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Community and Q&A

Galvanized roofing and rain water catchement.

user-757117 | Posted in Green Products and Materials on

I was involved in roofing a barn over the weekend with galvanized steel panels. The panels had been stacked next to the barn where they had been rained on. I noticed that water that had gotten between the panels had caused a reaction with the zinc coating, leaving a gritty white stain (zinc oxide?). I’m sure a good rain will wash off the grit (probably not the stain), but I wonder how good an idea it would be to catch rain water off this roof now – even if the grit has been washed off. Anybody have any idea?

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  1. Steve El | | #1

    I have no idea. On the subject of rainwater catchment, I just wanted to share a news story I caught in the paper when visiting Colorado a few years ago. It seems so many people are moving into the foothills and starting to catch rain that farmers out on the arid plains filed a lawsuit to stop the practice. Out west, the law of water is summarized by "First in time, first in right". Farmers who had tapped the flowing waters for irrigation and had water use rights made the argument that from the moment rain hit the ground it is flowing water, and subject to their irrigation rights. Therefore, they wanted the court to enter an injunction against rainwater catchment. I did not follow the case so don't know the outcome.

    This sort of story keeps me flipping between laughing at us silly humans, and crying at us stupid humans.

    Back to your original excellent technical question.... I'm interested in the answer also. I suppose you could ask the same question about any material. Slate and tile must be pretty benign.

    Sorry to butt in. I just had to share that story. Looking forward to your other replies.

    Steve El

  2. Riversong | | #2


    Galvanized steel or terne is not a good roof for potable water catchment, or any roof with lead flashing. Painted metal is OK, through natural non-reactive materials like slate, tile, copper and cedar are better. Surprisingly, a lot of rainwater is collected from composite shingle roofs.

    But, any potable rainwater system should include a First Flush diverter (or equivalent) to wash the roof clean during the start of every rain event before sending it into the reservoir, and some kind of filtering system.

  3. Interested Onlooker | | #3

    What is it that rainwater collects from a galvanised roof that is not removed by the filtering system?

  4. user-757117 | | #4

    I always figured that a galvanized steel roof would make an ideal rain catchment surface.
    Seeing an unusual example of how these roofs corrode has made me wonder.
    When a galvanized panel is new it is very shiny. In fact, I think I may have a bit of a tan under my chin from the weekend's work.
    After several months of being exposed to the elements these panels turns more of a dull grey. I see now that the dull grey is corrosion of the zinc surface in progress. So, at some level there must be little bits of that oxidized grit coming loose all the time.
    Question is, do I want that grit in my water in any amount?
    I'm not sure I do.

  5. Riversong | | #5

    I would have some concern about dissolved metals, such as zinc, particularly with acidic rainwater.

    We get all the zinc we need in a typical western diet and the natural zinc in ground and surface water. Zinc is a necessary micronutrient but excess zinc can cause immune suppression and other problems.

    The primary roof material to avoid is lead, which makes an excellent flashing material but is obviously unsafe for a drinking water source.

  6. gobug | | #6

    IIRC the Sierra Club's first legal victory was to prevent Aurora from taking water from the start of the Frying Pan River. Some of the pipe had already been placed.
    I am a Colorado native. I have some mountain property and catchment has always been an interest. Catching rainwater in Colorado is legal. There are limits, though. I called the state and was told they do not look for violations, unless the catchment system is blatantly in violation.
    I suggest taking a sample of the questionable catchment water to the state for testing. They have several different types of tests available - at different prices. Potability is the cheapest, and you can get more detailed for specific things, like lead, sulphur, etc.

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