If you do a bit of Googling most sites will tell you rainwater is safe to drink. ( You don't want to drink rain water from hot radioactive sites, like Chernobyl or around Fukushima though

)
I have a few rules I keep to
#1 Clean your urns /tanks every year.
#2 Wait until the rainy season is in full swing before you start collecting.
#3 What for at least 30 minutes of very heavy rain before you divert your your plumbing to your storage tanks,this allows for your roof and guttering to have a good flush through .
Then......free drinking water for the whole year .
