Deck on concrete

[TrixtanR]

Hi Everyone,

I've been a silent viewer of this amazing forum and finally decided to get my first D.I.Y going after a lot of procrastination. I have a small outdoor area and want to build a deck there.

I have looked through a couple of amazing projects and got quite a good understanding of how to proceed,

https://www.workshop.com.au/t5/Outdoor/Low-Level-decking-Over-concrete/td-p/44471

https://www.workshop.com.au/t5/Outdoor/Low-profile-deck-on-concrete/td-p/40882

https://www.workshop.com.au/t5/Outdoor/Merbau-decking-over-concrete-slab/td-p/31230

I do have a few questions which i'm hoping this forum would be able to help,

Do i need to anchor the frame to the brick? Will it suffice if i anchor the frame to the concrete floor using angle brackets and concrete dynabolts?

Its an open area so the deck will need to face the elements? Do I need to use H3 or H4 timber?

I want the deck to sit flush with the brick in height (200 mm). I plan to use merbau to finish the deck (https://www.bunnings.com.au/specrite-90-x-19mm-2-25m-fj-merbau-decking-5-pack_p0064530) which has an height of 19 mm. I need to make up another ~180 mm in height. I can't find a suitable H3 or H4 sleeper in that height. Does that mean i will need to get the 200mm sleeper and cut it to fit?

I did find cypress sleepers which are 150 mm added to the merabu of 19 mm. And for the rest, I plan to use packers. Is cypress a good option for the frame?

.

Many Thanks

[MitchellMc]

Welcome to the Bunnings Workshop community @TrixtanR. Many thanks for your questions, I'd be pleased to assist.

You do not need to anchor the frame to the brick, however, if the brick is square then it is a great place to start from as it adds rigidity to the deck.

H3 treated pine is suitable for above ground use. If it is in close proximity to the concrete you should use plastic packers to raise it up allowing water to run off and air to circulate.

You can use Treated Pine Outdoor Timber Framing 140 x 45mm,  Specrite 90 x 19mm 2.25m FL Merbau Decking 5 Pack and Dunnings 65 x 130 x 65 x 5mm M12 Galvanised Angle Bracket. That will leave you with a gap of 36mm for air circulation and drainage.

Please let me know if you need further assistance or had other questions.

Mitchell

[TrixtanR]

Thank you Mitchell for that. 

Two more questions if you don't mind,

Would I need to leave some space between the brick and the deck for drainage?

And how much space do I need to leave between each Merbau deck to allow for expansion?

Thanks in advance!

Tristan.

[MitchellMc]

@TrixtanR,

You will need to prevent moisture buildup between the ledger board and the brick. You can butt the ledger board up against the brick but this would require flashing and sealant. As the bricks have sunken mortar joints it would be difficult to achieve a great result with this method. Instead, I would advise you to leave at least a 25mm gap between the deck and the brick to allow for drainage and stop moisture build-up at the wall.

On a 90mm decking board, a 3-4mm gap between them is sufficient for expansion.

Mitchell

[Michael_W]

Hi Mitchell

Your suggestion for this project is to make the decking independent to the brick?

Is 5-mm gap enough for a 140mm merbau decking?

In the project, we cannot avoid cutting off a joist at somewhere by using a saw. Then two untreated cross sections are exposed to the air. Do we need to seal them for longer lifespan? And what type of product can do so?

Many thanks.

[MitchellMc]

Hi Michael_W,

The decking does not need to be independent of the brick, it can be bolted to it, but there needs to be a gap left in between them or a flashing placed over the ledger. 

A gap of five to six millimetres between 140mm Merbau decking boards will be sufficient.

You will need to seal any cuts ends to ensure the longevity of the timber. TWA Woodcare 300g Ecoseal Tanalised Timber Treatment is the appropriate product to restore any exposed ends to the original level of treatment.

Please let me know if you need further information or had questions.

Mitchell

[Sandeep]

Hi Mitchell,

Just wandering how do we cover the Gap of 36mm? What can we use there to lift up to that height? 

It's about ur suggestion to use 140 mm timber and 19 mm thick Merbau and 5mm angle bracket and to leave 36mm gap. I have the same situation regarding decking as I want to make bring the height of decking to 200mm on a concrete slab in Alfresco and the Alfresco is covered from 3 sides. Could you also suggest me if I need to bolt the frame to the bricks on any side? 

I will much appreciate your reply.

Regards,

Sandeep

[MitchellMc]

Hi @Sandeep,

These Builders Edge 25-40mm Pedestal Foot Minifeet would be a great way to establish a 36mm gap under your bearers. They are adjustable and can be set at the height you require. The 36mm is only being suggested here as it works out well for @TrixtanR's overall height using those timber sizes. There are also a couple of other sizes if you need a larger gap.

To achieve a 200mm overall height you could also use 90 x 45 outdoor framing timber, 19mm decking timber and a Builders Edge Pedestal Foot Bigfoot Xl 70 - 170mm adjusted to 129mm.

You can bolt the frames to the brick if you wish. You would normally bolt to the house if you are unable to support the deck structure from underneath.

Please let me know if you need further assistance or had questions.

Mitchell

[ChickenLittle]

These pedestal feet look perfect for my timber base that i'm building for my 3m x 1.5m tin garden shed.

Is there a rule of thumb when it comes to how many of these minifeet are required (spacing etc) ?

[MitchellMc]

Hi @ChickenLittle,

The number and position of feet are determined by your timber span ability. If your timber base frame is made from 90 x 45mm timber, then you'd need a foot every 1000mm to support it.

If you are able to post a picture or sketch of the timber base/frame I can help suggest locations for the feet.

Please let me know if you have any questions.

Mitchell