As posted above, even in arid locations, many thousands of gallons may be collected from rain.
If renewing or repairing roofs, avoid materials that taint the water.
Devices are available that divert the first XX gallons of rooffall to waste (or an oil drum etc.) and then pass the remainder to a large tank. This results in much cleaner water since the initial flow is liable to contain dirt, leaves, dead insects, and bird droppings.

If the collected water is acidic due to acid rain, this may be neutralised by placeing lumps of chalk or limestone in the tank, or by use of a cement tank.

If a flush toilet is used normally, it would be prudent to be prepared with a composting toilet or other water free system, for emergency.