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?

[MitchellMc]

Hi @IronStan001,

75mm would be sufficient; there's no need for 100mm.

Mitchell

[IronStan001]

Great.  Also for fixing the joist to the bracket, M12 x 65mm (including a washer) should do the job.  Or maybe 75mm?

https://www.bunnings.com.au/zenith-m12-x-65mm-hot-dipped-galvanised-hex-head-bolts-and-nuts-12-pack_...

For the noggins, would I be fine with 90x45mm considering my joists are 120x45?

[EricL]

Hello @IronStan001 

The Zenith M12 x 65mm Hot Dipped Galvanised Hex Head Bolts And Nuts might be cutting it a bit too close if the timber width is 45mm. If you take away the width of the timber which is 45mm you'll be left with 20mm, less the thickness of the bracket 5mm and the washers 4mm. You'll therefore have 11mm to thread on, I propose using the longer 75mm bolt so that you have room to move.

In regards to the noggins 90x45 should still work and provide enough support to the 120x45 joists.

If you need further assistance, please let us know.

Eric

[IronStan001]

I thought as much, so 75mm it is.

Probably the last advice I need is regarding the styrups. 

 I just got some levels with the laser level. The bottom of the bearer will sit roughly 50mm above the ground. I image in that case, using:

https://www.bunnings.com.au/pryda-post-anchor-full-stirrup-130-x-90mm-m10_p1070070

...in a footing that is 400mm deep should be suitable?

The holes I've dug so far are at least 400mm deep. The hole on the right is 300x300x400mm and took 15 minutes to dig. The middle hole just so happened to have a nasty big rock right in my way (which you can see in the below), so because of this, it's bigger than the first hole. This took me 4 hours to remove the rock. Once I clean this up, it'll hold a decent amount of concrete.

https://imgur.com/a/aqCGot2

The 120x45 joists would be fixed to the post anchors with some 130mm M10 bolts.

[EricL]

Hi @IronStan001 

That sounds like a good plan. Please keep us updated with your progress, we look forward to seeing your low-level deck assembled and ready for use.

Eric

[IronStan001]

With respect to angle brackets for 120x45 joists, are 40x40x80 OK?  Or should I go a little taller?

[MitchellMc]

Hi @IronStan001,

You want the fixing to be roughly halfway up the joist. If you can use a bracket where two fixings can be placed in the joist, all the better. Just be careful with the 130mm. If you have any less than 10mm under the joist, the top of the bracket will be proud and prevent you from attaching the decking.

I think the 80mm bracket will be fine, but you might like to increase it to the 100mm.

Please let me know if you have any questions.

MItchell

[IronStan001]

I have 140mm to play with.  120mm of that is taken up by joist with a 20mm airgap.  Looking at some of the brackets, regardless of what size I go (80, 100 or 130), I'd only be able to use the top hole anyway.  I sort of difficult to tell from the picture of the Dunnings brackets the actual difference between the 80 and 100mm brackets, with respect to the location of the holes.  I think I could do 130, but it doesn't leave much room, so I'll go with the 100mm instead of the 80.

[IronStan001]

When using 140mm deck boards, what is the minimum gap needed between them?  Depending on where I read, some say a minimum of 5mm, others say 8 to 9mm is the minimum.  So far, I've planned for 6mm gaps.

[MitchellMc]

You should be fine with 6mm gaps with hardwood decking @IronStan001. Going to an 8mm has the additional benefit of allowing more airflow through the deck.

Mitchell