How to build deck over concrete and soil?

[JesseS6]

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.

[JesseS6]

Perfect Thank you

[JesseS6]

Sorry @JacobZ ,

so would you compact the road base? 

[JacobZ]

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]

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]

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]

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]

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]

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