How to install decking over existing concrete?

[JoshuaS]

Hi all,

I am hoping to get professional advice from the community who have faced similar projects.

First thing, please excuse my little knowledge of the project in question. 

I am hoping to create a modern outdoor living space by installing decking on top of the existing concrete slab, as well as extending it a bit further out (as shown in the picture below) for more space to put an outdoor dining table.

The space that will need decking is 2680 x 4850mm with likely treated pine decking or merbau for the deck itself. The existing concrete is 485mm high, which means I can go up another 70mm before hitting the step to enter my house. Moreover, I will need a couple of steps to be installed as well, and railings which I have not yet planned. 

I have attached my proposed plan to implement the subfloor with the below materials:

- Treated H3 90x45 for the noggins and joists.

- Treated H3 140x45 for the bearers (this could be an overkill).

- 90x90 F7/H4 for the posts.

- 90x90 bolt down anchor with M10 dyno bolts

- 90x45 joist hangers and screws.

- Buildex 10x65mm decking screws x 500.

Hoping someone can advise if this is the best plan and if there are things to change to ensure proper structure and practicality. 

Thank you kindly.

[MitchellMc]

Welcome to the Bunnings Workshop community @JoshuaS. It's brilliant to have you join us, and many thanks for your question about building a deck.

It appears that you've put a plan together that would be similar to those used on a single-level low-level deck where there is no room for a joist-over-bearer system. For your plan to work, each and every one of those joist needs to be supported very 1.2-1.5m across the 2680mm span. This isn't really feasible. You'd be much better off running bearers across the 4850 width to support multiple joists. This will allow you to decrease the number of supports the deck requires.

This support is mainly required for the portion that extends off the concrete slab, and I've made a quick addition to your plan below.

Remember that this elevated deck could require council permission and will need handrails around it. It would be best to start by contacting your council to determine requirements, as you might need plans for the build produced by an engineer.

Please let me know if you have any questions.

Mitchell
 

[JoshuaS]

Hi Mitchel,

Thanks for your response and recommendation. Yes, I agree that makes perfect sense to add a bearer to provide support and to avoid the trampoline effects. 

Fortunately, my council approves decking below 800mm and 1m away from boundaries. 

I just wanted to confirm based on my initial materials to use the 90x45 treated pine for all the subfloor is all good?

In terms of deck railing and stairs, is there a recommended structure or best practice to follow? Especially prior to installing the subfloor is there anything I need to prepare for it? I would hate to finish it all and realize there were things I missed out to enable be to correctly install the railings.

Thanks again.

[EricL]

Hello @JoshuaS 

It is very likely that your local council will have their own rules and regulations regarding the installation of deck railings and stairs. You can refer to the NCC Part 11.3 Barriers and Handrails, but I propose doing a little bit more research locally regarding rail distance and rail mounting. I totally agree with you in looking ahead in regards to this part of your deck, the last thing you want to do is to modify it after you've assembled it. 

I recommend re-drawing your plan with the rails and stairs included so that you'll have a better idea of how they'll fit in with your decking frame plans.

If you need further assistance, please let us know.

Eric