How to build low-level decking over concrete slab?

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

For your setup, using L-brackets can be a good option where ground clearance is limited, such as Points A and B. When installing, aim to position the fasteners in the middle of the joist height, avoiding the top and bottom edges to prevent any splitting or weakening. Many DIYers go for heavy-duty galvanised brackets, which provide solid support and resist corrosion. A tip here would be to set up one joist at the required height first, then try a few L-bracket types to see what works best with your setup. Take a look at the Carinya 100 x 100 x 50 x 4.5mm Hot Dipped Galvanised Angle Bracket and Carinya 150 x 150 x 75 x 6mm Hot Dipped Galvanised Angle Bracket.

For Point C, pedestal feet would work much better here. Although they’re more of an investment, pedestal feet are highly adjustable, making them ideal for leveling decks over uneven ground or slopes. They’re sturdy and provide consistent support, especially over that height.

Regarding Dynabolts, a 70mm length should be adequate for attaching to masonry, and the thickness should match the hole in your chosen bracket. Ensure the bolts penetrate the concrete deeply enough for a secure hold.

For the brackets, try to avoid placing them near the edges of the concrete to prevent cracking.

Please let me know if you have any questions.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/44422#Comment_44422

Many thanks @MitchellMc ! Yes Big foot seems like way to go as I found the concrete apron has variable slope from 45mm to 25mm, was supposed to be 50mm but this is how it was poured seems like :).

I am preparing list of items I need to buy (e.g., dynabolts etc.) and will share. Would be great if you can have a look and suggest any changes.

Many thanks,

Sujoy

https://www.workshop.bunnings.com.au/discussion/comment/44422#Comment_44422

@MitchellMc I am summarising the items needed, can you please advise?

Angle brackets: Try these two and finalise Carinya 100 x 100 x 50 x 4.5mm Hot Dipped Galvanised Angle Bracket and Carinya 150 x 150 x 75 x 6mm Hot Dipped Galvanised Angle Bracket. Should be every 1 m (For each Joist - 3 angle brackets + 1 Bigfoot at the end near point C)
Dynabolts for fixing angle brackets to concrete: Use Ramset 10x75mm Dynabolt + Hex Nut bolt
Screws for attaching angle brackets to joists: Is M10x40mm galvanised screw okay? Do I need to pre-drill and then screw?
Brear attached to the brick (A-> B): Use 10mm shims between brick and bearer to allow for ventilation.
- Dynabolts - Which one should I choose between 10x75mm and 12x100mm?
- Angle brackets - Can I use Carinya 100x100x50x4.5 as mentioned above?
Joist hanger - I am happy to add this at both end of the joist if it makes the frame sturdier.
- Joist hanger - Pryda 45x90mm
- Nail or screw - I am preferring screw, please advise which one to use.
Shims - With above set up, do I need put shims/packers under the joists for added support?
Joists will be placed 450mm apart and I will be adding noggins as well.
Use Protectadeck Flexible PVC joist before laying the merbau.
Merbau - Will use 140mm x 19 mm laid from B -> A and use 12G x 50mm Stainless Steel Self-Drilling Decking Screws as highlighted by you in another thread. I can't find the one mentioned as the thread is 4 years old, can you please suggest which one to use?

Thanks again for all your help!

Regards,

Sujoy

https://www.workshop.bunnings.com.au/discussion/comment/84427#Comment_84427

Hi @Sujoy.

That all sounds about right. In answer to your questions, those screws will be fine for attaching the brackets to the joist. You will need to predrill first. A 12 x 100mm Dynabolt would be best for the bearer attached to the bricks. Using joist hangers is the correct way to hang joists. These connector screws can be used. The joist will be supported by the joist hangers and the angle brackets. As long as they are supported adequately, there is no need for packers. These Simpson Strong-Tie 12G (5.5mm) x 50mm 316 Stainless Steel DHSD No-PreDrill Hardwood To Softwood Decking Screws are suitable for the decking.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/107237#Comment_107237

Hi @MitchellMc,

Regarding using pedestal for resting joist over concrete apron in my decking project, can I replace it with below solution?
- Buy 100x100 H4 post and cut it into 50x100mm 1.8m pieces. Install these as bearer under the joists (only in concrete apron area)
- Use LOSP to coat the exposed cut surface.
- Use angle bracket or dynabolt to fix it on concrete apron. Install 10mm shim under it to allow water flow.
- This gives me 60mm high support to start with.
- Use packers between the joists and H4 bearer to level it.
Would this work?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/120880#Comment_120880

Hello @Sujoy

Between using the 90x45 and the 100x100, I would go with the 90x45 because of the gap that it provides. My other worry is that you'll be cutting into the 100x100 post and although its treated, exposing its centre core will weaken it even with the spray coating. The more space you have under your deck means better air circulation and no trapped moisture under the deck.

Let me call on our experienced members @Nailbag and @Dave-1 for their recommendations.

If you need further assistance, please let us know.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/129314#Comment_129314

Good Evening @Sujoy

I was just reading through the posts and I am impressed with your detail of knowledge 🙂 You are asking the right questions.

I agree with @EricL over its wiser to have air flow then not. Because you are right on the edge of the timber touching the concrete the more you can facilitate the air flow the better for the deck long term.

I have used concrete screws instead of dynabolts for the past few years and have found them so much eaier to use and also remove. With Dynabolts you may be left with the metal sleeve if you ever decide to chnge the deck area.

Dave

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/129314#Comment_129314

Hi @Sujoy as with @EricL recommendation, definitely stick with 90 x 45. It's a standard size, more cost effective and less work and wastage.

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/134651#Comment_134651

Thanks @Dave-1 @EricL . It’s all the posts and suggestions by you guys helping us to learn 😊.

Based on your suggestion I have just laid the bearer with shims under to give a rough idea what I am planning. Attached photo for reference, I am getting 35mm clearance from concrete and planning to level first and screw the shims under bearer before laying it. Let me know if this looks alright.

Few other questions-

1. Should I coat underside of the bearer with bitumen?
2. should I use these screws to fix the bearer to concrete?

https://www.workshop.bunnings.com.au/discussion/comment/138110#Comment_138110

Morning @Sujoy

Those concrete anchors are a little long 🙂 The deck is only low and wont have a lot of movement so id go with something shorter. either a 50mm or a 60mm anchor Ramset 8 x 60mm Galvanised AnkaScrew Masonry Anchor (box 10) Id be wanting 50mm into the concrete type deal.

With your bearer turned sideways I am not so keen. I know the span is short but my head baulks at the flex. How about taller styled right angled brackets and do away with the bearer entirely? Dunnings 60 x 100 x 35mm M10 Galvanised Angle Bracket

You can still attach the skirt to the ends of the joists. In reality you could use the bearer or the right angle. Im trying to think of a reason other then personal preference 🙂

Dave

https://www.workshop.bunnings.com.au/discussion/comment/138110#Comment_138110

What you're doing looks fine @Sujoy. As @Dave-1 mentioned, it is best to have the bearer in the upright configuration, though as long as you support it readily, there shouldn't be too much flex.

There's no particular need to coat the underside of the bearer with bitumen, as they are treated for above-ground use. However, if it comes in contact with the ground or is very close to it, you might like to.

Those screws look fine. I suspect you need the length to go through the bearer, the air gap and then into the concrete. Just ensure at least 30/40mm is screwed into the concrete.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/140446#Comment_140446

@MitchellMc @Dave-1 Thanks, I had no option but to turn the bearer sideways as total clearance was only 80mm. I first fixed the packers under the bearer and drilled 8mm hole through the gap between the packers and then screwed using the 8mm galavanised Masonry Ankascrew into the concrete. The screws are every 450mm, and has gone 45mm into the concrete making it very sturdy. Also added joist tape to avoid moisture accumulation on the top. Attached photo for reference, hope this is fine? The overall levelling seems to be alright, progressing well 😊.

Another question re the screw for attaching angle brackets to joists, is the screw shown in the 3rd photo ok to proceed with?

https://www.workshop.bunnings.com.au/discussion/comment/142150#Comment_142150

That's looking good @Sujoy. The screw you've pictured is suitable for attaching the brackets to the joists.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/143362#Comment_143362

Morning @Sujoy

My preference is a bolt 🙂 But yeah that screw should work.

(I like bolts as I always feel there is less chance of splitting and an overall stronger connection between pieces)

I know you probarly have but a pilot hole would be wise. 🙂

Dave

https://www.workshop.bunnings.com.au/discussion/comment/144271#Comment_144271

@MitchellMc @Dave-1 I am almost done with the decking, coming along nicely and very happy with it. Thanks to you guys for all your guidance 🙂.
there is a bit of glitch though at the end while laying the last merbau, I have only 60mm remaining but need to lay another 140mm as shown in the picture.

Can I add one more 90x45 bearer screwed to the first bearer? So that gives me 60mm + 45mm=105mm, 1 merbau fascia 19mm totalling to 124mm, this gives 21mm overhang for the last sheet.
I need some help to make that 45mm extension sturdy.

https://www.workshop.bunnings.com.au/discussion/comment/144938#Comment_144938

Evening @Sujoy

The deck is looking sweet as! 🙂 Love that term as it really encompasses so many things!

The deck is looking very professional.

Your solution is bang on what I would do to extend it that tiny bit more. Id paint it black before attaching so its super tidy.

Now here is a question for you... 🙂

Do you think you would have figured out how to impliment the "fix" if you didnt have the experience of building the deck? 🙂

You now have a huge amount of knowledge that you have learnt yourself!

Dave

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/144938#Comment_144938

Hi @Sujoy,

That deck is looking fantastic, nice work!

As @Dave-1 has said, adding that extra 90x45 is a perfect solution to your problem and will give you a nice little overhang that will look fantastic.

I'd suggest grabbing some ZENITH 14G - 8 x 75mm Class 3 Galvanised Bugle Batten Screws and using one every 600mm or so. This will give a nice sturdy connection between the new timber and the existing bearer.

Let me know if you've got any further questions.

I can't wait to see everything once it's finished.

Jacob

https://www.workshop.bunnings.com.au/discussion/comment/145462#Comment_145462

@JacobZ @MitchellMc @Dave-1

Finally done with the decking 😊!, took a break in between during holidays. Used 14G screws every 450mm as suggested by Jacob to attach the second bearer, seems to be holding strong. Finished the oiling with Intergrain Merbau decking oil after cleaning with tanin remover and Ultraprep. Added a screen using Ekodeck Castellated panel and Tuscan Garden screening. Overalll, very happy with the outcome considering my first DIY. Can’t thank you guys enough for all your help! Attaching some photo.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/145874#Comment_145874

Nice work @Sujoy, it has come up a treat.

The colour matches the red brick facade beautifully and the screens add a lovely bit of depth to the project.

Congratulations on your first D.I.Y. project. Hopefully, it will give you the confidence for many more to come.

Jacob

https://www.workshop.bunnings.com.au/discussion/comment/146187#Comment_146187

Afternoon @Sujoy

😀 Very nice indeed! Can see that smile on your face!

It looks great and I love the colouring!

Are you tempted to put the whole job into a Project? I would be 🙂 Def something to be proud of!

From the skills you started with to now 🙂 Oh yeah!!!

Dave