Installing Rainwater Tanks and Cisterns

Does anyone have any experience installing water tanks or cisterns for rainwater collection and using the water for a garden (gravity-fed or pumped)?

There’s a 60’ by 30’ shed near my garden that would catch 2300 gallons of water in a 2-inch rain. I figure one of those 2500 gallon tanks daisy-chained to a 500 gallon tank would be enough to handle even the biggest storms.

I’d want to raise the tanks a few feet off the ground so I can gravity-feed them to the garden. A 2500 gallon tank can be around 7.5’ across. What would be the safest type of platform to hold it 3-4 feet off the ground? Some kind of metal scaffolding on a gravel base or just cinderblocks? The surface is slightly sloped ground under some pine trees.

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Hi Brent!

Just a quick mote about those 2500 gallon tanks is that if they are painted a dark colour the water inside can get heated up to the point where you could scold your plants/seedlings when using the water. On the flip side, if your tank is at all transparent, the water won’t be as hot but you will get algal growth inside the tank. Just something to keep in mind.

As for pressure, I believe you get 1.45 PSI of pressure per meter above grade. So in essence I’m not sure a few cinder blocks would do the trick depending on how much pressure you want coming out of it. And we’re not even talking about pressure losses in the pipes on the way out to where you want the water to go.

If you have a strong desire or need to capture the water, I found Art Ludwig’s Water Storage a good reference.

Be sure to do your homework calculating the pressure head you will need for your intended purpose (remembering the that tank will not be full all the time). Then figure out the structure required: 2500 gallons weighs 4 tons and can do thousands of dollars of damage if your structure buckles. I found the costs and risks didn’t justify the benefits in my situation.

Yeah I may have to hit the books for this one to nail down an exact number on the water pressure. I think I may end up using a pump but it’s nice to have the option of gravity feeding as a backup. The tank is pretty tall and can only be raised so high before it reaches the roof so maybe the maximum height still wont provide very much pressure.

I assumed a dark colour would be best but then also have something viney growing up around the sides of it to help keep the sun off it. I’ll see what this Art Ludwig fellow has to say about it.

The main purpose purpose of the project is actually to get eaves troughs on this shed. Every year sheets of water come off of it and wash out the laneway. I figured if I’m putting on eaves I may as well direct the water into a tank that I can use for the garden behind the shed. Location-wise the tank would be conveniently close to the garden but in an area under some pine trees that I’ll never use for growing space. I’ve had the conservation authority out to look at it so that will obviously be a huge factor in whether I go through with it.

I would just put a whole bunch of wooden pallets under the tank to get it off the ground.

I think he mentioned he’s on a slope so that might not be a possibility without the risk of the tank slipping off or falling over.

Hi Brent. We’re planning on doing pretty much the same thing with our bank barn that is about 80 x 30 feet. Tons of roof and a lot of water that could be put to good use watering our market garden especially after that drought last year. We were actually thinking of burying the tank. That way it will stay cool. Then use a solar pump or two to get it where we want it. There’s no way there would be enough gravity-fed pressure to reach along our 1 acre rows. Have you looked into burying it?

Unsure about using pallets…wouldn’t they rot eventually? I could level the ground but it would take a lot of pallets to get it up high and form a wide enough base. Any good sources for getting free pallets? Or weather-damaged wood? In a book I’m reading now the guy gets all his wood free from a lumberyard from stacks that tipped over or don’t look saleable. I might try looking up local sawmills and see if I can swing something.

Kind of threw me for a loop there, Linda! I hadn’t thought of burying it. My thinking was try and get it up high for gravity-feeding as a backup for when the pump fails. So I guess by burying it not only would it be kept cool but it would be easy to cover to prevent algae growth. But then it would also be trickier to fix if something goes wrong. Is there danger of damage to a buried tank from frost-heaving in winter?

I gallon of water weighs 10 pounds. If you really want to install tanks this big, you had better have a very strong support structure. A few concrete blocks or wooden pallets will certainly NOT do the job. We use the 250 gallon reinforced tanks 6’ high on top of triple re-reinforced screwed together pallets. They are just strong enough. We use a pond aerator windmill to pump water from our dedicated drilled well to these tanks (I have two). We then use a 2" line from the tanks to the head of each greenhouse. At the head, we have a standard, 4-gang brass coupler connected to soaker hoses.

That’s an interesting setup. What’s the thinking behind running the water through the tanks rather than pumping it straight from the well to the greenhouse?

Water pressure. With this set up, the greenhouse waters itself. I have two that are 24 x 96. Twice + daily hand watering would take far too long.

What about algae in the tanks clogging up the soaker hoses, is that ever a problem? Or the water heating up too much?

For algae or other debris, I typically have a flush-out filter right as you come out of the tank that’ll catch anything foreign and keep it from clogging the drip tape or soaker hoses. If hour tank is shaded or underground I wouldn’t be worried about it heating up too much. But if it’s in sunlight all day long, that might be problematic. Mind you the heat sink potential of such a big tank might be high enough that it’ll never heat up to high temp during a single day of direct sunlight. Can anyone else confirm that?

When you are doing calculations, do yourself a favour and work in metric. The metric system has a fundamental identity around water: 1 litre of water takes up 1 cubic decimetre of space (10cm x 10cm x 10cm) and weighs exactly 1 kg at the maximum density of water (which is at 4 degrees Celcius).

So if you need to calculate how much a storage tank weighs or a footprint for a tank of a particular volume it is all FAR more handy and accurate than working in imperial.