How to build low-level decking over concrete slab?

[Sujoy]

Hi All, 

Newbie here and started doing small DIY stuff like making merbau planter boxes :). Now I am planning to start my decking and found this forum extremely helpful gaining the basic knowledge about decking, especially guidance from @MitchellMc in similarly low level decking threads. Here is my project details (please refer the photo attached -

• Total area - 6.3mx4m 
• Clearence at Point#A:140mm, house slab sloping towards Point#B:150mm and then concrete apron sloping another 40mm, so total clearance at C is 190mm. 
• Joists will be as shown in the photo and bearers just connecting end of the joists at A and C. 
• Will attach the bearer to brick wall A with 10mm gap (using shims) and planning to joist hanger and L-brackets to hold the joists. 
• Have granite pebbles in D, planning to use pavers + EcoDeck Bigfoot in that area to hold the joists. 
• Will be using 90x45mm H3 pine for bearer and joists and 140x19mm merbau, so total height for joist + merbau is 109mm. 

Questions: 

1. Ground clearance in A is 31mm (140 - 109), B is 41mm (150-109) and C is 81mm (190-109). Based on this, is L-bracket a good option? If yes which L-bracket should I choose? 

2. With L-backets, I probably have to use shims underneath the joists, isn't 81mm too high for using shims? what are the other options I got? Looking for options other than EcoDek foot (planning only to use it in the pebbles area D). 

3. Need help with detail specs of dynabolts, L-brackets etc. suitable for the above project. 

Many thanks :). 

[MitchellMc]

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
 

[Sujoy]

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

[Sujoy]

@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

[MitchellMc]

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 Deck... are suitable for the decking.

Mitchell

[Sujoy]

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]

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

[Dave-1]

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]

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

[Sujoy]

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?