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Low level decking over concrete

Hi everyone,

Firstly, sincere apologies for posting another low-level decking post. I was unable to find some details I was looking for. It's my first ever decking project and pretty much new to tools and products, please never mind if I don't use the correct wordings.

Here we go,

I am trying to build low-cost decking over an existing concrete slab. The height between the slab and the finish floor levels (FFL) is about 120mm. My first question is;

Is it a good idea to level the deck with FFL? Any issues/complications this will have in the future?

I am thinking of using "70 x 35mm MGP10" as for underneath timber with Hot dip galv. angle bracket for concrete fixings with about 10mm clearance between the slab and the frame mainly for water drain and air/flow.

Is 10mm clearance good enough?

What sort of bolts shall I use for angle bracket fixing to concrete?

Also, see a detailed timber layout for comments and suggestions on spacing and etc.

Community manager's note: Check out How to build a low-level deck for expert advice.

Timber Frame.JPG

I will be finishing it off with SpecRite 90 x 19mm 2.25m FJ Merbau.

Section detail.JPG

A quick mock-up of how it will look like if everything goes smoothly;

Overall GA.JPG

Current situation;

Thanks in advance 🙂

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Decking and Paving

Comments

MitchellMc

Welcome to the Bunnings Workshop community @Fadi. Many thanks for your questions, I'd be happy to assist.

It's great to have you join us and we look forward to following along with your project. There is no specific issue with finishing the deck flush with existing levels. Waterproof flashing should be used when butting up against the wall of the house. You will need to use treated pine and Treated Pine Outdoor Timber Framing 90 x 45mm would be suitable if leaving a gap for water drainage. When attaching brackets, alternate the sides they are attached to. You'll be attaching the brackets to the concrete with Dyna bolts.

Please let me know if you need further assistance or had questions.

Mitchell

Stuardo

Hi @Fadi, we all love the idea of a deck and low-level decks are a constant question on the site. Please DON'T use the timber you suggested as it is untreated and suitable for internal use only. At a minimum over concrete you would need H3 treated timber and H4 is better. Please refer below information copied from a Queensland Government Guide for H Code (Hazzard) treated timber.

H3 - The minimum level of preservative treatment required to protect the timber from attack by decay fungi and insects, including termites. H3 preservative treatment offers protection in situations above-ground, exposed to the weather or periodic wetting. Depending on the application, preservative treatment to provide protection in H3 conditions can involve one of two levels. The lower retention level must not be used in a load-bearing application where routine inspection and maintenance cannot be readily carried out.

H4 - The minimum level of preservative treatment required to protect the timber from attack by insects, including termites, and severe decay. H4 treatments are suitable for use in situations where the timber is in contact with the ground or is continually damp. H4 is appropriate for less critical ‘non-structural’ applications and/or where the decay hazard zone indicates a lower in-ground hazard.

As for the structural side of your deck I will defer to the professional builders on this site. I hope your deck ends up to be a thing of beauty that you will enjoy and be proud of for a long time.

Stuart.

Fadi

https://www.workshop.bunnings.com.au/discussion/comment/9070#Comment_9070

Thanks @MitchellMc for your prompt response really appreciate that.

I will take your suggestion on board and will be going to install Treated pine timber. However, I am still wondering if I can go with the 70x35 rather 90x45 framing option as I don't want the house floor and the decking floor to be exactly flush.

Also, you have mentioned that I will need to use waterproof flashing all around where it connects with the wall. Can you please advise what sort of product I can use here.

Many Thanks!

Fadi

https://www.workshop.bunnings.com.au/discussion/comment/9071#Comment_9071

Thanks @Stuardo for your valuable input many many thanks for that!

I will definitely be taking your suggestion on board.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/66798#Comment_66798

Hi @Fadi,

You can certainly go with the 70 x 35mm treated timber, I've only recommended 90 x 45mm to ensure there is no sponginess. Thinner calibre timbers can potentially bend under load.

If the timber is to be butted hard up against the house I would recommend Protectadeck in between the two. The best-case scenario is that you just leave a small gap and not have the timber hard against the wall. Anytime timber is placed against the wall it creates a point where rot or moisture penetration can occur.

Mitchell

rahil_xyz

https://www.workshop.bunnings.com.au/discussion/comment/9070#Comment_9070

@MitchellMcHi Mitchell, how are you.

with alternating the sides the bracket is attached to, do you mean alternate each bracket on the same joist?

or have brackets on 1 joist facing the same way and the next joist facing the opposite way?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/67086#Comment_67086

Hi @rahil_xyz,

I'm great, thanks for asking.

Yes, I would suggest alternating the side on which the brackets are placed on the same joist. The image below illustrates what I mean. Although not a necessity, I believe it ensures the joist stays at 90 degrees to the ground. You'll find if all brackets are on one side it can skew the joist in a certain direction especially if the ground is not perfectly flat.

Please let me know if you need further information or had questions.

Mitchell

pete_brig

https://www.workshop.bunnings.com.au/discussion/comment/96351#Comment_96351

Hi @Fadi

Have U started your "deck"? I only just saw your post &have a few comments that might b helpful, if U haven't started

Cheers Peter

rahil_xyz

https://www.workshop.bunnings.com.au/discussion/comment/96351#Comment_96351

Any tips on how to fill the gap where the raised brick area is. If my joists ends at the brickwork and the merbau sits on top until where the brickwork starts. There will be a small gap left between the door sills and the merbau. My deck will be at level with the floor. And the gap hieght ( from top of brick work to floor hieght)

is 60 mm .

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/114777#Comment_114777

Hi @rahil_xyz,

It will depend on what orientation you decking boards are laid. Are they running parallel to this doorway or perpendicular to it? If they are running parallel to it then you might want to start your first board there and offset it and have it hanging 10 mm off the side of the ledger board/bearer to close the gap. If they are running perpendicular you could do the same and have the boards slightly overhanging and butted up closer to the door sill.

Bear in mind that you need some type of gap here to allow airflow and prevent moisture buildup inducing timber decay.

I'd encourage you to start your own discussion about your deck that way we can delve further into your questions in more detail.

Please let me know if you needed further information or had any questions.

Mitchell

th-DIYER

Hey @MitchellMc and @Fadi when securing the angle bracket to timber what are you using? A nut and bolt or screws...? I’m building a small 1000 x800 timber platform/step on a concrete slab.
Thanks in advance.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/13738#Comment_13738

Hi @th-DIYER,

You could use Zenith 14G x 50mm Galvanised Bugle Head Batten Timber Screws or for an extended lifespan Zenith 14 - 10 x 50mm Stainless Steel Bugle Head Batten Timber Screws. You can also bolt the joists to the bracket, but this could be a time-consuming process depending on how many you are doing. If you were to go the bolt route, I'd highly recommend you purchase an attachment for your drill that will allow you to fit sockets to it. You can then drive all the nuts onto the bolts with a drill.

Please let me know if you have any questions.

Mitchell

Fatsquare

https://www.workshop.bunnings.com.au/discussion/comment/69393#Comment_69393

Hi, can I post my questions here because I am also going to diy low level decking on the concrete slab. But the clearance height is 60mm only.. any idea to which H3 treated pine is situable for this job. 70mm*45mm? Just want to confirm that the nails are using to connect the joists ( I am using 70mm*45mm H3 treated pine, 45mm is the height, not 70mm, because it is really low.), Right? I don't have nail gun( mine is electric nail gun which 35mm maximum length which is not suitable for this job) , so do you have any idea which size of nail is perfect for this job? And do I make the whole frame first( it may be easy for me to nail the joist by hammer), then secure the frame to the slab?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/99120#Comment_99120

Hi @Fatsquare

Thank you for sharing your question about your low-level deck project. I understand you want to use nails for your joists, but I suggest using Zenith 14G x 50mm Galvanised Bugle Head Batten Timber Screws instead. The screws will provide a greater amount of holding power due to the threads of the bolt. An ordinary nail when stressed will pull out much faster.

The assembly process you mentioned is one way to create your decking frame. However, I am concerned with your allotted space of 60mm. The 70 x 45, even when turned to the 45mm side and joined to the 19mm decking panel will give you an overall measurement of 64mm. It will be 4mm past the 60mm space which now becomes a tripping hazard.

I recommend re-assessing the space you have and perhaps considering either breaking up the surface of the concrete slab so that you will have more space or tiling it with a timber look profile on it.

If you need further assistance, please let us know.

Eric

Fatsquare

https://www.workshop.bunnings.com.au/discussion/comment/116348#Comment_116348

Thanks for information, Eric.

It maybe lots of job to break up the surface of concrete slab. I am now breaking up the surface of slate tile in my living room. It is a job I wll never want to do it again. I am exhausted , it is more than 20 years slate tile, uneven and tough to remove:)

It is OK for my decking. I have one easy option. The decking is 63mm(45mm+19mm) high. I put 10mm wide tile on the 60mm high area between the deck and the door. The tile will level the 60mm area to the same height as the door step. Tile color may be carefully considered.

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