There are simple old time solutions to the possible problem of collecting and using “polluted” roof gathered rain water.
Add a splitter, (an upside down Y shaped piece of pipe), to the downspout before the pipe runs into your cistern or rain barrel. One side of the “Y” goes to your catchment, the other to the ground or drain. Add a simple flip value to the inside of the splitter. During the first 5 minutes of any rain, turn or flip the valve to run the water out onto the ground or into the usual drain. Once the roof and gutters, (and air), have been washed clean of any dirt, debris or pollution, flip the value over to fill the rain barrel or cistern. Keep the barrel or cistern tightly covered so animals or mosquitoes can’t get in. At least once a year completely drain the barrel or cistern and scrub clean with bleach and water. Rinse well and drain. Then refill. This ensures a clean supply of “soft” water that should be safe to drink, (and incidentally is especially good for using in steam irons and washing hair ). – Jim Fry, Curator, Museum of Western Reserve Farms & Equipment, Ohio