Suburban rainwater tanks

Suburban rainwater tanks asks what volume you need to plan for home and garden, and gives thoughts on an underground reservoir, rather than above ground plastic or fibreglass storage chambers.

A fibreglass water tank is the best above ground storage chamber, but how many of these do you need?

By Bernard Preston

The factors to consider are the size of your roof from which you can harvest rainwater, and the length of your dry season.

  1. Bernard Preston
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Roof area

It's relatively easy to collect rainwater from about half your roof area; it's certainly possible to harvest more, but it does get more difficult.

So, let's say you have an average 200 m2 roof, and you are planning on harvesting from from half of it; just for nice round figures, that makes 100 square metres.

That's about 1100 square feet.

If you have 100 mm of rain for say 10 months, then you can harvest about 10 cubic metres of water every four weeks; 2649 gallons.

That's about enough for an average family, so assuming it rains at regular intervals, which it won't, you could get away with a 2,500 litre tank for the wet season months.

Two such suburban rainwater tanks, in total about 5,000 gallons would provide enough storage for two weeks. If the weather is fickle for a few weeks, and you can top up from the utility, then you would be fine.

If not, to be safe, 10,000 litre storage, or 2641 gallons, to be more exact, is necessary.

That's two large 5,000 litre tanks, each about five times the size of that shown above.

Dry season

But of course there's going to be a dry season, lasting say three months when you may not have a drop of rain; then you are going to need 30,000 litres of storage, if you are unable to access municipal water. In our case, we almost always get some winter rain, so we planned for 27,000 litres of storage.

That means you would need six 5,000 litre suburban rainwater tanks.

Only once in nearly seven years have we run short, when the Spring rains were later than expected; by one month. For this reason, we remain connected to the utility, though we use almost none.

Suburban rainwater tanks

Suburban rainwater tanks recommends you store your harvest from the skies underground because it's so much colder, and less unsightly.

Yes, the temperature of the water in above ground storage chambers, even if they are made of concrete, rises quite high, and bacteria will flourish, and it will certainly not be potable. 

You will have to rely on bottled water for drinking.

Underground reservoir

Our complete rainwater reservoir.

Whilst I'm not a disbeliever in suburban rainwater tanks, we have three at our home, for mass storage we think an underground reservoir is a cheaper and better solution.

One would need to get an engineer to draw up plans, or I might in the future provide ours as an ebook, dig a large hole and have a contractor lay a foundation with reinforcing and build a double brick wall, which then must be plastered. It was surprisingly inexpensive.

Admittedly I did the plumbing and the electrics from our solar generator, but even so, it's not a lot of money to put in an underground reservoir and ;connect it into your home and garden. It's a sublime supplement to your suburban rainwater tanks.

Underground the water is seriously cold, which means it's probably potable. We drink it without reservation, and have done for more than six years. It's lovely and soft, and free of chlorine. One could put in an ozone device to sterilize it.

Backyard permaculture

You cannot grow vegetables and fruit trees without water. Harvesting and using suburban rainwater tanks for storage, or a reservoir, is absolutely essential for any form of backyard permaculture.

Working with nature, rather than against it, means having a reliable source of water in the dry season.

Ours is in the winter, lasting about three months long, though the odd shower is not unusual; without some form of suburban rainwater tanks for storage it would be a hopeless pipe dream.

There would be no lettuce, or cabbage, broccoli, kale, or broad beans. All demand a copious supply of water.

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