Word of advice, do not underestimate the diameter and placement of the breathing/vent pipe. A proper installation results in a dramatically improved flow performance.

Down here there are a lot of people of of scarce resources that have water tanks installed atop the roof of their houses, with no electrical equipment or pumps whatsoever. They have to rely on gravity alone to pull the water down into their showers and toilets.

This is the recommended installation for a pump-less rooftop water tank:

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Notice how the breathing pipe is installed vertically inline with the main output pipe, and has a diameter at least equal to the main distribution pipe.

 

Here's another diagram. It has an obvious mistake in its main title. It reads "How to obtain more pressure from your water tank?", when it should actually read "How to obtain more FLOW from your water tank"

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Just got done unloading the truck with the supplies.

This is the recommended installation for a pump-less rooftop water tank:

My roof is 4/12 pitch. And it wasn't engineered to have a water tank on top of it. My tanks are mostly for use in the garden. If an emergency should arise - I have a few gallons I could filter and boil. The kind of disaster that will be catastrophic is an asteroid strike or a super volcano erupts. Anak Krakatoa, they say, has been growing 12 feet per year. That suggests pressure is building up inside. When the crust cracks that's when 200 extra gallons of water isn't going to make much of a difference. When that thing blows it's going to change life for a whole lot of people. Those who survive will have a struggle. I won't survive. Even with a thousand gallons of water on hand.

 

55 gallons of water is 456 1/2 pounds. I wouldn't want to put that up on the roof and have the roof collapse in just so I could save a little rain water.

Farmers used to have water tanks high above their land. They pumped water into it from a stream and a ram pump. If you've never seen a ram pump - it's pretty ingenious!

 

I wasn’t suggesting that you put the tanks above ground. I was pointing out that a well-placed breathing pipe is important to obtain a decent flow out of them when the time comes.