rain barrel and vegetable garden
Your question is being asked all over the US by everyone who is harvesting rain water. There is a consensus from websites that unless the roof is designed with materials and methods intended for rainwater collection, there is a possibility that toxic substances will end up in the water. If you would not drink the water you catch, you should not use it on the vegetables you eat. Deep slow sand filters can do a great job but I can't say that it is enough to drink. Have your filtered water tested before trying to drink it. If you were just watering flowers, you would be ready to go without a flush diverter/slow sand filter.
You should never allow your sand to go dry as it will remove the oxygen and kill the micros in your filter--in other word, it will kill your system. Check out U Tube on filtering rain harvested water. There are some true experts out there and trust me you can tell who really has a system and knows their stuff!
Good luck, A