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Round deck post placement advice

Hello! I'm building a round, free standing, low profile deck and was hoping to please have someone with a bit more experience double check the specs, especially regarding the spacing of bearers and joists and the post locations. Many thanks in advance!

Details are as follows:

3m diameter
free standing (i.e. not attached to the house or anything else)
low-profile (i.e. minimal clearance to the ground, so about 150mm + bearers and decking)
joist are inline with bearers (not on top as per usual so that I can minimize height)
bearers and joists are all H3 90x45mm treated pine
decking timber will be 90x20mm pine
bearers are affixed to the stirrup posts with cuphead bolts

I've been referring to this article by Softwoods SA (linked to by others on Workshop posts) and a few other resources to check that the spans between bearers and joists are ok. To the best of my knowledge I think they are, but it's always best to check 🙂

round deck.PNG

The complexity is that it is a round deck which is also low profile – the joists (aqua) are integrated between the bearers (red). Blue indicates the locations where posts will attach and yellow are additional joists. Pink reflects what is essentially packing timber to allow me to affix the cladding around the perimeter. The cladding will be thin rusted steel which is positioned lower than the lip of the decking so there isn't risk of cutting your feet on it. I haven’t included this cladding in the drawing, nor have I included the decking planks.

My questions are:

1) Are my spans of 850mm between bearers and 450mm between joists acceptable?

2) Is using 90x45mm H3 timber for all the bearers and joists will be ok?

3) Is using 10 stirrup posts enough?

4) If i use 90x45 bearers there will be a 10mm (or so) gap between the flat horizontal top of the L-shaped stirrups and the bottom of the bearers. This is due to where the pre-drilled holes are in the vertical part of the stirrups. How should I resolve this to maintain the strength of the joint? Alternatively, should I just use H4 posts instead to simplify things?

Thanks for your feedback and advice!

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Comments

Jason

Sounds like a fantastic project @midas. We are really looking forward to seeing what you can achieve. I'm sure our ever-helpful community members like @EricL, @Adam_W, @Tara86, @tom_builds and @TedBear will be happy to assist as much as they can.

Welcome to the Bunnings Workshop community. It's wonderful to have you join us. As we have clever and creative members sharing exciting projects and helpful advice on the site every day, we're sure you'll fit right in and get plenty of value from regular visits.

Please don't hesitate to reach out if you ever need a hand getting the most from the site.

Jason

Adam_W

Hi @midas wow-eee! That's going to be very cool.
Okay, a quick look and my thoughts would be to do two things;

- tighten up the spacing on those joists (aqua). Because they don't have the support of bearers beneath & they are not continuous span each is carrying all the load rather than spreading it. When you have 'enclosed' joists the load & stress becomes different too.
See the attached diagram I quickly drew but I think you'll get the point. With traditional bearer & joist arrangements the load travel from joists to bearers to posts. With an enclosed-type the load is being carried mainly by the saddles so increases the risk of failure. As someone said to me... the art of engineering is about transferring & distributing loads... I find it helps to picture in my mind that load is like something flowing, the further it spreads the better as each part is then carrying less.
- just me over engineering but I'd probably add more posts. Maybe concrete in the ones you have proposed & then use surface mounted Tuff Blocks for some extras with suitably compressed soil beneath of course.

Stirrups will give you the longest lifespan. Can you find some that are a better fit? If needs be you can pack underneath as you want the load on the head of the post, not the side. Shims are useful as they come in very accurate thicknesses but just check if they are okay for exterior use.
I'm sure others will have some feedback too.

EricL

Hello @midas

Thanks for sharing your question about your round decking project with us.

It's great that you've received fantastic advice from @Adam_W. I totally agree with his recommendation of added posts. I recommend having a ponder on the possible number of people that will be on the deck. This is where FLW and Adam's mantra of flowing weight distribution comes in. The materials you use will directly affect the strength and longevity of your deck.

Using H4 posts will simplify things as you will not have to make adjustments and level the shim or packer to make sure they are all the same height. The good part about adding more posts is that it does not affect the weight of the deck yet strengthens it by adding more support. We look forward to seeing the start of your deck.

If you need more advice or information, please let us know.

Eric

tom_builds

Looks like an exciting project @midas, can't wait to see the finished project.

I think @EricL and @Adam_W have pretty much covered everything, the only other thing I can think to suggest is that if the stirrups you are planning to use are too large for the 90 x 45mm bearers, maybe you could consider upgrading to 120x45mm. This will mean the bearers sit on the base of the stirrup as intended, and will also give the bearers greater strength, making your proposed spacings more achievable. This will still allow you to use the 90 x 45mm for the joists as you are planning on doing

Really looking forward to seeing update photos of this one, would be great to see some work in progress pics as well as the finished product.

Best of luck!

midas

https://www.workshop.bunnings.com.au/discussion/comment/14313#Comment_14313

Thanks so much @Adam_W @Jason @EricL @tom_builds! I really appreciate each of your advice, thank you.

@Adam_W Thanks for the explanation about force transfer. I'm thinking I'll increase the number of joists so there is only about a 315mm space between them (instead of 450mm). That feels like a lot of joists but as you say, I'm not dealing with continuous joists. Adding more posts too is a good idea. I've updated the diagram to show these alterations.

round deck v2.PNG

@EricL Good thinking about using H4 posts instead. I had been thinking of using stirrups so I could have smaller footings because the deck will be close to an established ash tree. However, timber posts might just be easier in the long run.

@tom_builds That is another great suggestion, thank you. This might work rather well.

May I ask, do any of you have any concerns with using angled joists for the curved sections (shown in yellow in the diagram)? I can't find appropriate 40 degree joist hangers. I could change the design to 45 degrees but even then I can't find angled hangers. I guess this means I'll have to use four large coach screws at each of points when the yellow joist connects to the other joists and bearers? Will that be enough support?

Thank you again! I will be sure to take plenty of photos along the way and share photos of the finished product.

Joel

Adam_W

https://www.workshop.bunnings.com.au/discussion/comment/69710#Comment_69710

Good stuff @midas and yes! Please make sure you share pics as you progress.

tom_builds

https://www.workshop.bunnings.com.au/discussion/comment/69710#Comment_69710

While joist hangers don't seem to come in an angled variety @midas, you might find that you can bend them sufficiently to achieve what you need, and then use coach screws to connect through the bearers into the joists. The joist hangers will make the coach screws easier to fix, and while bending the joist hangers may affect their strength, combined with the coach screws they should help strengthen the joint.

Brad

https://www.workshop.bunnings.com.au/discussion/comment/97900#Comment_97900

Mitek 45 Degree Joist Hanger

They are wrap around style, not sure on sizing for your project but they have a diagram and phone numbers listed for most states.

midas

https://www.workshop.bunnings.com.au/discussion/comment/115640#Comment_115640

Thanks @Brad ! I found those online but struggling to find a local supplier.

@tom_builds i had the same idea. I'm thinking i might do it that way. The hanger (or maybe even just a nail plate) will just hold thinks in place while the coach screws do the heavy lifting. If in doubt I'm sure i could also put a post in the middle too to help hold the weight.

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