How to build a low-level deck on a concrete slab?

[mike1360]

Hi everyone, 

This is my first time building a deck. I'm planning to demolish the pergola and the bricks to build a low-level deck on a concrete slab. As you can see there's an unusable spa in the middle and filled with soil. I'm planning to use 90 x 45 treated Pine joists sitting on galvanised brackets with about 10mm ground clearance, I'll put a few H4 posts in the middle to support the frame.

Is it workable? would this cause air ventilation and drainage problems?

  

[MitchellMc]

Welcome to the Bunnings Workshop community @mike1360. It's terrific to have you join us, and many thanks for your question about building a deck.

It is always best to have your deck elevated as high as possible. Unless there is some reason to have only a 10mm gap under your structure, I would advise against it. Having only a 10mm gap would be a risky choice only used on extremely low-level decks where something is governing the maximum height, like a door sill. In this case, I'd recommend having an additional step up to the deck and raising the structure to an appropriate height. Having a gap of 100mm under the timber would be a better solution which will allow airflow around the structure. It looks like you've removed some timberwork from the area, was this due to decay, and how close was that structure placed to the ground?

I'd need to know more specifics on these H4 posts you're planning to place in the middle to support the frame before commenting on their feasibility. I recommend using a calibre of timber that can span the distance across the spa instead of placing support within it.  

You should find this guide useful: How to build a low-level deck.

Please let me know if you have any questions.

Mitchell
 

[mike1360]

Hi Mitchell,

Thank you so much for replying back to me🙏. I removed the decking due to the timber underneath is rotten and the whole deck needs to be replaced. This is my first time doing this sort of project, so I'm unsure what's the proper way to do it. You recommended to use a calibre of timber that can span the distance across the spa. What do you mean by that? What sort of timber would you use to build the frame and how do you fix it to the concrete?

Regards,

Michael

[mike1360]

Can I lay  90 x 45 treated Pine H3 directly to the concrete? Would this cause some sort of ventilation and drainage problems?

[EricL]

Hello @mike1360

My colleague was referring to a more robust piece of timber that can carry more weight such as the 140 x 45 or even the 190 x 45. This is to save you from having to put excessive multiple footings and extra support underneath the deck frame. In reference to your other question putting the timber right next to the concrete will create moisture damage and shorten your deck frame life considerably.

As my colleague had mentioned it is recommended to place a 100mm gap between the frame and the concrete. This will facilitate ventilation under your deck and will not trap rainwater or moisture underneath. I suggest having a look at the DEKO 250 x 150 x 150mm Instant Concrete Foundation Block as a possible candidate for your deck frame footing. 

Here is a link to the DEKO brochure: DEKO instant foundation block

Here are some amazing low-level deck projects our members have shared:

• Low-profile deck with screening and steps by @MGusto
   
• Low-level hardwood deck built over concrete by @DIYgals
   

If you have any other questions we can help with, please let us know.

Eric

[mike1360]

Here is the finished product:)

[MitchellMc]

Sensational results @mike1360! I'm sure you're super proud of your efforts, and rightly so, as the area now looks amazing.

Many thanks for sharing your finished project, and I look forward to seeing what you tackle next.

Well done.

Mitchell

[Jason]

Congratulations @mike1360 - great result!

You should be really proud of your efforts.

Looking forward to seeing what you tackle next.

Jason