Environmental corrosion of copper

What can slow corrosion on roofs?

Editor's note: Following are the author's opinions about copper corrosion based on his observations. Views expressed are not necessarily those of NRCA.

Copper sheet metal has been and continues to be a beautiful, durable roofing material. Copper interacts with the environment to form a patina that protects itself from further corrosion, and the color of uniform patina is one of the glories of architectural roof systems.

However, during the past 40 years, especially the past two decades, I have observed rapid corrosion of copper and lead-coated copper roofing and flashing materials exposed to rainwater runoff from other building surfaces and roof surfaces. The phenomenon I describe is something that isn't yet conventional wisdom in the roofing industry, but you should be aware of this problem.

Since first becoming aware of the problem in Connecticut, where I live, I have looked for similar occurrences in other parts of the United States. I have observed copper corrosion as far north as St. Johnsbury, Vt., and St. John, New Brunswick, and as far south as Asheville, N.C. In Chicago and cities west of it, I haven't seen the same sort of corrosion because of a lower concentration of acid aerosol and acid rain.