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Best metal roof finish for safe rainwater collection
Q. I'm looking for factual, specific information about the safety of collecting rainwater from different metal roofing products to be used for human use. Which finishes are safe for collecting potable water? water for food crop irrigation? livestock water?
Has there been any testing done on the toxicity of run-off water from metal roofing finished with fluorocarbons vs urethanes vs silicon polyesters vs standard polyesters, and so on?
It's very difficult to find a definitive answer. Any information is appreciated. Thanks.
Designer, Homeowner - Hancock, Michigan, USA
March 29, 2009
A. Hi Barbara,
Many (many) moons ago I came across a factory where there was corrosion of the steel reinforcing girders, etc... they had tried out all types of panaceas.
Suddenly someone (a bright guy) had suggested using ordinary pitch, i.e., asphalt. This worked like a charm.
Seeing that pitch, etc., is used in many houses, that them-there rainwater is 100% drinkable, the bond is good whereas with Urethanes & others, you'd need clean metal etc.
The product fumes were from tanks containing sodium chlorate.
This concept saved me ... I was working on some hot sulfuric fumes and opted for PP ... in those days the UV stabilized PP didn't exist, so I painted the outdoor ducting with pitch. Worked like a charm.
Hope that this helps you somewhat.
Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
May 8, 2009
A. Rain water is not pure by most definitions. If I were going to put on a metal roof for the first time, I would use one with a baked on Kynar finish. It costs a bit more, but if you do not walk on it with dirty boots, it will normally last a very long time.
Note that pitch tar is not exactly the same as asphaltic tar. They are close, but pitch is nastier to put on and requires a hotter temp. There are a number of qualified roofers (but not all) that can apply pitch.
- Navarre, Florida
May 13, 2009
Q. It seems there has been a misunderstanding of the original question.
DO NOT USE PITCH !!
That is the last thing you want in water you are going to drink. But I have the same question about metal roofing: Will harmful chemicals leach out of the paint and contaminate your drinking water?
- Grove, Oklahoma
August 26, 2013
A. Hi. Although my first thought echoed Earl's, after a quick review I'm not so sure. Browsing 4 or 5 websites I see coal-tar pitch "not listed" under their lists of toxins, and apps.echa.europa.eu/registered/data/dossiers/DISS-9d93573a-8e5f-0b85-e044-00144f67d249/AGGR-faa5a4cd-fb57-4452-a19f-760b73a19a28_DISS-9d93573a-8e5f-0b85-e044-00144f67d249.html says "Pitch is water insoluble and thus not bioavailable or only to very limited extent".
Freeman isn't around anymore to ask for clarification whether he misunderstood the question, but I think a general point is that we are a much more squeamish society than 50 or 100 years ago, and I can't really picture Americans drinking cistern water or roof water anymore. And for "food crop irrigation", I've now become pretty confident that the water from a pitch roof is harmless.
I don't think the FDA considers any paint food-safe. So why not a stainless steel or stainless steel clad roof?
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 26, 2013
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