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I'm planning to put a 7m X 4m deck over my concrete alfresco. I will be using a Merbau deck board and H3 treated pine for joist & bearers with joist placed every 450mm and bearers 2m apart.The maximum height for me to work with is 135mm. If I use 70x35mm T/P, the total working height of the deck board (19mm) + Joist (70mm) + bearer (35mm) will come to 124mm. That leaves me with 11mm for the spacers below bearer and for the flashing below deck board.
Q1. Will that spacing (~11mm) below bearer be enough?
Q2. Do I need to seal the concrete before putting the frame?
Q3. I'm not planning to attach the deck to the brick wall or to the concrete floor as there is a pillar in the middle which I am expecting to hold the deck in its place. Does that approach sound right?
Community manager's note: Check out How to build a low-level deck for expert advice.
Welcome to the Bunnings Workshop community @karim. It's fantastic that you've joined us, and many thanks for jumping into the conversation.
I trust @Jithesh will have some helpful advice for your project. Please feel free to start your own discussion so we can delve further into your project and answer any questions you might have about it. I trust you'll find loads of inspiration just like this project within the community as our amazing members contribute their projects here every day.
I'll be looking forward to hearing more about your project and would encourage you to share it with us.
Mitchell
Great work on the list of materials and tools! It's good to know what you're going to be up for and you can see if you've missed anything 
Just remember with the timber lengths that you can't buy a lenth that is exactly 4m long (probably 4.2m or 4.8m is the closest and depending on stock availability). And also with the 7m length, timber does not come that long (probably 5.4m or 6m) so you'll have to buy two lengths and join them in some way. If it were me I'd join them using a half lap joint and coach bolts, making sure the joint is close to the bearer/support point below the joist.
Best of luck.
Edit: My comment about the lengths doesn't have as much relivance now that I see there is more than 1 page to this thread... dough. Looks great though.
Welcome to the Bunnings Workshop community @ChrisNocker. It's fantastic that you've joined us, and many thanks for contributing to the discussion.
Available timber lengths need to be accounted for in the design stage and can add to the cost of a project if there is going to be significant wastage. It's always something to consider when budgeting. Your suggested method of joining lengths is also a great option.
Many thanks for sharing your knowledge, and we look forward to hearing all about your projects around the house and garden. We'd encourage you to let us know if you ever need assistance or have something to share with the community. I trust you'll find loads of inspiration for your projects here as our amazing members contribute their ideas here daily.
Mitchell
Fantastic forum! Cant believe I've only just found it.
I'm in the process of doing a low ground deck over a concrete slab too. Seeing your questions relate to my situation 100%.
My main question is this: will bolting your treated pine directly onto the concrete increase the chances of rot?
My deck will be directly in front of my front door and only slightly shaded from the elements. How can I keep a low deck while also leaving room for airflow and avoid the timber touching the concrete slab directly?
Thanks in advance 
Aiden.
Welcome to the Bunnings Workshop community @aiden. It's terrific that you've joined us, and many thanks for your question.
You must leave as much space for airflow and drainage under your timber as possible. Bolting any timber that is not rated H4 and above directly onto concrete will reduce its lifespan. If a gap is not left, moisture will constantly be trapped, which starts the rotting process. Deck structures are commonly constructed from H3 outdoor framing timber, which is suitable for aboveground use. It is not suitable for in-ground or in direct contact with the ground.
You might like to start your own discussion so we can cover your project more extensively. If you include some area images, a sketch of your deck plan and details on the height between the concrete slab and the door sill, we can further discuss what options are available.
Mitchell
Thanks for the prompt response. I'll go ahead and try to take some pictures later on for a separate thread. I just thought that this was so similar I almost didn't need a separate thread.
From the deck example in this thread, did you use H4 treated pine directly on top of the concrete slab? Thanks
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