joints tight. Proceed around the periphery, with no spacing between planks. From time to time you will have to rip a board to keep the progression even. Keep the scrap, it will fit an inner circle. (Note upper left of picture!)

Step 15: The Centre Post Clamping Bolt

As you get closer to the centre of the deck, you will need to insert some scrap joisting to provide a surface to mount the decking. Additionally, since the deck planking is firmly holding the joisting together, we have the opportunity to modify the centre post clamping screw to asure that the final pieces of decking can be fastened in the centre, without interference.

Step 16: Clamping Bolt Clearance

To get the clamping screw and nut below the bottom of the deck surface, we need to chisel out the edges of the main joists around the clamping screw and nut to accomodate the big washer. In Fact, at this point I installed a second smaller washer between the clamping nut and the original large washer to reinforce the larger washer. Follow the image here and cut the deep enough to accomodate screw and washer(s) and wide enough to JUST accept the largest washer. Mark the excess bolt length above the clamping nut, disassemble the nut and washer structure and then hacksaw the bolt to length. You will probably have to try this a couple of times to get it right. 

Step 17: One of Several Mods

The connection of the Octagon deck to the house, required a small alteration in the plan. Keep the octagon shape, but deck the space between the wall and the deck and make it obvious. Here is the framing between the deck and house.

Step 18: Between House and Deck

Here is the decking between the octagon and the house wall.

Step 19: Decking Complete...Sort Of.

The deck looks great but my skills with the saw left rather wavy lines along the planking over the main joists. So my answer was to cover the joints with an inlay of cedar decking. Two reasons for this: 
  1. Wavy lines
  2. Inlays clearly define the triangular segments of the octagon

Step 20: Second Mod.. Inlays

To do the inlay,
-Rip a 6 foot piece of cedar decking about 3/8 inches thick.
-Using the 7-1/4 inch saw to cut the channel straddling the saw joins along the main joists to about 1/4 inch depth.
-Fasten all joints in the channel with a deck screw and washer
-Use contractors glue (waterproof) to glue the ripped strips into the channel.
-Use a hand plane to bring the level of the inlay to the deck surface. Sanding takes too much time.

(Sounds like "all ya gotta do is blah blah blah")

Let's start

Step 21: The Power Saw As a Router

Using the ripped piece of decking mentioned above, straddle the saw join with the cedar strip so that it is centred above the join. Draw a line along the right edge of the strip. See red dotted line in the diagram.

Measure your power saw edge guide. What is the measurement to the saw tooth closest to the edge guide? It should typically be about 1-1/4 inch. See yellow rectangle in the diagram.

Use a piece of scrap wood no thicker than 1/4 inch and tack the scrap wood onto the deck to the right of the drawn line a distance equal to the measurement of the guide to saw blade, and parallel to that line. See the brown rectangle in the diagram.

Make a cut from the vertice of the join to the centre of the deck. That's 1/8". put the cedar strip between the scrap wood guide and the saw. Make another cut. That's a total of 1/2" channel. You got it! proceed until you get close to the line on the left of the saw join. Insert the strip, or make another cut. BE CAREFULL. MEASURE TWICE, CUT ONCE. PLEASE TRY ON A SCRAP OF WOOD BEFORE DOING THIS! When finished, clean out the uncut pieces with a wood chisel.

Step 22: Inlays Completed

This shows four inlays in place.
It doesn't require a genius to see the improvement of the design with the inlays. The lines at the main joists are dead straight, the triangular features of the octagon pop right out.
Be sure to review this whole procedure. You don't want to glue a strip down before screwing the joints down. In this picture (aside from my wife's geraniums) notice that I had not decided how to finish the centre. I enventually decided on a one piece cedar octagon.

Step 23: The Tent Mod

In our county, a hard roof on a gazebo or any deck changes all the rules. The setbacks for a deck move from 4 feet to 23 feet the minute your designs include a hard roof. The result is that I had to design for a tent roof. This design started with the intention of cedar rails and a semi-permanant roof. Aside from the cost the local engineering department said NO!
As it happens, my wife found a really great buy on an octagonal garden tent (about $575 Cdn.) The problem was that the outfit selling the tent had little or no documentation and were not sure of the size. They guessed 14 feet. Does that mean 7 foot edge to edge or 7 foot radius? We bought it and found that the radius is about 13 feet 6 inches. They must include the diameter of the support poles in the 14 feet. Regardless here is where the last design mod is described. 
I couldn't increase the current radius, but I could put in extensions to the vertices to support the tent poles. 
  Each post support is constructed of 4 pieces of wood. 
Two sides at 8 inches, cutting both ends at 22.5 degrees, and
two centre pieces at 4", again with 22.5 degree cut both ends.
Assemble as shown here, glue and screw and mount at each vetrice below deck level. Fasten deck planks following the pattern of the deck with a 1-1/2 inch overhang.

Step 24: The End

That's it guys. I am not going to talk about the stairs, we all have been there.
Any comments good or bad are appreciated. Suggestions are also welcome but I don't plan on another octagonal deck.
This is my first instructable and it has been interesting. 

A couple of things;
-I sanded and refinished the deck in 2009. I probably will do it again in 2012.
-The reason for pushing the deck planks tight together came from my local lumber yard person who noted that shrinkage will occur  regardless, but less with cedar than PT. His opinion has been vidicated since very little shrinkage has ocurred. My fence pressure treated fenceplanks have shrunk about 1/2 inch in width in the last 6 years. 
-The deck has been very sturdy, even when a full band with instruments has been playing on it, no shaking or bouncing. (good lattice.)



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    33 Discussions


    16 days ago on Introduction

    Hello from England. Just wanted to thank you for the instructions and send a pic to show that they worked out great!

    1 reply

    Reply 15 days ago

    MarkU33: The deck you have made looks super, congratulations on a fine job. Glad the instructable was useful to you.

    Rudy Puff

    Question 1 year ago

    Once you have attached one end joist, how do you connect the adjacent end joist?

    1 answer
    cobourgdaveRudy Puff

    Answer 1 year ago

    Hi Rudy, I am not sure what joist you are referring to. If you are referring to the end joists, they are shown in step 10 with the cut angles shown in step 9. If you are referring to the joists meeting in the center post, they are held down by the clamping screw and braced into a lattice with the cross members, as shown in step13.

    Regardless, if I don't quite get your question, please ask again.


    2 years ago

    Thank you so very much for sharing, me and my significant other are looking forward to build one ourselves.


    Reply 4 years ago on Introduction

    Hi, thanks for your comment. The cost was about 1/5 of the cost of getting someone else to do it, and besides, I'm cheap


    7 years ago on Step 11

    I never use a ledger board when I build decks or steps, this will trap moisture between the ledger board and the house. With a wood construction house this causes rot and with a masonry house like yours the trapped moisture is hard on the brickwork during freeze/thaw cycles. The deck you have built would be stable as a free standing structure with a small 1 - 2 inch gap between it and the house to prevent trapped moisture. People don't even notice the gap and it prevents some painful repairs 10 - 15 years down the road.

    5 replies

    Reply 7 years ago on Introduction

    Thanks for your warning. I will in fact check for damage this summer.
    Since the brick is very roughly textured, I am HOPING that sufficient air flow has taken place to prevent spalling on the brick. There is certainly not a 1" gap, more like 1/4".
    If damage has occured, or even shows a possibility, I can pull the ledger board
    and take your suggestion. The deck is very sturdy as a free standing structure, and since all parts are screwed together, I can disassemble the areas adjacent to the ledger board very readily for any changes required. It was built in 2005, so any incipient damage will be fairly obvious.
    Thanks again for your thoughtful feedback.


    Reply 7 years ago on Introduction

    The mortar between the bricks is most likely to suffer.

    I recently discovered an unexpected benefit to building decks as free standing structures. Someone was backing a truck up to a deck that I built several years ago and accidentally hit the deck. Instead of doing a bunch of damage to the house the deck just moved off the concrete weirs that it was on. I was called to fix it and all we did was come with a bunch of guys and just put it back!

    Most "professionals" will say I am wrong, that you should use flashing and caulking to keep the ledger board dry but omitting it is easier and more effective as caulking and flashing always fail on level surfaces like decks.

    you should check with local building codes.. where i live it is required by the county to have all decks or other such structures attached to the house WITH a ledger board and they have regulations as to the size of fastening screws attaching the ledger board as well...

    Beautiful pogoda btw... thank you for sharing.


    Reply 7 years ago on Introduction

    Your input definitely would have helped me about 7 years ago.. we noticed damage caused by moisture last year because of this. 10-15 years like you suggested may be average, but depending on the size and circumstances of the build, we can see negative effects as early as 6 years in my case.

    Otherwise WONDERFUL instructable, with detailed instructions and photos.


    Reply 7 years ago on Introduction

    Appreciate the comment, thank you. As 67spyder has noted, trapped moisture is a problem. Your comment of similar damage underlines that we can't just build and leave, we have to spend some time in checking for things like water damage, trapped moisture and so on. Thanks to 67spyders comments I will be spending time checking for this and other types of damage when I sand and restain the deck surface this coming summer.


    7 years ago on Introduction

    What a beautiful addition. I had to build a deck for my wife, but fortunately for me there was no complex math (mine is rectangular)

    Great Job

    1 reply

    Reply 7 years ago on Introduction

    Thank you for the comment. I am surprised by the number, but definitely appreciate the numbers of positive comments on my first instructable. You guys have really been supportive.


    Very, very nice. I might have considered doing something like this on my side if I didn't already have the cement deck to begin with.

    3 replies

    Could be. But after the work I did on the deck (see my instructable; I'm sure you can sympathize), I don't think I'm yet ready for another big project like that.

    Actually, I had looked at your project before, when I was browsing a large number of instructables to get an idea what was the type of project Instructables was looking for. When you mentioned it now, I went back for a second look. Indeed, you spent as much time, or more on your project than I did on mine. As you indicate, a lot of work. I have to congratulate you on your's, and would agree it is not yet time for another bigee.