watering edible plants from composit roof water?

I have a 2500-gallon above-ground plastic cistern which is filled with water caught from my composition roof. I was using it to irrigate my mostly-ornamental garden. I replaced most of the ornamentals with a food forest. I've been told that the water isn't safe to use on edible plants
because of the heavy metals in the shingles. It's a 30-year roof which I had installed in 1999 before I discovered permaculture.

I've read about sand filters that are used in the Third World to purify drinking water. They don't seem to be legal for sale in the US. I also heard about a paint that could be used on an asphalt roof to reduce the toxic effects of the heavy
metals in the shingles. Does anybody have experience purifying water caught from a composition roof? If possible, I'd like to use my own water to grow food.

Thanks,

Susan
Seattle

Personally I would not worry about it. I plan to do the same thing. I think some folks get a little too paranoid about such things. I agree, don't use insecticides, chemical fertilizers or herbicides on your garden but I think a lot of the phobias over anything chemical get taken to irrational levels.

I bet the first three hard rains washed 90% of anything you have to worry about off that roof already. The minute volume of anything left that will leach into that water will probably be so small it would be near impossible to even detect.

Just my personal opinion.

I have several 185 gallon cisterns that I collect rain water with off my composite shingled roof and use for my gardens. I agrree with Ray. I would think anything to worry about is gone especially since 1999. All I have is a screen filter on mine. Besides, I am relatively sure that a sand filter would take care of biological impurities but not touch any residual heavy metals.
My personal opinion also.

Karl
Portland Maine

I tend to agree with the other replies, and just wanted to add that in general, plants do not want to take up chemical compounds that they don't use for their normal metabolism, so even if some undesirable compounds are present in the soil, that's where they will stay.

Sand filters are sold and used for swimming pools, plus a homemade filter could be made fairly easily. They will filter out particles, but any dissolved ions and/or organic compounds will pass right through.

I agree that your roof was mostly, if not entirely rinsed off a good while back, but where did that go? Was it washed into the soil where you will be planting? Or did it flow somewhere else? If you still have concerns, you can reduce the possibility of exposure by only harvesting the above ground parts, like leaves, berries, nuts, fruit, as opposed to the underground parts like roots and tubers.

In general, I think that if plants are grown organically and look healthy, they are healthy to eat.

if you want to filter, use activated charcoal.

charcoal burnt in anerobic conditions. will pull out almost all additives.
thats what all the inline filters for water treatment and ice machines have in them.
they make some expensive "de-chlorination" filters that homebrew makers use, and it's the same activated charcoal.
add iron fillings, and it will even clear arsenic.

if you want to try some, these guys aren't too bad, and some good info on their site for the differences in charcoals.
easy to make at home with an old wheelbarrow, just tip it over a fully glowing fire.

http://www.buyactivatedcharcoal.com/bulk_activated_charcoal

If you don't mind having a white roof, you could coat the roof with a hydrated lime/water solution. When wet, hydrated lime reacts with CO2 in the atmosphere to produce calcium carbonate (limestone), so you are coating your roof with a thin layer of limestone, which should help to seal in whatever chemicals are being leached out of the composition roof. Lime also has a high pH, giving it an antiseptic property, reducing the amount of fungi and bacteria that might be growing on the shingles. And the white roof color reduces your roof's surface temperature in the summer, reducing your A/C bill. Depending on the acidity of your rain, the lime will erode off in a year or two and have to be recoated, but hydrated lime is very inexpensive (I coated my roof with $14 worth of hydrated lime).