Can a floating deck hold an inflatable spa?

[dnlbignami]

Hi all,

i started to build a floating deck in the backyard and i am planning to put on top of the deck an inflatable spa (https://www.bunnings.com.au/bestway-1-80m-x-66cm-lay-z-spa-tahiti-airjet-inflatable-spa_p0262778). I am not sure however if the deck can hold the weight of the spa plus two people? If yes, anything you recommend to keep in mind as i go ahead with the project and to reinforce the deck. Dimensions below

Deck Size - 2.8 mt x 3 mt

Pool Size and Weight - 1.80 mt x 0.66 mt (inflated size); 1.32 mt (inner size); 696 kg (filled weight)

Two People - 150 kg (approximate weight)

[MitchellMc]

Welcome to the Bunnings Workshop community @dnlbignami. It's wonderful to have you join us, and many thanks for your question about creating a spa base.

These types of spas are designed to be installed over a solid surface like a concrete slab. 850kg is a fair amount of weight, so if you build over soil, the first step would be to clear the area, excavate down at 200mm, compact the area, fill with roadbase and then compact again. This will give you a firm working surface. You'd then be able to start building your deck. If you don't compact the soil and add roadbase, there's a decent chance that you'll see subsidence of the structure.

Any low-level deck that is going to be holding that amount of weight needs additional structural members. With your design, I'd recommend installing joists across the span at centres of no more than 300mm instead of the standard 450mm. The joists could be more sleepers every 300mm across the width. You can then nail off your decking boards.

Please let me know if you have any questions.

Mitchell
 

[dnlbignami]

Thanks Mitchell.
This is much appreciated. Can I just clarify:

- What you mean exactly with “The joists could be more sleepers every 300mm across the width. You can then nail off your decking boards”; and

- Why is important to build a “firm working surface”?

Thanks

[MitchellMc]

So the decking boards need something to support them across the span @dnlbignami. These supports are your joists. The joists can be sleepers, and you'd space them every 300mm across the span. I've created a rendering below to illustrate. 

If you place an 850kg object onto uncompacted soil, then there is potential for the soil to compress under the weight, and your spa become lopsided. The amount at which the soil compacts could be unnoticeable, or in extreme cases, it could subside so much that water escapes the spa. Any spa installation guides I've come across that mention the spa can be installed over soil require you to compact the ground before the installation. Whether it's a path, shed or spa, starting with a solid foundation of compacted soil and roadbase is important.

Mitchell

[dnlbignami]

Thanks, Mitchell.

It all makes sense. Rather than sleepers, can I use joists that are 140 x 45 cm?

Thanks

[MitchellMc]

You could, but it's only H3-rated for above-ground use @dnlbignami. If the timber is in contact with the ground, it is preferable to be H4-rated, such as the sleepers. Having the sleepers in contact with the ground would probably be the best approach as it transfers the spa's weight directly to the ground. With the 140 x 45, you'll have to support them at suitable points if they don't touch the ground.

Mitchell