Low-level decking construction

[Amin88]

Hi All,

I am inspired by the other decking projects posted in this group and also the technical support prvided the expert group. I am planning to my deck project and would like to ask the suggestions as my situation is a bit different from all previously posted decking projects.

my outdoor entertainment area is currently concrete floor sloped with a drop of 180 mm from house level. Attached is the rough cross-section drawing. I prefer to go for the deck finished level same as house level, however, this will allow a floor to ceiling height of 1950 mm at the outermost rafter level which is too low. 

Thinking of dropping the level by 100 mm to allow 2100 floor to ceiling height at the middle rafter level ( I will still only have 2050 mm for the outermost rafter level). But, then the clearance for substructure will only by 60 mm (80 mm - 20 mm decking material) at the house entry level. I am seeking a suitable substructure plan for my decking project.

Cheers,

Amin

[MitchellMc · Accepted solution]

Welcome to the Bunnings Workshop community @Amin88. It's fabulous to have you join us, and many thanks for your decking question.

With 60mm to work with, you'd be looking at using 90 x 45 H3 Pine on its 90mm face. So, it will be 19mm for decking, 45mm for joist and a 16mm gap underneath for airflow and drainage. Since you'll be working with a sloped slab, the use of Builders Edge Pedestal Feet would be ideal. They are easily adjustable to suit your slope.

If you haven't already read through @Adam_W's article on How to build a low-level deck, then I'd suggest that's a great place to start your project.

What direction do you plan to lay your decking? Parallel to the house wall or perpendicular to it?

Please let me know if you have questions.

Mitchell

[Amin88]

Thanks @MitchellMc  for your prompt reply. I never heard of laying 90 x 45 by the face of 90 mm. Having said, it’s a viable option for me. What would be spacing of supporting the 90 x 45 you reckon? Also, I’m planning to lay the decking parallel to the house. The length of the area is 9 m. Thanks in Advance!

[MitchellMc]

Generally, it's best not to lay timber on its face as it doesn't have as much structural rigidity @Amin88. However, there really isn't enough room to build the structure conventionally close to the house in your case. I'd suggest that the joists should be supported at least every 700mm, and they would be spaced 450-600mm apart. 

Once you move further away from the wall and the height allows, you could switch the orientation of the joists to be sitting on their 45mm side. In fact, when you get to the outside of the concrete slab nearing the 270mm mark, you'll need to switch to post supports as even the largest pedestal foot will run out of height adjustment.

Mitchell

[Amin88]

Thanks @MitchellMc . That makes sense! What’s the best way to support the joist to concrete near house end where the joist is only 16 mm spaced from concrete floor? Also, how to switch the orientation of the joist, use different piece of timber? Thanks 

[MitchellMc]

@Amin88,

You can pack out under the joist with shims and then Dynabolt through the face of the timber directly into the concrete. 

To switch the timber's orientation, you'll simply turn the 90 x 45mm joist from its 90mm side onto the 45mm side. By that stage, you'll likely have enough room to use the pedestal feet instead of the shims.

Mitchell

[Amin88]

Thanks @MitchellMc . That gives me a starting point to get my work. My question is can I use the 12 mm bolts and nut instead of dynabolt? Because, with dynabolt, it’s hard to manage the height, particularly at a sloping surface? Also, is it a bad idea to choose 35 x 90 treated pine with their 90 side is faced up? That will give me additional 10 mm space. 

[StevieB]

Great to see that you're well on your way to getting started on your deck @Amin88

@MitchellMc will be happy to help with your question when he's back on the site on Friday.

Stevie

[MitchellMc]

Hi @Amin88,

I apologise for the delay in my reply. 

Could you please provide a little more detail on how you would like to use the 12mm bolt and nut in this application? When using Dynabolts, you would manage the timber's height with shims. Pack out under the timber with shims until you reach the required height and then Dynabolt through the face of the timber directly into the concrete. 

Having an additional 10mm under the timber would be beneficial, but you need to weigh that up against the thinner timber issues. It is already not a recommended building method to turn joists on their face side if any other option is available. If you then go from a 45mm to a 35mm, you risk having a springy deck, and you'll also need to have the joist supports closer together. 

Mitchell

[Amin88]

Thanks @MitchellMc . I will consider the 45 mm timber in this case for structural stability. With bolts, I thought of using the bolts exactly same as dynabolt. Driving the bolt through the timber and into concrete floor (like how they connect the ledger using the bolts in the attached photo) after fixing the height using shims.The bolts will be countersunk into timber. I prefer bolt instead of dynabolt because it can be removed later. Please share your opinion.

Thanks