How to Build an Octagonal Deck

here are several ways to design and build an octagonal deck.

We outline the simplest way in this article, with more advanced designs and octagonal deck construction articles to follow.

Please read the whole article before starting - and also again during your session of building your own octagon deck.

The most important thing to realize - and remember - about building an octagonal deck, is the fact that it is basically a rectangle with the corners cut off. Remember that, and base you design on it, and you should end up with a well constructed octagonal deck.

The octagon can either be with regular sized sides, or having sides of different dimensions. The two basic shapes being either based upon a square or an oblong. Whichever of these tow basic shapes you decide upon, it is essential to get the rectangle 'square' and with precise dimensions to ensure this.

Study the images below, and read the text so that you are absolutely familiar with all you have to do.

Image shows basic framework in first stage of building an octagon deck

Build the rectangle frame as in the image to the left. Make sure that the outside dimensions of the deck frame - from corner to corner - are exactly the same. It is a good idea to have the dimensions of your frame in multiples of the deck board width. For instance, with 150mm (6in) deck boards, the frame should be built to match. A 6 metre long deck for instance, will match the overall width of 40 deck boards. Just a little detail, but it will make your octagonal deck look more professional.

Ensure that the deck frame is 'square' then put a few strips of timber diagonally across the joists and screw into place to hold the frame 'square'. Make sure that your deck frame is on a level surface to prevent movement. A few packed up 6x2 joists will help under the frame.

If you are unsure about building a deck frame, there are many pages of resources on this site to help you. The basic knowledge of building a deck frame is important before you attempt to build an octagonal deck.

Image showing how the basic octagon deck sub base i worked out

Once you are happy that you have a well formed rectangle, you can decide how far you want your corners to be cut off to form your octagon. It need not be a regular octagon. In fact, unless it is a large deck, it is best to have the corners cut back just a little. This will give the same effect, but allow more space for you to enjoy your deck.

There is plenty of scope for experimenting at this stage. You simply lay the corner frame timbers across the top of the octagon until you get a pleasing dimensions that firstly make your deck look good, and secondly, allows you space you require - without falling off the edge of the deck, when you start to enjoy the fruits of your hard work.

You can either form your angles by using a 'square' that will allow you to draw a 45deg line, or better (safer) still, simply mark back from the corner an equal distance along the two edges. You can then use the same dimensions on the other corners.

IMPORTANT When you make your pencil mark for the cut, don't forget to lay your bit of joist timber across and then make your pencil mark on the INSIDE edge of the joist - not the outside. This way, you will end up with your octagonal deck the size you want. If you make your mark along the outside of your angle joists, then your angle will be the width of the joist plus extra for the angle.

Make sure that your deck is braced exactly 'square before you make any measurements or cutting marks!

The actual saw cut for the angles of your corners will probably be the most difficult task - especially if you are not well used to using a hand saw. Go and buy a new saw to do the job. A hardpoint saw costs little This image shows how the cuts are made to the sub deck frame of the octagon deckand makes the job much easier. These cuts will determine how successful your deck will be.

After you have marked your angles, then also use a set square to make your cut marks down through the joists. Make sure that you get your eye 'over' the set square, and not make these marks by looking from the side.

At this point, and before you even think of making a cut, make sure again that your rectangle is braced as before, and also ensure that each of your angle-cut joists will also be braced into position.

Just cut one corner at a time - fixing your angled joist after each one. Screws would be the ideal fixing - and make sure that the first ones are at right angle to the angle joist that you are fixing and not in line with the main deck joists. When fixing the first screws, make sure that they do not force the deck out of square.

Do NOT proceed with any of this until you are 100% sure of what you are doing. If it starts to go wrong STOP and retrace your steps.

So now your few bits of joist should start to resemble the octagon deck that you wanted to build. Hopefully The finished frame for the octagon deck projectyou are happy so far, and found this little exercise on building your own octagon deck useful. If so, you should now have a framework for your octagonal deck, that does not wobble (too much). The bracing strips should be left in position until you start laying the deck boards.

Any uneven cuts at the corners will be hidden when you place your decking fascia boards into position (I didn't say that!) That is another article which will happen soon!

Lay your deck boards in the normal manner, with those on the angles overlapping, so that you can cut to the right angle after all deckboards are positioned. Any slight discrepancies with the angles can be righted at this time, for it is the cut of your deckboards at the angles that will be the deciding factor on shape. If an angle is a little bit out - is does happen - then simply cut the deckboards with a little bit of overhang to right it. It's ok - we all do it at times. The only time you cannot get away with this, is if your octagonal deck requires balustrades - and hence newel posts!

The Octagonal Deck below was constructed using a different framework - not for the average DIY 'expert'.

This is another way to build the frame for an octagonal deck! Not easy The octagonal deck around the tree takes shape

The framework above for this octagonal deck was designed and built because the client thought that it would be nice to have the deck boards radiating out from the tree - running parallel to the edges of the octagon. There was a change of mind, and it was decided to run the deck boards across the octagon shape - mainly for purposes of strength.


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