All the gardens, shown on this this website, used water harvested in the catchment system shown above. This isn’t the best setup; however, it has worked for me and my needs. There are lots of great websites that go into great detail about rainwater catchment systems. I’ll only cover the basics and things that are most important on this page.
Barrels: I highly recommend getting quality barrels suitable for storing drinking water. You want to be able to keep all light out of the interior of the barrel. You also want to be able to close off your barrel, so insects and other creatures don’t fall into the water. I personally prefer smaller barrels because I can easily flip them over and rinse them out if need be. Large barrels work fine if you already have one.
First Flush: I highly recommend having a downspout that allows you to do a first flush. A first flush is when you let the rain wash your roof first before catching the water in the barrel. This is one of the best things you can do to get the top-quality water. You can buy first flush devices that attach to your downspout, or you can do it manually.
Roofing Material: The best roofing material for rainwater catchment is a quality metal roof. You can still get excellent quality water off other roofing materials. If you are planning of using your rainwater for drinking or growing food, you really need to do some research on if the roofing material being used is safe, or treatment options to improve quality.