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Low-level deck on concrete slab

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Budding Contributor

Low-level deck on concrete slab

PXL_20201115_012759871.jpgHello 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?

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Bunnings Team Member
Bunnings Team Member

Re: Merbau Decking over concrete slab

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

 

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Budding Contributor

Re: Merbau Decking over concrete slab

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.

Deck_Frame.jpg

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Bunnings Team Member
Bunnings Team Member

Re: Merbau Decking over concrete slab

@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

 

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Budding Contributor

Re: Merbau Decking over concrete slab

@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. :smile:

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Community Manager
Community Manager

Re: Merbau Decking over concrete slab

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

 

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Budding Contributor

Re: Merbau Decking over concrete slab

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 ?

 

NameI/NQtyPriceTotal
Frame
Treated Pine Outdoor Timber Framing 90 x 45mm @4m803216280$5.29$423.20
Treated Pine Outdoor Timber Framing 90 x 45mm @7m803216221$5.29$111.09
Pryda Joist Hanger Suits 45 x 90mm107166850$1.80$90.00
Pryda Timber Connector Screw 12G x 35mm Hex Head Box 25000847531$57.00$57.00
Carinya Heavy Duty 75 x 75 x 40 x 3.5mm Zinc Plated Angle Bracket004691770$2.90$203.00
Fastener (Dynabolt) for angle bracket? 150  
Macsim 75mm 600 Piece Mixed Window Packers01649361$39.90$39.90
TWA Woodcare 300g Ecoseal Tanalised Timber Treatment09601071$15.90$15.90
Flashing over joist?? (& nails for the same)    
     
Deck
SpecRite 140 x 19mm 2.25m FJ Merbau Decking - 3 Pack006453235$64$2,240.00
SpecRite 90 x 19mm 2.25m FJ Merbau Decking - 5 Pack00645302$58$116.00
The Deckhand Easifit Template Guide - 2 Pack24103031$19.50$19.50
Macsim 10G-10 x 65mm 304 Stainless Steel Torx Decking Screws - 500 Pack2400118259$118.00
OR    
Zenith 12G x 65mm Stainless Steel Self Drilling Decking Screw - 500 Pack 24000442$150$300.00
     
Tools
Ozito PXC 18V Drill and Impact Driver Kit62905241$169$169.00
bit for Timber connector screw?    
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Bunnings Team Member
Bunnings Team Member

Re: Merbau Decking over concrete slab

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

 

 

 

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Budding Contributor

Re: Merbau Decking over concrete slab

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.

 

 

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Bunnings Team Member
Bunnings Team Member

Re: Merbau Decking over concrete slab

@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

 

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