Can I safely water my vegetable garden with roof water that has been purified by a slow sand filter?

Probably, depending on what type of roof you have (read faq number 16 above) and what wildlife is present in your area. As for chemicals that leach from a roof surface, the key concept here is "bioaccumulation". Plants that are regrown each year from seed won't accumulate much poison, unless you have an unusually polluted environment. Fruit trees or perennials like berries may accumulate poisons over time. Use a first flush diverter and have a metal roof with a non-toxic coating to get the purest water. Any other type of roof (excluding possibly some types of tile roofs) will add toxins in proportion to their age and manufacturing. It is also important to note that many carcinogens can be present in extremely small concentrations and only a lab can detect them - and as they build up over time (months or years) in tissue they become quite toxic. The toxins in roofing are varied and expensive to identify. There are coatings available that will vastly improve the quality of water taken ...  more

Probably, depending on what type of roof you have. (read faq number 16 above) The key concept here is "bioaccumulation". Plants that are regrown each year from seed won't accumulate much poison, unless you have an unusually polluted environment. Fruit trees or perennials like berries may accumulate poisons over time. Use a first flush diverter and have a metal roof with a non-toxic coating to get the purest water. Any other type of roof (excluding possibly some types of tile roofs) will add toxins in proportion to their age and manufacturing. It is also important to note that many carcinogens can be present in extremely small concentrations and only a lab can detect them - and as they build up over time (months or years) in tissue they become quite toxic. The toxins in roofing are varied and expensive to identify. There are coatings available that will vastly improve the quality of water taken from a roof (see number FAQ 16 above). Also, if you have a "carport" or seperate garage, ...  more