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How to build a low-level deck over concrete?

Hi!

Only discovered this community the other day - what a great resource!

I will be building a deck under my pergola/undercover area and it will sit on top of the concrete. There is probably 120mm between the concrete and the entry to the house so I was going to only put joists on the concrete.

I have a few questions...

- what is the best way to attach the joists to the concrete? I know you can buy those tuff blocks but the cost of those will add up. Do I just use an L bracket and some dynabolts?

- how far off the ground do I need to pack it?

- do I still flaah the joists even if it's undercover?

- the concrete will be on a slight slant. Do I just use bigger packers on the spots that are falling down?

Many thanks in advance

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @Lockmc. It's sensational to have you join us, and many thanks for your question about installing a deck over concrete.

You've come to the right place, and dozens of community projects show exactly how to do this. I suggest you have a look through some of our low-level deck projects to get a good idea about how to go about this. Here's a great example: Low-level deck over concrete by @Backyardtradie.

In answer to your questions:

Typically our members use angle brackets that are Dynabolted into the concrete and screws into the side of the joists.

You'd raise your joists as high as possible so the decking board are flush with the door threshold. So, 19mm for decking and 90mm for the joist means that there will be an 11mm gap under the joist.

I presume that might be 'flash'? Flashing the joists might be unnecessary if there is no rain exposure.

As the concrete slopes, a larger gap will be under your joists. If this slope isn't too significant, then it shouldn't affect your ability to screw the angle bracket into the side of your joists.

Also, check out this helpful guide on How to build a low-level deck and our Inspiring low-level deck builds.

Please let me know if you have any questions.

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/28226#Comment_28226

Thanks for the response Mitchell. I have a few more questions if you don't mind.. Just trying to decide if this is a DYI job or if I need to get someone.

Would I use dynabolts and timber screws for anchoring the L bracket?

I just measured it then and have some definitive measurements:

There is 160mm from the concrete to the door sill.

There is a fall from one end of the deck to the other (the direction that the joists would run) of about 130mm.

How would I pack the low end of the deck to have it level?

And last question.. What is the regulation height before a step is needed?

I've uploaded a photo just for context.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/76678#Comment_76678

Hi @Lockmc,

That's correct that you would Dynabolt the bracket into the concrete and then screw it to the side of the joist. Or, you could bolt into the joist.

The same bracket method will work with 160mm versus 120mm, you'll just have 50mm clearance under the joists, and that's even better. More clearance equals more airflow. The more airflow, the less chance of decay. 160mm isn't quite enough room to increase the joist width to 140mm, and it will just about be resting on the ground, and that's not fantastic.

So if there is a fall of 130mm over the span, then the 50mm clearance under the joists turns into 180mm. This is too high to do with brackets, and you might have to look at adjustable pedestal feet down the far end. Their max height is 170mm, so you'd still require some plastic packets under the joist.

The regulation step height is a minimum of 130mm and a maximum of 225mm.

Please let me know if you have any questions.

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/102310#Comment_102310

Hi Mitchell.

Sorry for the late reply but here again!

Is there any alternative to the adjustable feet? At 16.50 each, these will add up very quick, rough calculations was heading upwards of 2k.

Ive seen these Klevaklip adjustable joist supports.

With a 90mm joist, it suggests the maximum height to the top of the joist can be 200mm (or a few mm over).

I measured again to top of joist and it appears to be bang on 200 (see attached). So should those be ok?

And if there are any sections that are slightly more, I'm sure I could pack it just a little bit or use some of the large builders edge feet?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/118282#Comment_118282

That sounds correct @Lockmc.

The other alternative would be angle brackets, as @Backyardtradie used in their Low-level deck over concrete. However, you previously mentioned it was 120mm from concrete to threshold, and brackets would suit that. If it's now 200mm and there is a 120mm gap under the joists, then brackets wouldn't really be a suitable option as they don't provide adequate lateral support.

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/127534#Comment_127534

Thanks Mitchell and sorry for the confusion with the heights.

Yes, it runs down to about 200 to top of joist.

So in this situation, is using feet or adjustable height the best? What would a chippy do (as opposed to a DIY weekend warrior) to account for the height difference? Put in concrete footings?

Another question around heights.. There s a gap between the brick sill and the the underside of the decking board. Should I just put in some packers?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/133609#Comment_133609

No need to apologise, I just didn't want you using the angle brackets like I'd previously mentioned @Lockmc.

The Klevaklip adjustable joist supports sound like they would work well and are similar to what a professional installer would use.

Packers would be the way to go for that last board. Please be aware that I believe, technically, a deck should finish before the house wall and not encroach over a brick sill. This has to do with waterproofing and the ability for excess water to be blown across the deck and onto the sill. A 'trim' piece to finish off the deck might be acceptable.

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/137370#Comment_137370

Thanks Mitchell, that makes sense and thanks for the patience.

Given the deck will be under cover (and the brick sill is right against the house so it never gets wet), I think it should be OK.

Another couple of questions if you don't mind!

You can see from this image that the concrete has been cut a little bit too short and my joist that was going to run straight along that brick sill. Because at the end there is no concrete to attach the support too, is it fine if the joist is moved over to the right so it can sit on the concrete? Just means the decking boards will over hand on the left by 100mm or so.

And final question!

To set up for the pool fence (existing one will be removed) , the pool fence installers said to put in triple joists. But the joists are running the wrong way.

Would I be best running joists the other way at that end and maybe use something different for supports so I can get 3 joists nice and close together? Or maybe just a different timber rather than 3 90x45s which I can talk to the fence guys about.

Hope that all makes sense!

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/139956#Comment_139956

It's not great to have unsupported decking boards @Lockmc. 50mm might be alright, but 100mm is pushing it.

You'll note the Klevaclip bolt is in the middle of the foot, so you should be able to get the bracket quite close to the brick sill. I'd imagine closer than 100mm. You could always bolt another joist onto the side of the last one to provide timber under the ends of the deck boards. The bracket extends up the side of the last joist, but you should be able to pack out the additional joist to accommodate.

As you've mentioned, I'd just switch direction at the end and run your joists as needed. Maybe splurge and use pedestal feet in that area.

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/141761#Comment_141761

Thanks Mitchell.

I think I confused you with my question about being close to the sill.

I don't see any issue in putting the joist close to that sill - but where it runs off to the end, the concrete has been taken out (see my image below where the arrow ends).

So what I was saying - is it ok if the joist is moved to the right by 100mm or so, so that the far end of the joist (near the pool fence) will be supported on concrete?

Or is the best to get some concrete put into that section that was taken out?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/143120#Comment_143120

Oh, ok, @Lockmc. You're going to just move the issue then, aren't you? Currently, the decking would be overhanging at the end where there is no concrete, but if you move the joist back, the decking will be overhanging the whole way along. Regardless, the same information applies, as I stated earlier. A double joist will need to be used. I've created the rendering for you below to illustrate. Does that look like it would work?

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/144075#Comment_144075

Perfect! That is what I am asking, and that looks like it will solve it. Thankyou for all the help 🙂

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/144798#Comment_144798

All good @Lockmc! Happy to assist!

Mitchell

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/145352#Comment_145352

Hi @MitchellMc.

Another few question if you don't mind.

The triple joist that will run length ways (the opposite way to the joists) that the pool fence will sit on... This will be at the lowest point. I was going to attach the 3 of them together with some batten screws. Using those KleverKlip adjustable hangers, I don't see any way that I can use them for that triple joist. Any thoughts on how the triple joist can be positioned at the right height? Perhaps just use some timber that comes high enough and use L brackets and bolt to the slab?

And one last one. The board that will run along the stacker door sill... This will be running along the brick sill (but about 20mm higher).

Hoe do I attach that board as the joists won't be running over the brick sill? Picture below might help explain what I mean.

Or do I have to run the joist over the brick but notch it out so part of the joist runs over the brick sill?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/145781#Comment_145781

Hello @Lockmc

I'm sorry, but can you please explain further when you mentioned the triple joist? At this particular point of your planning stage, I suggest drawing up a decking frame plan so that you can address these type of assembly issues. The direction of your frame will dictate the direction of your deking panels. By finding out your panel direction you can then sort out the installation issue with the KlevaKlip 88mm Nylon ModWood Decking Concealed Fixing Clip.

I also suggest thinking about increasing the height of the decking frame so that it lines up with the panel that you'll be installing on the angled brick. Since there is no timber to anchor onto, I suggest cutting a timber wedge to go underneath the decking panel and you can anchor it onto the brick with masonry fixings. The tricky part is to line up the panel with your actual decking and have the correct gap at the same time.

Once you've drawn up your plan, I suggest sharing it with community. You can then post questions about the framework and other installation concerns you might have.

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

Eric

Lockmc

https://www.workshop.bunnings.com.au/discussion/comment/146107#Comment_146107

Hi Eric.

I've drawn the attached.. Hopefully it makes some sense.

The triple joists were requested by the pool fence installers to hold the pool glass and are at the lowest point of the deck (top of the below diagram).

My question is that because I am using the joist hangers, they work fine for a single joist but having 3 batten screwed to each other...those hangers won't work with 3 joists bolter next to each other. So I am wondering what other option I have for raising the triple joist to the right height.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/146381#Comment_146381

Hi @Lockmc

Thank you very much for posting your deck framing plan. In regards to the triple joist that will be sandwiched together, I suggest using a nut and bolt assembly to bind them together. I also recommend putting them in a staggard pattern so that the hold on the timber will be spread out.

As for the footing, I recommend using Builders Edge 70 - 170mm Bigfoot Xl Pedestal Feet. However, if there is not enough space for the pedestal foot, I recommend using 100 x 100mm Post H4 Treated Pine Wet CCA 1.8m dug into the soil to provide proper footing.

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

Eric

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