How to build a low-level deck?

[IronStan001]

I have an alfresco area which I'd like to deck.  The joists (Green lines) will most probably fix straight into the concrete and ideally, I'd like to go 1500mm beyond the concrete (Red line).  I have a storm water pipe (White line) about 800mm out from the concrete.  800mm out from the concrete, the ground is at its lowest point, which is about 200mm lower than the alfresco.  With regards to the footings and bearers, what are my options?

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

[IronStan001]

Would using double joists rather than extra footings be sufficient to reduce bounce?  The more I can avoid digging footings, the better.  There is A LOT of rock underneath.

[EricL]

Hi @IronStan001 

If anchored very well together it should theoretically be strong enough. Another possible way that comes to mind is to create two frames, one over the concrete and another over the soil.

Eric

[IronStan001]

Actually, since I'm not using pedestals, I could probably do 120x45 joists which I assume would be way more than enough to not have any bounce over the 1400mm gap.  I'd just have to find another method to support joist at the end of the concrete.  If the gap is 25mm, I could squeeze a pedestal in there.

[EricL]

Hi @IronStan001 

I'm thinking that with a separate frame the heavy weight of the double frame will not affect the frame on the concrete side should there be any movement from the frame on soil side. 

Eric

[IronStan001]

Would a taller joist like a 120mm eliminate the prospect of bounce?  My original idea of using at minimum, 90x45 F7, I'd get a maximum continuous span of 1500mm.  MGP10 is up to 1700mm.  120x45 however for my needs, starts at a minimum of F5 with a 2200mm span.  Then F7 @ 2400mm and MGP10 at 2700mm.  Even using 120mm MGP10 is almost double the span I need.  I think to save a bunch of hastle, the 120x45 would be better wouldn't?  It would make the joist structure simple and straight forward.

[IronStan001]

This is what I have so far.  120x45mm joists at 5000mm long.  Fixed at 20mm above the concrete (or whatever it needs to be level with the brick) using https://www.bunnings.com.au/dunnings-40-x-80-x-40-x-5mm-m12-galvanised-angle-bracket_p1070088 (150mm in from the ends, spaced at around 980mm) on alternating sides.

Depending on what the height is at the very edge of the concrete at the jump to the bearer, I might be able to slip a pedestal under there for support, which might also require shifting the angle bracket over by another 50mm.  The bearer will be at 1400mm using a double 120x45mm supported by 4 posts.

@EricL and @MitchellMc, thoughts?

[IronStan001]

I guess my next design question is, how do I handle the column with downpipe to get a joist there?

[MitchellMc]

Hi @IronStan001,

You basically just need to create a bit of framing to support any decking boards that can't span across to the last joist due to the drain impeding them. I've added an image below that should help illustrate this.

You'll likely need to place a pedestal foot under that structure to support it.

Please let me know if you have any questions.

Mitchell

[IronStan001]

Would I perhaps use a ledger board that's fixed to the slab?

[MitchellMc]

That's certainly an option, @IronStan001 and it could be an easier option.

Mitchell