How to build a timber deck for a spa?

[BrettSa]

I have built some basic decks for a garden shed & outdoor furniture in the past so have a basic understanding without being too concerned about weight limits. This is different.

I plan on building a 2400x2400 deck on fairly level ground using treated pine bearers, joists & decking boards. The spa with water & 4 persons will weigh no more than 1600kg. Using 140x45 joists & bearers, & builders edge adjustable feet (which have a weight capacity of 220kg each), was my plan. I was going to have 3 pedestal feet (each side & the middle) supporting each joist, which will be hung from the bearers every 400mm, so 7 joists in all. This would be a total of 21 pedestal feet.

Does someone with far more knowledge & experience than me think that this is a feasible project?

[EricL]

Hello @BrettSa 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about building a deck for your spa.

There was a similar query to this a while back, I suggest having a quick look at the discussion - Can a floating deck hold an inflatable spa by @dnlbignami. From the discussion it was suggested to have the bearers in direct contact with the ground. By doing this you won't have to bother with footings for your heavy spa. Please keep note that your spa is almost twice as heavy as the inflatable spa in the discussion, so not only does your decking frame need to be able to hold the weight but spread it out evenly along its structure.   

Let me call on our experienced members @Nailbag, @Dave-1 and @AlanM52 for their recommendations.

If you need further assistance, please let us know.

Eric

[AlanM52]

Hi @BrettSa,

Would you be willing to compromise and have the spa sit lower in the deck on a raised concrete slab.

Just saying... because that's what we did when I helped a family friend.

There is a lot of plumbing work which will likely involve a licensed plumber.

Cheers

[Dave-1]

Good Morning @BrettSa 

I hesitate to put that much weight on any deck. I justhad a very quick search on your spa size and aproximatley how much the water would weigh for a 2400mm * 2400mm spa. It ranges from 1600Kg to 2000kg depending on the depth and makeup of your spa. This is without adding any bodies and not including the spa itself.

If you did go the route of a deck to support a spa I would go with an over engineered deck for sure, I would double the supporting feet and suggest to put the actual feet on stirups in concrete footings more so then the adjustable stirups.

I have had a spa for several years and currently installing another spa, the amount of peopel that will climb into a spa together and the lateral loading that will take place when someone ios over excited and takes a running slide into the spa pushing whatever weighted water around by the impact is where I am heading with my thinking of doubling your support. 

I would suggest to sketch up your plan, keeping in mind the type of timbers in a wet area that will stay wet even after the surface has dried (under the spa) and post them up here, There are span tables for decks, not sure about decks with a ton, a ton and a half or more weight on the, but we can give you a ballanced idea.

Dave

[MitchellMc]

Hi @BrettSa,

Decks are typically designed to support the weight of people and lighter items like furniture or a barbecue, not the concentrated load of a full spa. Since a spa can weigh over a tonne when filled, you would need to build a structure that’s at least twice as strong as a standard deck. That means more footings, additional bearers, and smaller spacing between them to properly distribute the weight. Given the significant loads involved and the non-standard construction methods required, I’d strongly recommend engaging a professional carpenter or builder to assess the site and design a deck that meets the correct load-bearing requirements. This will ensure the structure is safe, stable, and long-lasting.

Please let me know if you have any questions.

Mitchell

[BrettSa]

Thankyou all for your suggestions & caution about my proposed project. I think I will go away & re-strategise. I certainly don't need this thing collapsing. It is a relatively small spa (200kg empty with an additional 830kg when filled with water), but with up to 4 big people in there, could be close to 1600kg. I'm not keen on a concrete slab, but it maybe the best way.