How to build a deck platform for a spa?

[su51q]

Hi, I am a newbie, so be kind to me.

I want to build a deck over concrete for a Lazyboy inflatable spa.

This is what I am planning:

1. Build a square using treated wood.

2. Use a screw pedestal system to level the square - what are these called?

3. Add joists every 300mm.

4. Cover with decking

Questions I have:

1. How big should the deck be for a 1.80m diameter spa? - I don't want to do any cutting! Also would like to put a portable gazebo over it.

2. Do you use the same wood for bearers and joists?

3. Do you need to use brackets to attach the joists or can the simply be screwed in?

4. How high off the ground should I build it.

5. Do I need to put noggin things in? If so where?

Thank you for you help. I can do this!!

[MitchellMc]

Welcome to the Bunnings Workshop community @su51q. It's wonderful to have you join us, and many thanks for your question about building a base for your spa.

Your plan sounds quite reasonable, but the first step is to check with the spa manufacturer to determine whether a decked platform would be a suitable base. Many spas must be placed on a compacted soil or concrete base, not a raised platform. You should consider placing the spa on a compacted base and then building a deck around the outside. Standard decking practices are not appropriate when dealing with the weight of a spa. You're really constructing a customised platform that needs far larger calibres of timber and more frequent support to bear the weight.

The deck size is really up to you and determined by whether it's purely holding the spa and the aesthetics you want to achieve. The minimum-sized platform for an 1800mm wide spa would be 1800 x 1800mm. However, I think you'd need a portion at least on one side to stand on whilst you get in, so 1800 x 2200mm. If you wanted the deck to extend out 400mm around the whole perimeter of the spa, you'd be looking at 2600 x 2600mm.

In a single-level platform, you'd have a frame around the perimeter, then joists and no bearers. Or were you thinking of creating a joist-over-bearer platform? With low-level platforms, we typically don't use joists over bearers as they raise the height. H3-treated Pine is suitable for all joists/bearers above ground.

With the huge amount of weight a spa has, joints would best be fixed with screws and brackets. It would be in the ballpark of 1.8 tonnes when full. As this is not a deck carrying standard deck weights, pedestal feet are not designed for the additional weight. 

The height off the ground is up to you. It would be good to have some clearance under the deck for airflow, but given the weight of the spa, having the weight transferred through the joists directly into the ground would be wise.

Noggins would be required, and they should be placed readily along the joists.

Please let me know if you have any questions.

Mitchell