Can you build a low-level deck with just 5cm clearance?

[nirmal1988]

Hi All,

I am thinking of decking my alfresco which is already concreted. My alfresco is 3m wide and 4.2 m long.

I have been reading various threads about building a low-level deck and they were all very helpful.

Some of them being:
https://www.workshop.bunnings.com.au/t5/How-To/How-to-build-a-low-level-deck/ba-p/45715

https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-Merbau-deck-over-concrete/ta-p/9...

https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-Merbau-deck-over-concrete-slab/t...

https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-deck-with-picture-framing/ta-p/7...

But, when I measured my clearance, it is hardly 4-5 CM from the concrete to the top of the brick below the alfresco door.
Is it even viable to build a deck in this area ? If yes, what type of framing timber should I use ?

See picture below.

See arrow

Kindly advice.

Thanks,

Nirmal

[EricL]

Hello @nirmal1988

It's great that you've seen @Adam_W's guide on - How to build a low-level deck. Let's say that the distance from the bottom of the brick to the edge is 50mm. Plus the edge of the aluminium track which let's say is conservatively 25mm. You would have a working distance of 75mm. Unfortunately, the smallest timber framing you can use is 70x45. If you add the 19mm thickness of Merbau decking that would come to 89mm which is 14mm past the aluminium track. If you add a gap of 10mm the overlap now becomes 24mm. This now becomes a tripping hazard.

What are some of the options? Break up the concrete alfresco to get back some space. You'll need to remove 50mm of concrete so that the timber frame and decking panel can line up with the lip of the brick. You'll need to make sure that the area is level so that you won't have a hard time levelling your frame. I suggest considering using tiles with a faux timber finish on them. This will save you from having to break up the concrete and you still get that timber look you're after. 

Let me tag our experienced members @DIYImpressions, @Neo19, and @Jithesh for their recommendations.

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

Eric

[nirmal1988]

Thanks @EricL for the advise.

Can I ask whether we can use the 70x45 timber lying down on the 70mm side, as the bearer and joist and place the decking on top of it ? so that the overall height from floor including the gap is around 74mm. And does using composite decking help ?

Also, do we have any faux timber finish tiles available at Bunnings ? and do we fix them in place or do we just lay it over the alfresco floor ?

Thanks,

Nirmal

[MitchellMc]

You can certainly use 70 x 45mm timber placed on its face to build your deck within the limited height @nirmal1988. However, you need to be prepared to accept that the lifespan of your decking timber could be reduced due to poor ventilation. Without adequate airflow, your deck can not dry out sufficiently after rain, and you will experience an early onset of decay.

Even composite decking needs to be spaced at a minimum of 40mm from ground level with good drainage or 90mm without adequate drainage.

Unfortunately, we don't currently stock any faux timber tiles. You can check out our range of floor tiles on the Bunnings Website.

Please let me know if you have any questions.

Mitchell

[nirmal1988]

Thanks @MitchellMc

After giving a long thought, I have decided to give it a go. I am planning to use 70 x 45 mm placed on its face as bearer and joists & top it off with an ecko-deck.

Would you please advise me on the spacing requirements of Joists & bearers (both in single layer, or skip bearers as the depth is less) ? My dimensions are 3m width and 4.2m long.

[MitchellMc]

Hi @nirmal1988,

You'll just be working with joists and a frame as there is no room for bearers. Joists are spaced 450mm apart.

Please let me know if you have more questions.

Mitchell

[Vis-á-vis]

If you don’t have bearers then what will you attach your joists to?

[nirmal1988]

Hello @Vis-á-vis 

something like this:
https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-Merbau-deck-over-concrete/ta-p/9...

https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-Merbau-deck-over-concrete-slab/t...

[Vis-á-vis]

I doubt you'd find a professional who would build a deck like that for many good reasons.

The issue with moisture is, among other things, it will make your timber want to cup and bend and the closer to the ground you are the worse this will become. 

Aside from all of the moisture issues, turning a piece of 70x45 on it's side is an absolute no. Regardless of whether you are 5cm or 5m off of the ground, the load stresses will be the same. The example you have given has the joists hanging on brackets so they will have to take the load of the deck and anything on it.

Now my memory may fail me on the exact maths but roughly speaking the following holds true. The strength of a beam to resist bending comes from the relationship between the height and the width. The most important number is the cross-section height and the thin side down is much much stronger, not just stronger by the amount you have increased the thickness. In other words, if you had a beam that was 100mmx50mm (to keep the maths simple), and you placed the 100mm side horizontally as opposed to vertically then you would be forgiven for thinking that it would be half as strong in resisting bending when in fact what you have done is reduced the strength and increased the force exerted by doubling the width of load-bearing side. Again, I forget the maths but the required width would be something like nine times greater than what it is when placed thin side down so your 75x45 would have to be around 650x45. 

There is a good reason why 100% of structural flooring and load-bearing timbers never has joists or bearers with the thick side down.

[Vis-á-vis]

This is pretty clever. You can get the top of an aluminium joist as low as 35mm and up to 68mm. You will still have decking boards close to the ground but at least you lose all the issues of also having a damp timber sub-floor.

https://klevaklip.com.au/patio-deck-kit

Or timber decking tiles that are slightly raised of off the ground there are loads of different types including composite and hardwood.

https://www.northernriverstimber.com.au/decking-tiles.html