Adequate deck framing for outdoor Spa sitting on a slab?

[Standleyana]

Hi all, Looking at installing a 3.0x3.0m merbau deck directly on a 2.4x2.5m slab for our new spa. The Spa's dimensions is 2.3x2.3m and Weighs close to 2 Tons (1800kg) when full. I was going to have joists of (90x45 treated Pine) and 90x90 treated pine for the posts. I was wondering what the adequate joist and post spacings for such a load would be? As the standard deck spans wouldn't suffice. Cheers!

[KatieC]

Hi @Standleyana,

First, let me extend a warm welcome to the Bunnings Workshop community! It is wonderful you have decided to join and hit the ground running by asking a great decking question. Our resident DIY expert @MitchellMc will be back online on Friday to further assist, but let me tag Workshop members @Jewelleryrescue, @DIYImpressions, and @Neo19 to see if they are available to lend any advice. While you wait, check out our top 10 most popular deck projects for inspiration. 

We would love to see how your project progresses so please update the community with photos. 

Katie

[Jewelleryrescue]

Hi @Standleyana  and @KatieC 

Maybe you can send a photo of the area and surrounds,  I am not sure there is any need to put timber under a  spa weight as a timber deck needs  periodic servicing.  If you wanted the spa to be higher I would probably recommend cement pad under the spa as this wont rot or need servicing but you have your reasons if you could put us in the picture more thanks.

Another  option with out   hearing your reply is use a cement slab and tile it with  extremly realistic timber looking plank tiles?

[MitchellMc]

Hi @Standleyana,

It sounds like your best bet would be to extend the concrete slab, so the spa sits solely on it. If you then wanted a deck, you could build it up and around the spa. 

If you wish to place the spa on the deck, you should first reference the spa installation guide and speak with the manufacturer to find out if this is possible. The spa's weight will need to be directly transferred to the ground, as a deck will not bear its weight. This means you'll need solid timber under all points of the spa that need support. This is where you'll need to reference installation guides. Can joists support the spa at 450mm centres, or does it need to be smaller, or does it need a flat concrete base or compacted earth?

A standard deck built to code supports around 200kg/m2. So if you double-up on all the frame timbers, you'd be getting close the right ballpark. However, as mentioned, you first need to check with the manufacturer to determine whether having the spa on a deck is feasible.

Please let me know if you have any questions.

Mitchell