Harvesting rain is a great way to have an alternative and low cost source of water, and is one of the oldest and well know one (of course!).
The quality of rain water depend on many factors, but usually isn't bad and rain water can be used for a lot of things, even as a potable water paying a little bit of attention.
Even in drought areas like Cape Town harvesting rain can give you a good quantity of more water. In pre-drought years the medium rainfall in Cape Town was around 788mm per year. With a 100 square meters of roof, a medium size for a private home in this area, will give you up to ~ 79000 liters of water. In 2017, the worst year ever, the rainfall in Cape Town was only 153mm of rain in whole year. Even with such low rain, 100 square meters would give you 15000 liters of water. Not a huge quantity in a year, but not so bad.
The best way to collect rain water is of course to maximize the surface you use to catch the rain, and usual this mean to use your roof. This has the advantage that you can also use the gutters you already have, and just modify them to collect all the water you can in some tanks.
