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?

[IronStan001]

To get the last joist in, I'll need to use a Pryda Framing Bracket:

https://www.bunnings.com.au/pryda-joist-hanger-to-suit-45-x-120mm_p0294195

From the downpipe, to the edge of the pillar, I have about 105mm to play with:

What is my best option for fixing the bracket to the brick (considering how close it will be to the edge) without cracking the brick?  10G 65mm Screw Bolt?  Pryda themselves recommend 12G.

Otherwise, my next best option is to fix something to the slab and fix the joist to that...

EDIT:  Come to think of it, a framing bracket fixed to the brick (which is hollow) probably might not be the best.  After a bit of reading, I think a 90x90 H4 treated post that is fixed to the slab using 2 (maybe 3) chemset anchors would solve my problem.

EDIT2:  I've dug away the soil and exposed slab which gives me 200mm of concrete to work with.  The downpipe does actually deviate towards the stormwater pipe, but this is at about 210 to 220mm down.  So, I think what I'll do is fix a 90 x 90mm H4 post into the concrete with 2 fixings.  The holes will be about 65mm from the edge of the slab, so I think a 220mm galvanized threaded rod (M12, maybe M16 of which around 110mm will be in the slab) chemset into the concrete will be more than sufficient to hold the joist up.

[EricL]

Hi @IronStan001 

Using a 90x90 H4 post in combination with Chemset sounds like a better idea. Drilling several holes into a single brick will definitely weaken it. Plus, there is the possibility that with that many holes the brick will crack.

Eric

[IronStan001]

What do you think I should fix the post with?  3 x M12, or 2 x M16 threaded rod?

[EricL]

Hi @IronStan001 

I think 3 x M12 should be sufficient to hold the post in place. It's better to spread the load rather than concentrating it in two spots only.

Eric

[IronStan001]

OK, so with respect to fixing the joist to the post, how would I best acheive that?  Consider that I plan on finishing the deck with a fascia board (140x19mm).  The easiest thing would be to fix the post and joist flush, then I can fix the fascia to the joist as needed.  Could I use a joist strap (or similar) the right side (red arrow) of the joist to fix it to the post and maybe pop a couple of nails/screws from the joist to the post (purple arrow)?

[EricL]

Hi @IronStan001 

That's an excellent idea using the joist strap in combination with screws. It should be strong enough to hold the joist in place. I would also suggest placing a nogging near this spot to add extra support to the joist.

If you need further assistance, please let us know.

Eric

[IronStan001]

Yeah, I'll be adding a noggin just in front of the downpipe to help support the decking board that's going there.  I've gone and purchased all the brackets, bolts, screws and fixings today.  I upped the brackets from 40x40x80, to 50x50x100.  That'll put the hole for the bolt half way up the joist rather than in the bottom half like it would have been with the smaller bracket.  I'm also going to use 50x50 M12 square washers to fix the joists nice and firm.  Maybe overkill, but id rather over engineer than under.

For the footings, I've dug 3 holes to a depth of 450mm.  2 holes are good.  The bottom of them is firm ground.  The 3rd hole however has soft, moist clay.  It's down to just over 450mm at the moment, so how do I handle this?  Drop some aggregate in there?  Or just dump the concrete in 'as is'?

[EricL]

Hi @IronStan001 

A couple of mid-sized stones and some gravel should be enough to provide drainage for the footing. Place your builder's line for the footing and test for height. When you are confident that the footing is at the correct height you can then put the concrete in. 

I suggest mixing the concrete in a wheelbarrow to get the consistency right before tipping it into the hole. In this manner you'll know that there are no unmixed portions of your footing. 

If you need further assistance, please let us know.

Eric

[IronStan001]

I have a few chunks of rock I dug out from one of the other holes, so I can probably smash that down a little and drop it into the base of the hole as suggested.  I'll cover that with a bit of gravel, then I should be good.

For the stirrups, I got some measurements using a laser level and the bottom of the bearer would sit approx 50mm above the ground, so my original idea of using a 130mm stirrup was too short.  The leg would have been pretty shallow in the footing.  So I got 200mm instead which embeds the leg more.

As for the concrete, since that's probably one of the last things going in, the bearer and stirrup will obscure most of the hole, so I think I'll just use regular concrete that I can mix in a barrow and shovel into the hole.  I'll lay down about 4 or so joists to start with to make up my base frame, enough to support the bearer, then fix the stirrups and set them in concrete.  Give it a day or so, then I can proceed with the rest of the build.

[IronStan001]

For the span from the slab to the bearer, considering I'm using 50x50x100 brackets, should I instead use 65x65x130 so I can hopefully use 2 bolts rather than 1?