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How to build deck over concrete and soil?

Hi All,

I think I have finally finished my design for my deck which will be half over concrete and half over soil.

Over the concrete I have 180mm to the top of the brick. I will be using 140x19mm merbau. For the joists I plan to use 140 x 45 Pine set 21mm off the concrete using Zinc Plated Reinforcing Angle Bracket with the joist. I will Dynabolt these using a DynaBolt Plus Hex Nut to the concrete every meter or so.

The joists have 355mm centres, I will just use a nail gun to fix these every 1m along the joists.

Over the soil, I will dig out about 300mm and put down crushed rock and ag pipe for drainage. The posts will concrete in 300mm and out 320mm. With bearers cut in. I will be using 190 x 45mm Bearers with 1218mm spacings. The posts will have 1000mm spacings.

The deck will also be a picture frame deck.

I have drawings below, please provide any comment, or issues, corrects, errors you may notice. deck latest.png deck latest (3).png deck latest (2).png deck latest (1).png

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Comments

MitchellMc

That's looking pretty good @JesseS6. With the brackets connecting the joist to the concrete, alternate the side they are on going down the joist. The first one on the left side of the joist , the next one on the right side. This will make it much more stable than all being on one side.

For the noggins, you should space them more evenly along the length of the joist. You've got them all in the middle, whereas they would strengthen the structure better if they were spaced out as per the below.

Mitchell

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/50429#Comment_50429

Is it fine to just use nail gun to fix the noggins?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/87322#Comment_87322

Hello @JesseS6

It's perfectly alright to use nails for the noggins. However, if you have a choice in the matter, I suggest using screws instead. Screws are strong and are built to last. This is a stark contrast to nails, which may come loose over time. This happens when the materials they connect with begin to expand and shrink over time. Screws are built to resist this problem.

If you need further assistance, please let us know.

Eric

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/109157#Comment_109157

Thanks @EricL

Also am wondering if the post need to be sunk in the concrete. Are there any issues with setting a post anchor and attaching the posts to those? or something similar? what other options are available?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/121905#Comment_121905

Hi @JesseS6

From a construction point of view, it would be best to sink the post into concrete. Building a concrete footing and anchoring into it is like double handling and you have to spend for the hardware to attach the post to the concrete footing. Building it as one piece means that won't have to go back to it again. The other options are pedestal bases, concrete foundation blocks and Tuff block footings. These options will require you to add paving bases and shims to level and square.

If you need further assistance, please let us know.

Eric

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/129985#Comment_129985

Hi Eric,

In regards to the other options highlighted. Are these all seperate options? and would I require posts for the bearers or have these level so the bearers can sit straight on.

I assume by paving bases, its not just a paver but roadbase compacted then paver? do you have any illurstrations of these?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/135065#Comment_135065

Hi @JesseS6,

Yes, the pedestal feet, concrete foundation block and TuffBlocks are all different, but almost identical in their function.

It's usually best to add compacted road base as it is an easy way to ensure the pavers don't move and create a level surface for your pedestal feet, concrete foundation blocks or TuffBlocks to sit on.

Getting the foundation for your supports nice and level will make the supports level, which in turn makes your bearers level. Fine adjustments can then be made with packers on top of the pedestal, foundation block or TuffBlock and underneath the bearers.

Let me know if you have any further questions.

Jacob

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/138425#Comment_138425

@JacobZ Thanks for the response.

The road base would need to have a slight fall for drainage. Would using mortar under the pavers assist in keeping them level?

When you say packers, what are you referring to?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/140659#Comment_140659

Hi @JesseS6,

No, the road base would not need to have a slight fall for drainage. Road base, as a material, is porous, allowing water to drain through it, so there is no need to add a fall for drainage.

Yes, adding mortar under the pavers would help you to adjust them to level your pavers. You don't need to add mortar, but it can make the levelling process a bit easier.

When I said packers, I was referring to window packers. They are small pieces of high-density plastic that can be sat underneath your bearers to lift them that little bit to make things perfectly level.

Let me know if you have further questions.

Jacob

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/142298#Comment_142298

Perfect Thank you

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/142298#Comment_142298

Sorry @JacobZ ,

so would you compact the road base?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/143468#Comment_143468

Hi @JesseS6,

Yes, I would compact the road base, but I would compact it level. There's no need for a fall in the road base.

Jacob

JesseS6

Hi All,

You are probably sick of me asking deck related questions...

I am confused when it comes to building a low level deck over concrete and soil.

If I'm building a deck over concrete and soil, keeping appropiate space for airflow underneath when it is over the concrete. Can I put down compacted road base, and use tuffblocks to install bearers over the road base and extend the joists onto the bearers? Removing the need to dig down and install posts for the bearers to be fixed too?

or do I need to dig down to get a 400mm cleareance between the deck and ground?

Thanks Again!

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/53336#Comment_53336

Hi @JesseS6,

Absolutely, you can build your low-level deck using compacted road base and TuffBlocks without having to dig down and install posts — but whether that’s the right solution really depends on the relative heights of your concrete and soil.

If the soil and concrete are level, and you add road base directly on top of the soil, you’ll actually raise that area, reducing the clearance between your joists or bearers and the ground. That reduced airflow can trap moisture and lead to long-term issues like timber rot. In that case, you’d typically want to dig down about 100mm into the soil, then backfill it with compacted road base. That way, you’re creating a stable, level surface for your TuffBlocks while also maintaining airflow beneath the deck. The key here is clearance under the timber, not necessarily the total height of the deck.

While 400mm is a generous guideline, for low-level decks over well-drained, compacted surfaces, you can usually get away with around 200mm of clearance between the underside of your timber and the soil or gravel. If your current design only leaves about 100mm, it's definitely a good idea to dig some of that soil away and replace it with gravel — that keeps the timber from sitting too close to damp earth, which is what you're trying to avoid.

Also, make sure the areas where you place the TuffBlocks are very well compacted to avoid future movement. We hire compactors for this purpose.

Please let me know if you have any questions.

Mitchell

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/88858#Comment_88858

Thanks @MitchellMc very helpful.

The concrete is roughly 180mm below where I want the top of the deck to be, and the soil is probably 200mm below the concrete. I'm thinking of laying 100mm of road base, compact this down then place tuffblocks, bearers and then run the joists. might need to play around with the size of bearers and the soil depth to get everything to line up, but this should save some time from digging out soil to then dig holes for posts to get to 400mm

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/110166#Comment_110166

Hi @JesseS6,

It certainly sounds like a good plan that will minimise the amount of digging you'll be required to do.

If you are adjusting your bearer or joist size, just make sure you confirm everything fits within the boundaries of the relevant Bearer and Joist span tables.

I'm excited to see your project get underway.

Jacob

JesseS6

Hi,

I'm about to start building my deck, and wasn't sure if the joists and noggins should be fixed using screws or using a nail gun?

I've seen both and have no idea what should be used, and which screws, or what nails? I have a framing gun. Joists are just 90x45mm

any help would be amazing

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/53544#Comment_53544

Good Evening @JesseS6

Thinking back I used 4" nails... into hardwood lol I can remember the ding din din ding sound as they went into the predrilled holes. I dont think it matters really as long as they are secure.

Dave

EricL

https://www.workshop.bunnings.com.au/discussion/comment/53544#Comment_53544

Hello @JesseS6

I suggest having a look at this guide - How to build a deck by @MitchellMc. He used Titan 14G x 125mm CL3 Gal Bugle Batten Screw - 100 Pack for the framework. You can use the same screws for the noggins. My best advice is to drill deep pilot holes to prevent snapping the heads of the screws.

If you need further assistance, please let us know.

Eric

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