Untreated Bonderized Roofing

Michael Blazewicz
Local metal distributor is selling "bonderized G90" for roofing and siding. It is sold without a warranty but I am surprised that they are not recommending that it needs to be painted. I am also surprised that several large projects in town (the high school for one) are using it. We like the grey/blue color as opposed to the shiny galvalume and in our dry intensely sunny climate painted metal roofs fade quickly. Can someone give us a concrete answer? Is bonderized an acceptable "finished" metal to use for a roof in a dry climate? How does it weather over time? Is the rainwater running off of bonderized metal toxic? (we would like to use it to water plants and have as a backup domestic supply (after treatment) in an emergency). Thank you for clarification.
David Stermer
Hi Michael, Thanks for your question. My understanding is that bonderized is a galvanized coated steel with a zinc phosphate coating, ready for field painting. It certainly has an interesting/appealing aesthetic. I believe it would perform well, though typically without warranty. If field-painted, the bulk of the corrosion resistance is provided by the zinc galvanizing, rather than a paint. I would expect some weathering as the zinc phosphate coating slowly weathers away, but the G90 should provide many years of protection. Water running off of any roof is considered non-potable because of potential contaminants in the rain (acid rain), debris on the roof and chemicals leeching out of the roof. I do not know of any hazard associated with zinc with regards to watering plants, other than food plants that have been watered with this source should be washed before being consumed. With appropriate treatment, roof runoff from metal roofs can be brought to a potable status for domestic supply. Regards, David

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