deck footings into rocky ground

[wigfam]

Gidday people's,
I am looking to find out what the acceptable approach to providing adequate footings into very rocky ground may be for a 7m X 5m X 3m high timber deck. I have done some Ø400 X 600deep concrete into natural earth with hot dip galvanised stirrups on my previous back yard but this one doesn't have any access for machinery like a bobcat or dingo. I cannot drive a large crow-bar into this earth. I haven't used a motorised or manual auger drill before but I don't think it would penetrate this earth? I am thinking about hiring a jackhammer or a friend suggested driving a few Ø16mm reinforcement rods into the ground and boxing up a concrete foot above the ground to encompass the galvanised stirrup.
ps- oh yeah, how close can I build this deck to my back fence? we back onto bushland and don't have any neighbours as such. speaking of that, I want this deck to look as natural as possible and am trying to convince my wife to let me use cca treated coppers log type materials. if we do go with that type of construction, how do you guys go with doing those neat looking joints say where an Ø80mm horizontal rail meets an Ø150mm vertical post type thing. I plan to 'check-out' the main posts to take the beams etc. and use Ø12mm and or Ø16mm galv. bolts etc on my joints. this style of construction I think would be more appealing to the position of the deck and environment as opposed to a standard rectangular beam / joist construction especially since I want to leave it as the cca colour and not have to paint it up as you'd expect to with normal decks?

thanks and we look forward to any of your thoughts,
Mark & Lizzy


kind regards

[journeyman Mick]

Whatever else you may decide to do, DON'T drive rebar into the ground and then concrete over it. The steel in the ground will rust, the rust will travel up into the steel in the concrete, rusting steel expands, your concrete will spall/crack/blow out, end of footings. Distances from boundaries to structures will be covered under the local planning laws. It may be as little as 1.5M or even less if the structure is fire rated, or it may be determined on a sliding scale of the higher it is, the further it needs to be from the boundary, best to check with your local council. Footing size is calculated on a number of variables like the post spacing/static loads, dynamic loads (wind, elephants dancing on your deck), height of posts and soil characteristics. If you have rock it may be possible to glue (chemset) gal. stirrups into place. If it is rocky soil you will probably need to jackhammer holes into it for footings. Those motorised augers are a waste of time IMHO. Only worth using on nice friable soil where there is no machinery access. Are you sure you can't get a micro excavator with a rock breaker into the yard? This may work out at about the same as hiring a jackhammer for a few days (and certainly a lot easier ).
If you want to go for timber I would suggest round posts and rectangular bearers and joists as it is a lot easier to get neat joints. Good luck, let us know how you go.

Mick

[bob w]

As Mick stated you will need to jackhammer into the rock and install post supports or piers. I also agree that it is far easier to get neat joints using rectangular bearers and joists. As far as leaving the timber untreated is concerned this is not advisable as cca treatment is for the prevention of termite infestation (white ant or borer) and is not a barrier against the elements. You may get some residual protection but it is not intentional and is short lived.
Keep us posted
Bob

[wigfam]

thanks guys for your sensible advice. i have yet to finalise and will get back to you then. see you later on.

[griffo]

I used a clear sealeant on my cca treated timber. It still looked good when i left the place a couple of years later. Can't recall the name of the stuff but it was recommended by the timber guy.

g