A reader's General Construction question: Deck building?

Matt S   from   Australia   had this Deck building question.

what a great site, thank god for Google- "DIY, Concrete" and here I am.

I am planning to build a deck out the back of my house. We are on bushfire prone land so the council wants everything non combustible.

I have specced up a Stratco Tuffloor deck.

For visual reasons, I want to build some of the supporting structures in masonry, and would like to use poured concrete as I can keep a better control on cost this way. I am building on top of a solid sandstone outcrop and an existing concrete slab (the house is a highset 60's Fibro box).

Do you have any advice for reinforcement requirements- how deep into the rock / slab the bar has to be drilled, the spacings, and how far up the columns and walls the bar has to go.

The supporting structures I was intending to build are: 4 columns 400mm square by 2.4m tall, a hollow pier 2m long by 1m wide by 1.4m tall with 200mm thick walls and another hollow pier 2.8m long by 800mm wide by 1.2m tall.

The 2 piers sit on the rock and the columns on the slab.

Oh yes also when building formwork onto an uneven surface such as rock, what is the best method for sealing the gap between form and rock? I will be talking to an engineer eventually for the DA but any advice you have will help me to start planning this out.

Bill's reply

Hi Matt,
Thanks for the kind words about the site.

1.) Existing slab, check for thickness. Dig around it in a couple of spots. Check original drawings if available. Your engineer will want to know. If it is thin, or if not reinforced, then you hire a concrete saw, cut out the slab where the walls are and put in a footing.

2.) The rock, hard to say but you will need some sort of footing I guess. I have never done a wall straight out of rocky ground. Wall starter bars will be cast into the footing. This way you start from a known quantity.

3.) The walls. I'd be doing the walls out of Besser blocks. Definitely cheaper than poured concrete in the sizes that you are talking about. Say Y12 bars each corner and at 600 ctrs elsewhere. Epoxy starter bars into the existing slab. Starter bars lap to main bars min 600. If you can't get a block layer you can do it yourself, very slowly, say only two three courses a day. All cores with rebar in them are filled with 15mpa concrete. Mix it yourself.

4.) Concrete walls. If available I'd be using Acrow "U" form. Very easy and quick to set up. If using ply I'd get something cheap and throw away.

5.) For the odd gaps at the bottom, this is another advantage of the footing. You dig in a bit and come up a bit to get a level surface for the next stage. Sometimes we cut the throw away ply to the shape, and sometimes, (as long as it is below the surface when stripped) we nail bits of 6mm Hardflex or similar to the inside of the ply, to fill up gaps . We backfill a bit of soil around them before pouring the concrete.

6.) You are building the supports with a bit of overkill as far as strength goes, but nothing wrong with that if you are trying for a certain look. For instance we used to build complete high set houses on 225 x 225 x 2700 high concrete piers at say about 3500 ctrs. each way. Now we do the same on 75 x 75 or 100 x 100 RHS steel. Cheaper still.

7.) I'm not familiar with the Stratco flooring system. I guess you may be covering it with 16 compressed cement sheets.

8.) It depends on a lot of things, but you may want to check out costs etc. on using Bondek permanent steel formwork and pouring a concrete slab up there.