How to build a low-level deck over existing concrete?

[benny4]

Hi I’m looking at starting a low level timber deck in a recently built house but just need some advice if I’m on the right track or someone to steer me in the right direction.

I’m thinking single height subfloor and will be built on existing slab and on ground. I have 160mm height to work with on the alfresco slab but can dig out some additional ground to get more clearance. Currently this is what I’m proposing.

- 120x45 wailer bolted into brick work

- 2/120x45 bearers (pink lines) supported by adjustable feet (on concrete) and posts in ground. I was going to span approximately 1600mm.

- 120x45 joists @ 450 centres in between bearers fixed with joist hangers. Max span 1500mm

- 140x19 spotted gum decking with 5mm spacing. Deck direction North/South looking at my sketch

- I will set deck out for a picture frame boarder

- On the natural ground laying approximately 50mm of stone to help with drainage.

Am I on the right track? Im not 100% sure on the spans and also the section of the deck on my sketch top left there will be a 600-700 cantilever I’m not sure will work.

I have also seen frames set up with just joists at 450 centres supported by feet and posts with noggins nailed in between.

I hope to get a start soon so thanks to anyone who replies!

   

 

 

 

[EricL]

Hello @benny4 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your decking.

Doing a bit of quick math 120mm plus the thickness of the decking panel which is 19mm will give you a total of 139mm. This means that you'll have a gap of 21mm underneath your decking frame. Please let me know if I understood your plan correctly. Have you considered using 90x45 for your decking frame? You will technically be using more timber pieces, but the gap underneath your deck turns to 51mm which is much better for airflow.

I suggest having a quick look at this discussion - Low-level composite deck over concrete and soil by @oninpena. It's an excellent example of a deck that uses 90x45 and mixed footing for the decking frame.

In regards to your questions, I propose building an independent/free standing deck with no attachments to your brick wall. In this manner it will not put any load or stress on your brick wall. To answer the rest of your queries, can I please request that you finish your decking plan with joists included. Can I also please ask you to put your measurements for the proposed footing locations. To help with your planning, I suggest having a look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc. These guides will give you very good ideas on what to take into consideration while planning your decking frame. 

I also recommend having a look at the Bunnings deck building guide:

• How to plan out your deck
   
• How to set out a deck
   
• How to install stumps
   
• How to install bearers
   
• How to install joists
   
• How to lay decking
   
• How to add a step to your deck.

If you have any other questions we can help with, please let us know.

Eric

[benny4]

Hi @EricL thanks for getting back to me!

I can change to 90x45 to have greater clearance. The low clearance was over the alfresco slab and under cover so I thought maybe I could get away with the 120x45 but I’ll change that and I will look at the L brackets so the deck isn’t attached to the house.

Do you have information on how far I can span 90x45? If I get that info then I can work out my supports.

I did come across that project from oninpena but I was unsure about the footings on the natural ground. Is I had compacted crushed rock with concrete blocks or pavers would that be sufficient?

Thanks!

[JacobZ]

Hi @benny4,

Following the Hyne Deck Joist Span Table, 90 x 45mm H3 Treated Pine joists at 450mm centres can span 1400mm between supports in a single span, and 1800mm in a continuous span. Let me know if you need some assistance working out your supports, I'm happy to help.

Compacted drainage gravel with pavers on top would be fine over the soil.

Let me know what you think, and if you've got any other questions or need some clarification, don't hesitate to ask.

Jacob

[benny4]

Hi @JacobZ thanks for the reply that will help me work out my supports. I’ll draw up a new plan based on all this info.

One more thing how are the joists fixed to the L brackets. Is it screws or bolts?

[EricL]

Hello @benny4 

It's great you've received excellent advice from @JacobZ. Oninpena asked us about the footing positions at the time, and we actually suggested placing the footing to 1-meter to prevent getting a bouncy deck. At 1-meter distances the joist is very secure and stable. You'll also notice that the angle bracket footings were placed on opposite sides of the timber to provide even better stability. 

For the soil section we suggested using pavers over the compacted soil and then the pedestal footings to go on top of them.

If you need further assistance, please let us know.

Eric

[JacobZ]

Hi @benny4,

Bolts are the way to go. Bolts are often used in conjunction with a nut and washer, which helps distribute the load more evenly across the surfaces being fastened. The nut adds tension and creates a clamp that holds the parts together securely, resisting forces like shear and tension better than a screw.

When you grab your brackets, take one with you to the bolts and find the thickest bolt that will work with your chosen bracket. This will ensure the load is supported by the thickest bolt possible.

You should also grab a flat washer to go with your nut and bolt to ensure the load is spread out as evenly as possible.

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

Jacob

[benny4]

Okay so this is an updated sketch. Rough and not to scale but the basic are getting there I think. I’ll add in more framing on parts of the perimeter because I would like to make a picture frame decking. I think I can start setting it out and preparing the area over ground now. Thanks for your help so far @JacobZ @EricL im sure I’ll be touching base again with more questions as I progress

[EricL]

Hi @benny4 

The plan looks good. Please keep us updated with your progress, we look forward to seeing the decking frame built.

Eric

[benny4]

Hi I’m starting the frame this weekend (finally). Another detail I’m curious about is what spacing should I have the decking off the bricks of the house. I was going to do 5mm gap everywhere for 135x19 spotted gum but have also read 10mm should be the gap off existing brickwork.

Also can any point me to what I need to use to seal the ends of cut decking boards? Or can I use the stain I plan to use on the rest of the deck?