Is there a formula for working out how close to a retaining wall you can put a water tank? I'm concerned about the weight. I spoke to someone on the phone who confirmed that this needs to be taken into consideration but when we were ready to start getting quotes I couldn't find him again and I haven't been able to get a clear answer from anyone else.

The last time I googled to see what's available I didn't see anything like that first one, the Thin Tank. That looks like a great way to collect water from the parts of the roof that aren't able to feed into the main tank. (Something occurs to me though, how warm would the water get in a tank that thin if it was placed in a location where the sun shines on it all day?)

As for the main tank we want to install, we have a reasonable amount of space and having stewed over various things I realised that the main starting point for a decision is how far back from the retaining wall does it need to be for safety? Another thing that confused me for a while was what to put the tank on. The area has a slope and we've heard that you can just level it and spread sand, or gravel, or place it on treated pine, but on the other hand someone on a gardening show said if it is a slimline type tank and not a round tank it must be on a concrete slab, but local installers looked blank when I asked about that. Anyway, I'm inclined to go with the slab, no erosion, no weeds. If only we could decide where and how big. Which brings me back to the question of how far back from the retaining wall must the tank be?

Hi Mac,
as you're in Canberra, the "ask the council" solution won't work (unless things have changed since I lived there) ...

I believe the final word used to rest with the ACT Planning Authority (now the ACT Planning and Land Authority) - and there should be pamphlets or website info ... >> let's see - "Google is your friend"<< ...

from - ACT Planning and Land Authority (clickable link)



So, if you don't want the paperwork hassle (and fees), your size maximum is 20,000 litres, and height is 2.45m;
it can't go in the front yard, nor within 1.5m of a side or rear boundary unless it's the only such structure etc ...
it mustn't block reasonable access (eg, firefighters etc) ... it mustn't block natural light to windows (the old "ancient lights" clause) ...

Other than that, considerations might include what you want the rainwater for (ie, how much mains water use do you intend to replace?), and safe access for maintenance and repairs (they will be needed over time) ...

with that in mind, I'd be inclined to treat a retaining wall as a natural boundary (ie, be not less than 1.5m away from the structure) ...