Low-level deck on concrete slab

Hello there. I was doing some research around DIY decking over concrete and stumbled across this forum. What a lovely thread. Many thanks for some of the tips here which I find very useful. I still have few questions in my mind which I am hopeful that the community here will be able to help me with some guidance in the right direction.

I'm planning to put a 7m X 4m deck over my concrete alfresco. I will be using a Merbau deck board and H3 treated pine for joist & bearers with joist placed every 450mm and bearers 2m apart.The maximum height for me to work with is 135mm. If I use 70x35mm T/P, the total working height of the deck board (19mm) + Joist (70mm) + bearer (35mm) will come to 124mm. That leaves me with 11mm for the spacers below bearer and for the flashing below deck board.

Q1. Will that spacing (~11mm) below bearer be enough?

Q2. Do I need to seal the concrete before putting the frame?

Q3. I'm not planning to attach the deck to the brick wall or to the concrete floor as there is a pillar in the middle which I am expecting to hold the deck in its place. Does that approach sound right?

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

Find more posts tagged with

Decking and Paving

Building and Construction

Comments

MitchellMc

Hi @Jithesh,

I'm glad you have found this discussion useful. You might also like to check out this tutorial: How to build a low-level deck by @Adam_W

You'll find as this is a low-level deck many people do away with a joist over bearer system and simply use joists. I would suggest Treated Pine Outdoor Timber Framing 90 x 45mm spaced at 450mm. That way your height of the joist and board will be 109mm leaving you with 26mm clearance underneath for air-flow and drainage. You'll need to pack out under the joists with shims to establish this gap. You can then Dynabolt the joists down with brackets and I'd suggest one for every meter of the joist.

You don't need to seal the concrete before laying the frame.

Traditionally, decks are attached to the substrate. Timber is a natural product and it can behave in unusual ways due to its tendency to absorb water and then dry out. Fixing a deck down minimises the risk of the timber bowing, crooking, kinking, cupping and twisting.

We look forward to hearing more about your decking project and would encourage you to let us know if you need further assistance or would like to share it with us.

Mitchell

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/10619#Comment_10619

Thanks @MitchellMc .

I have gone through the tutorial How to build a low-level deck.

In the article they recommend using a bearer bigger than joist. Am I OK to use the same 90x45mm T/P for my Joist & Bearer? If I dynabolt the joists to ground using the L brackets, do I need to use them on both side of joists? I'm also considering options using noggings every 1m instead of dynabolting the joists to ground. I will also use double joists on the sides for added strength.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/67675#Comment_67675

@Jithesh,

90 x 45 treated pine is fine for the joists and bearers. In this low-level deck, you are not using the bearers in the traditional sense of bearing weight. They are simply finishing off the end of the joists and the L brackets are bearing the weight.

You'll only need to use the L brackets on one side of the joist. However, I would advise alternating the side of the joist that you fix them on. In your plan, I would put the two end brackets on the left-hand side of the joist and the middle bracket on the right-hand side. This will help stabilise the joist and stop it trying to pull towards one side.

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

Mitchell

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/96677#Comment_96677

@MitchellMc Thanks for your reply

"In your plan, I would put the two end brackets on the left-hand side of the joist and the middle bracket on the right-hand side. This will help stabilise the joist and stop it trying to pull towards one side."

Did you mean to use the two L brackets on the left side end of the joist to bearer and the joist bracket on the right side? Which L brackets you recommend here?

Apologies for too many questions. This is my first DIY work with timber, making sure I get it right. 🙂

Jason

https://www.workshop.bunnings.com.au/discussion/comment/114973#Comment_114973

Hi @Jithesh,

Please don't apologise for asking questions, that's what we are here for.

@MitchellMc will be back on the site on Friday and will be more than happy to assist you further. If you need urgent assistance in the meantime, please let us know and I can tag other helpful community members who might like to help with your deck.

Jason

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/125304#Comment_125304

Thanks @Jason .

Few more questions regarding tools & materials:

I will be buying this Ozito drill & driver set, and to use this to drive the timber-connector-screw to the joist-hangers , can you suggest which bit to use?
Which Dynabolt to use with angle-bracket on concrete and joist?
Do I need to flashing over joist?
The above Ozito drill works on concrete?
I have created a table with list of items. Did I miss anything here ?

Name	I/N	Qty	Price	Total
Frame
Treated Pine Outdoor Timber Framing 90 x 45mm @4m	8032162	80	$5.29	$423.20
Treated Pine Outdoor Timber Framing 90 x 45mm @7m	8032162	21	$5.29	$111.09
Pryda Joist Hanger Suits 45 x 90mm	1071668	50	$1.80	$90.00
Pryda Timber Connector Screw 12G x 35mm Hex Head Box 250	0084753	1	$57.00	$57.00
Carinya Heavy Duty 75 x 75 x 40 x 3.5mm Zinc Plated Angle Bracket	0046917	70	$2.90	$203.00
Fastener (Dynabolt) for angle bracket?		150
Macsim 75mm 600 Piece Mixed Window Packers	0164936	1	$39.90	$39.90
TWA Woodcare 300g Ecoseal Tanalised Timber Treatment	0960107	1	$15.90	$15.90
Flashing over joist?? (& nails for the same)

Deck
SpecRite 140 x 19mm 2.25m FJ Merbau Decking - 3 Pack	0064532	35	$64	$2,240.00
SpecRite 90 x 19mm 2.25m FJ Merbau Decking - 5 Pack	0064530	2	$58	$116.00
The Deckhand Easifit Template Guide - 2 Pack	2410303	1	$19.50	$19.50
Macsim 10G-10 x 65mm 304 Stainless Steel Torx Decking Screws - 500 Pack	2400118	2	59	$118.00
OR
Zenith 12G x 65mm Stainless Steel Self Drilling Decking Screw - 500 Pack	2400044	2	$150	$300.00

Tools
Ozito PXC 18V Drill and Impact Driver Kit	6290524	1	$169	$169.00
bit for Timber connector screw?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/132233#Comment_132233

Hi @Jithesh,

I apologise for the delay in my reply.

I would advise alternating the side in which you place the L brackets as per the picture below. This L bracket connects the joists to the concrete. You have them illustrated as the green marks on your plan.

The Pryda Timber Connector Screw 12G x 35mm Hex Head is driven in with a 5/16 Nutsetter.

The Ozito PXC 18V Drill and Impact Driver Kit does not include the hammer function required for drilling into concrete. A suitable cordless alternative would be the Ozito PXC 18V Hammer Drill Kit.

You might like to consider purchasing a corded hammer drill for this project as they are far superior at drilling into concrete than the Ozito cordless variety. The combination of the budget-friendly XU1 500W 13mm Hammer Drill for the concrete and the Ozito PXC 18V Drill and Impact Driver Kit for the screws would be a good choice.

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

Mitchell

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/136455#Comment_136455

Thanks for your reply @MitchellMc .

It makes lot of sense now. I have couple more queries before I finalise the plan & head over to Bunnings.

Is flashing required over joist?
I'm worried that its bit too much work to Dynabolt on concrete (haven't done anything on concrete before) & I might mess it up. What is your opinion on skipping that part and using noggings to keep the joists in its place? I will still be using shims below joists for drainage.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/139361#Comment_139361

@Jithesh,

Flashing is not required over joists. There is the Protectadeck PVC Joist system which protects joists from severe rotting and effectively seals around nails beneath decks.

I have not heard of nor do I have experience with decks that are not fixed down. Unfortunately, I don't believe there have been enough examples of people constructing floating decks to format an opinion on whether it will work.

Let me mention @ProjectPete, @Adam_W and @Brad to see if they have opinions on whether you need to fix a low-level deck to the substrate.

Mitchell

Brad

https://www.workshop.bunnings.com.au/discussion/comment/141310#Comment_141310

I am in the fix it down camp.

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/142773#Comment_142773

Thanks @Brad and @MitchellMc . Goong with your advice, I have decided to dynabolt the frame down to concrete. What dynabolt can I use with carinya-heavy-duty-75-x-75-x-40-x-3-5mm-zinc-plated-angle-bracket_p0046917 and the drill bit for that please? I will use coach screw for the joist.

Is sleeve anchors recommended to use on concrete?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/143810#Comment_143810

Hi @Jithesh,

I believe Ramset 10 x 75mm Dynabolt Plus Hex Nut Bolts would be adequate for fixing the brackets down. Any sleeve anchor in those sizes would suit. A Sutton Tools 10 x 120mm SF TCT Masonry Drill Bit would be suitable for drilling into the concrete.

Please let me know if you have further questions.

Mitchell

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/144602#Comment_144602

Hi @MitchellMc . I have started the working on the frame and here is the progress photos. Will continue on the frame over weekend and hoping that the deck will be ready in couple of weekends.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/145185#Comment_145185

Hi @Jithesh,

That's looking fantastic! Make sure you take plenty of images as you progress as I trust our members will be interested in following along.

Many thanks for sharing your update. Please let us know if you need any assistance.

Mitchell

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/145646#Comment_145646

Finally the deck is ready!!! It took 3 full weekends to complete it. Can't be more happier with the outcome. Now on to oiling part.

Question: Do I need to weather the deck or use Intergrain UltraPrep Tannin And Oil Remover and the deck clean & apply the oil straight away? My son's birthday is coming and I am hoping to have a little party there.

PXL_20201115_095507005.NIGHT.jpg PXL_20201115_095709312.NIGHT.jpg

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/146000#Comment_146000

Hi @Jithesh,

That looks amazing! You should be very proud of your efforts.

For a new deck, you'll need to use the Intergrain UltraPrep Tannin And Oil Remover and once you have completed that process you then use Intergrain timber cleaner. After the two products have been used to their specifications you can coat the deck once it has dried.

Congratulations on your result and please let me know if you have further questions.

Mitchell

Ari

https://www.workshop.bunnings.com.au/discussion/comment/146000#Comment_146000

@Jithesh Thanks for sharing your plans and photos. Your deck looks great. I'm working up the courage to build a 5.7 x 5.2m low level deck over uneven pavers, and reading your posts was really helpful. Thanks again.

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/146292#Comment_146292

Thanks @Ari . Glad that I that you found my post useful. Good luck on your project.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/146533#Comment_146533

Hi @Ari,

You might like to consider using Builders Edge Pedestal Feet as they are adjustable to suit your uneven pavers.

Please let us know if you need any guidance with building your deck. We're here to help.

Mitchell

karim

https://www.workshop.bunnings.com.au/discussion/comment/146000#Comment_146000

@Jithesh , thanks for your post. Very helpful. I am planning similar to what you have done. The only difference is mine will be half over concrete and half over the soil. Keen to see the photo near the sliding doors. Also, keen to know how much space left from the fence?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/146295#Comment_146295

Welcome to the Bunnings Workshop community @karim. It's fantastic that you've joined us, and many thanks for jumping into the conversation.

I trust @Jithesh will have some helpful advice for your project. Please feel free to start your own discussion so we can delve further into your project and answer any questions you might have about it. I trust you'll find loads of inspiration just like this project within the community as our amazing members contribute their projects here every day.

I'll be looking forward to hearing more about your project and would encourage you to share it with us.

Mitchell

ChrisNocker

https://www.workshop.bunnings.com.au/discussion/comment/132233#Comment_132233

Great work on the list of materials and tools! It's good to know what you're going to be up for and you can see if you've missed anything 😉

Just remember with the timber lengths that you can't buy a lenth that is exactly 4m long (probably 4.2m or 4.8m is the closest and depending on stock availability). And also with the 7m length, timber does not come that long (probably 5.4m or 6m) so you'll have to buy two lengths and join them in some way. If it were me I'd join them using a half lap joint and coach bolts, making sure the joint is close to the bearer/support point below the joist.

Best of luck.

Edit: My comment about the lengths doesn't have as much relivance now that I see there is more than 1 page to this thread... dough. Looks great though.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/136526#Comment_136526

Welcome to the Bunnings Workshop community @ChrisNocker. It's fantastic that you've joined us, and many thanks for contributing to the discussion.

Available timber lengths need to be accounted for in the design stage and can add to the cost of a project if there is going to be significant wastage. It's always something to consider when budgeting. Your suggested method of joining lengths is also a great option.

Many thanks for sharing your knowledge, and we look forward to hearing all about your projects around the house and garden. We'd encourage you to let us know if you ever need assistance or have something to share with the community. I trust you'll find loads of inspiration for your projects here as our amazing members contribute their ideas here daily.

Mitchell

aiden

https://www.workshop.bunnings.com.au/discussion/comment/143810#Comment_143810

Fantastic forum! Cant believe I've only just found it.

I'm in the process of doing a low ground deck over a concrete slab too. Seeing your questions relate to my situation 100%.

My main question is this: will bolting your treated pine directly onto the concrete increase the chances of rot?

My deck will be directly in front of my front door and only slightly shaded from the elements. How can I keep a low deck while also leaving room for airflow and avoid the timber touching the concrete slab directly?

Thanks in advance 🙂 Aiden.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/144626#Comment_144626

Welcome to the Bunnings Workshop community @aiden. It's terrific that you've joined us, and many thanks for your question.

You must leave as much space for airflow and drainage under your timber as possible. Bolting any timber that is not rated H4 and above directly onto concrete will reduce its lifespan. If a gap is not left, moisture will constantly be trapped, which starts the rotting process. Deck structures are commonly constructed from H3 outdoor framing timber, which is suitable for aboveground use. It is not suitable for in-ground or in direct contact with the ground.

You might like to start your own discussion so we can cover your project more extensively. If you include some area images, a sketch of your deck plan and details on the height between the concrete slab and the door sill, we can further discuss what options are available.

Mitchell

aiden

https://www.workshop.bunnings.com.au/discussion/comment/145204#Comment_145204

Thanks for the prompt response. I'll go ahead and try to take some pictures later on for a separate thread. I just thought that this was so similar I almost didn't need a separate thread.

From the deck example in this thread, did you use H4 treated pine directly on top of the concrete slab? Thanks 🙂

MitchellMc

@aiden,

I believe @Jithesh use H3 treated pine attached to the concrete slab with L brackets which allowed a gap under them.

Mitchell

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/146295#Comment_146295

@karim - Sorry for the late response. I have built just over the concrete. My concrete ends 20cm off fence and so is my deck. It is flush on level near the alfresco door.

Jithesh

https://www.workshop.bunnings.com.au/discussion/comment/145660#Comment_145660

Hi @aiden I have used H3 treated pine over the concrete as joists and attached to the concrete with L angle brackets as @MitchellMc mentioned already.

BAditya

Hi Jitesh i am planning to do DIY decking need your support

Child Item