How to build a free-standing deck for spa?

[jubjp]

I'm wanting to build a free standing, low-lying quarter circle deck to bear the load of a 1680kg spa (1680kg assuming that the spa is full of water and people) with a diameter of 1.9m. Each side of the quarter circle will be 3.9m long, so the curved section would also have a radius of 3.9m. The spa will sit in the back corner. Along the sides, I also want to build a privacy screen to reach 2.2m high from the level of the deck. I've checked my local and state planning codes to ensure I can do all this without a DA where I am located. I've attached a crude birds-eyediagram and 3D visualisation of what I'm wanting to do below.

My questions to the community are:
-Should I be running double bearers (currently opting for 120mm x 45mm MGP12 treated pine)?

-Best way to fix my bearers to posts, assuming I'm using double bearers - so far I plan to special order 150mm x 150mm F7 sawn pine posts and notching double bearers to both sit directly onto post, with 2 M12 coach screws to fix.

-Should I build my joist frame to sit ontop of my bearers, or would a low-lying, load-bearing deck of this scale be fine to use joist hangers?

-Can I use the same posts to hold my privacy screen and bearers? Assuming I am using 150mm x 150mm posts. I.e could I notch the bearers into the post and have it still be the height of the fence? Or do I need to lay completely seperate posts for the screen.

Thank you in advance for your time and help.

[JacobZ]

Hi @jubjp,

Welcome to the Bunnings Workshop community, we are excited to have you join us.

As I am sure you are aware, adding a hot tub to your deck will create a large point load, which is outside of the realm of a standard deck design. With this inclusion, standard design practices are no longer reliable.

I can say that double or even additional rows of bearers, additional posts and enhanced concrete footings are likely part of what would be required, but I couldn't tell you how many and what calibre would be required.

Unfortunately, if you are looking for firm answers, you would need to speak with an engineer for their assistance in designing a deck that is adequate to support the additional weight.

The larger 150x150 posts would be a great option that would allow for double bearers to sit on notched-out sections. They could likely be held in place using ZENITH M10 x 200mm Stainless Steel 304 Grade Cup Head Bolts And Nuts, but this would need to be confirmed with the designer of the deck.

Generally speaking, it is best to keep posts for the deck and screening separate, but if it is deemed ok by the designer, then it might be possible. 

I'm sorry I can't be more assistance. My knowledge of how to build and design decks is fairly good under normal conditions, but due to the complexity of your plan and the additional variables outside of what is considered normal, I am not qualified to answer your questions. I hope you understand.

Allow me to tag our experienced members @TedBear, @Nailbag and @Dave-1 for their thoughts.

Jacob

[Dave-1]

Afternoon @jubjp 

Now that will be some weight for sure. Tho I think your weighting is a little low for a full spa. 

I know there are dips and cuve spaces for seats in a spa but the easiest way is 

L *B * H

1.9 *1.9*0.8 (height ish)

2888L which equals 2888kg. minus the bits and pieces but really the deck needs to be able to hold that at least.

You want a low lying deck... Is there any chance you can have the spa set on pavers/concrete? I have the same type of deal at the moment, I went with the idea of spa on a compressed earth, gravel then paver base. My spa is 3m by 3m ish and im guessing 800mm ish in height Thats a lot of weight to be playing around with.

I would suggest an engineer to work out the loading if you stay the route of wanting it on a deck, the amount of piers, sizing and how the weight is transferred to the ground, plus lateral forces is something to consider. (Ive had a spa before and yeah everyone piles into it and there are solid forces going every which way.)

If you are happy to loose some height from the deck to the top edge of the spa so the spa sits on the compacted ground with paving/slab the job becomes manageable and will still look a lot like your plans.

Dave

[jubjp]

Thank you both @JacobZ and @Dave-1 for your replies and time!

The spa is not a traditional spa, it's actually a small woodfired spa and the weight figure I have provided comes direct from the suppliers specs. The weight of the spa empty (dry) is only 280kg. It has a water capacity of 1070L and can fit a maximum of 4 people, hence the 1680KG figure I've got. Most of the time it will just be sitting dry and empty on the deck and only weighing 280kg. But obviously I'm designing this with the 1680KG figure in mind - an overestimation of the maximum weight. Sorry probably should have clarified that in the post. 

If it was a larger, traditional spa that was going to weigh in excess of 2.5 - 3 t then yes there is no way I'd be trying to DIY my own deck to support it without first having engineering design/approval.

As I will be over-spec'ing post spans, joist spans and post holes, I was hoping to still be able to do this with the spa sitting on top of the deck.

[JacobZ]

Hi @jubjp,

If it were me, and I couldn't confirm with an engineer, I would massively over-engineer things to ensure there is no possibility of issues.

I would use the largest bearers possible and would double the number of bearer runs under the spa. I would not attach two bearer runs to the same post; I would instead have two separate runs with separate posts for each. Adding extra bearers is only going to be worthwhile if there are additional posts to transfer the load from them down into the earth.

Unfortunately, this is as much advice as I could offer. I'd love to be able to give you a firm answer, but unfortunately, there isn't enough readily available information about decks supporting increased point loads for me to say it would be fine. If you wanted firm answers, based on calculations of the relevant forces, you would need to speak with an engineer.

Jacob

[Dave-1]

Afternoon @jubjp 

No probelms and thanks for the clarrification.

Id still be heading for a ground based solution with a deck around the spa that you step down into. It just feels an easier way to engineer things. Ive over enginered my deck so it probarally would be able to carry a ton, however there is no way I would want to put a ton in a smallish area on it type thinking.

Do you have a photo of the area you are going to install the deck/spa in?

Dave

[jubjp]

Thanks again @Dave-1 @JacobZ for your time.

Here are some pictures of the site, where the inner white circle represents where the spa will sit and the white lines on either side show where the privacy screen will run and the curved shows where the decking will reach to.

[Dave-1]

Afternoon @jubjp 

Thak you for the pics, It will be a nice area for sure, so where will the chimeny go? I have the same thing in my head as I am building an alternativly heated spa via a wood heater type deal (look up dutch spa) and I do worry about my neighbours and smoke and how it will impact them. Fair enough it wont be often but its in the back of my mind for my spa.

I still think it would be better to sit the spa on a solid base and build the deck around it. Making sure in the process that you could convert it to all decking if needed down the track and you wante dto get rid of the spa.

Just had a look at your pics again, how far off your noundry line is the spa privacy screen? Your local council may have a ruling about that, genrally found on their website. (If you know before hand then if your neighbours arnt happy, you will already know the answer )

Dave

[jubjp]

Thanks @Dave-1 ,

The chimney will sit inside the spa (looked up Stoked Woodfired Spas). Fortunately I'm based in the snowy mountains and just about every house around us has a fireplace/chimney so the smoke isn't an issue around here. 

I've already made sure of that, state planning indicates a 900m buffer between boundary and screen/deck with no local planning interfering with that.

I will put some more thought into building the deck around the spa rather than under the spa. 

Thanks all for your help.

[micool]

How’d you go ? Any final photos?