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Copper/stainless steel interaction

We installed a corrugated copper roof with stainless steel screws using neoprene washers. At a small percent of the screws there appears to be rust developing. Why? What is the solution? (the bronze screws we initially selected were not available in long enough lengths)

Barbara Winslow
an architect - Berkeley, California, USA

Copper exposed to outdoor conditions will eventually develop a greenish rust known as patina.
This is considered not only harmless but appealing. Is this the corrosion that you try to avoid?
Or is the iron in the stainless causing an orange powdery rust?

If you can tolerate patina you can copper plate new SS bolts to replace the bare ones. They will develop patina as well. If no rust is acceptable, then you will have to consider an organic copper color paint or transparent long lasting lacquer.

G. Marrufo-Mexico

Guillermo Marrufo
Monterrey, NL, Mexico


It is possible that the screws that are rusting have come into metal to metal contact with the copper. When wet, this can set up a galvanic situation that could cause some rusting. If the rubber washers are protecting the steel completely, and there is not possibility of water setting on the fastener in such a way as to allow electrical contact through the water, there should not be a problem.

You still always have a possibility of rusting of the stainless steel, depending on grade, environment (salt spray, etc.) and age of the screws.

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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A friend is considering a copper roof on his 1910 era house. A question has come up about the type of fastener to use. I told that he should use copper, bronze, or stainless. He seems to think that any fastener will do. I think that will lead to leak problems in the future. Is there any standard that should be followed when installing a copper roof?

Bob Walsh
hobbyist - San Antonio, Texas

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