Over time, untreated copper gutters will acquire a patina, slowly turning gray to greenish. In Minnesota, copper gutters are growing in popularity because homeowners want that durability. They also want the look that the gutters give. For instance, after about 4 to 5 years the gutters will start to turn a greenish color. This is called the “patina process.” The copper oxidizes and turns green.
Typically allowed to weather naturally, copper develops a blue-green coloration resulting from the formation of a protective copper oxide patina. During the initial weeks of exposure, particularly in a humid atmosphere or in areas of frequent rainfall, radical color changes often take place with iridescent pinks, oranges and reds interspersed with brassy yellows, blues, greens and purples. During continued exposure, these interference colors fade and are replaced by relatively uniform russet brown shades referred to as statuary or oxidized finishes. In industrial and nautical atmospheres, the natural patina generally forms in approximately five to seven years. In rural atmospheres, where the quantity of air-born sulfur dioxide is relatively low, patina formation may not reach a dominant stage for 10 to 14 years. In arid environments, the basic sulfate patina may never form due to the lack of sufficient moisture. Similarly, exposed horizontal surfaces develop the patina more rapidly than sloping surfaces which, in turn, patinate more rapidly than vertical surfaces. The critical variable, in all instances, is the dwell time of moisture on the exposed surfaces.
The progressive oxide, sulfide and sulfate films which develop on copper exposed to the atmosphere are quite thin, two to three thousandths of an inch and highly adherent, but with relatively low abrasion resistance. Neither the oxide nor sulfide films are particularly corrosion resistant. The sulfate patina, on the other hand, is highly resistant to all forms of atmospheric corrosion, once it has had an opportunity to form completely. It significantly increases the durability and, hence, the service life of copper roofing and flashing.
The natural weathering cycle of copper is illustrated by the 12 sequential color plates in the Weathering Chart below.
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Natural Weathering Color Chart.pdf 75K Download |
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| Unexposed | 4 Months | 8 Months |
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| 1 Year | 2 Years | 3 Years |
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| 4 Years | 5 Years | 7 Years |
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| 10 Years | 15 Years | 25-30 Years |
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