Laying deck on concrete slab

[TK87]

Hi all. I'm looking to lay decking over an existing concrete slab. The slab has existing water proofing membrane and is 4.8x5.7m.

I'm keen to use a composite panel product (e.g. Ekodeck).

Are there pedestal products that I can use that substitutes the need to build a joist frame structure? 

Would love to hear from advice. 

Cheers!

[Jason]

Hi @TK87,

Welcome to the Bunnings Workshop community. We're really pleased to have you join us and are looking forward to seeing your deck come together.

I'm sure our resident Bunnings D.I.Y. expert @EricL will be happy to assist as soon as possible, but in the meantime, have you seen the helpful resource How to build a low-level deck by experienced member @Adam_W?

You might also get ideas, information and inspiration by searching for the term "low-level deck" and checking out out Top 10 most popular deck projects. This previous project might be particularly helpful for you - Low-level decking construction by @Amin88 

Hope that helps,

Jason

[EricL]

Hello @TK87

Thank you for sharing your question about pedestal products for low-level decking.

It's great that my colleague @Jason has pointed you to some very useful guides. I've seen a few low-height aluminum joists and pedestal systems online such as the one featured here on Outdoor Design Source as an example. But if you are looking to make your own you can use Builders Edge Pedestal Foot Smallfoot 35 - 60mm and your choice of framing for the deck. 

The other frame I'm aware of is the Steel-Deck. The sub-floor frame is comprised of channel bearers and joists, built using corrosion and fire-resistant GALVASPAN® steel. 

Using composite materials and plastic footings will provide you with a measure of protection from mould and mildew. But I still recommend having adequate space for ventilation under the deck. Would it be possible for you to provide us with the proposed height for the deck? It will give our members a better idea of the proposed installation and provide you with other possible recommendations.

If you've drawn up a plan, we would love to see what you have in mind. 

Eric

[TK87]

Thank you so much @EricL and @Jason. Revving back up here. I’ll share some plans tomorrow but appreciate the guidance so far. 
I had seen the Builders Edge pedestals and thought they looked great. I’ll share some details as would love to find out the appropriate timber and spacing for the joists. 

Cheers!

[TK87]

Thanks again @EricL and @Jason.

To confirm, the dimensions of the space to deck is 5700mm x 4100mm.

Height of the deck

I'd like to keep it all as low as possible, but obviously with enough space for ventilation. Using the pedestals, is it adequate to lay the frame at say 50mm base height. Then if the frame is 70mm high, the boards would be laid with 120mm gap between the concrete floor. Is that adequate?

Building the frame

1. Timber dimensions - would 70mm x 45mm be appropriate? 

2. What 'type' of timber should be used here? I'm guessing a treated pine, any specifics?

3. Could you please shed some light on the frame 'design', particularly the adequate spacing between joists? I've drafted a diagram below, where I've allowed for 11 joists across with a gap of 520.5mm (assuming using 45mm width timber frame). Is that about right? 

Any guidance would be greatly appreciated. 

Cheers, 

Tom

[StevieB]

Hi Tom (@TK87),

@EricL will be happy to help further when he's back on the site on Friday. In the meantime, let me also tag @Adam_W, @tom_builds and @DIYgals who may be able to offer suggestions.

Stevie

[tom_builds]

Hi @TK87 ,

Your deck project looks great and I think you are just about set to start. 120mm between board and slab will be great clearance for ventilation, and 70 x 45 mm should be adequate provided you use a suitable number of feet for each joist. H3 treated pine is what you will need, this is treated for above ground outdoor use and can tolerate moderate wetting.

On framing, deck joists are usually spaced at 450mm centres (from the centre of one joist to the centre of the next), however I built my deck on 500mm centres and from memory building codes allow up to 600mm. Your siggested 520mm spacing will be fine if you are using long lengths of decking, but if you buy pre-cut boards you may find that they are cut to suit the standard 450mm spacing.

Your framing sketches look like you plan on boxing in the frame by attaching a perpendicular span at each end of your joists, and this is a good idea as it will help keep your joists from twisting out of shape.

If you could keep us updated with progress pics as you complete it that would be great!

Best of luck

[StevieB]

Thanks for being so helpful, as always @tom_builds.

Really appreciate you contributing your expertise and sharing from your experience.

Stevie

[TK87]

Awesome - thanks @tom_builds and @ST Really really helpful!

Yes, using long decking boards (5400mm lengths) so think I'll be ok here. 

One final question is around how many pedestals I will need... Could you advise the interval space to each pedestal - both around the edge and along the joists?

Thanks again!!

[tom_builds]

Hi again @TK87 ,

I've had a look at the specifications doc for the pedestal feet and for 70x45mm joists spaced at 500mm centres the maximum span between pedestals is 650mm. With one at each end of a joist you're looking at 8 (or 7 at a pinch) per joist for 11 joists for a total of 88..which could end up costing you a pretty penny.

A cheaper solution might be to use lengths of 70 x 45 (or even 90/130 x 45) on their short side as a sort of bearer, and the attach joists perpendicular to these. You could aim for 4 of these 5.7m lengths and the timber would set you back about $165 @ 70x45mm size.

Obviously depends on your budget, I think the pedestals would create a neat finish, and you wouldn't need to store as much timber either.

Hope this helps!