In the 1860's, the options for metal roofs were copper, lead, tin-coated iron, and terne-coated steel. Tin-coated malleable iron was disappearing at the time. Copper and terne rolled roofs were very popular during that vintage--terne more so because it was less expensive.
Terne is an alloy of lead and tin that provides excellent corrosion protection for steel. It was recently taken off the market due to the politics of lead, although no specific health threat was ever established. Terne roofs can last a very long time. A lot of terne roofs are a good 100 years old. To say a terne roof would last 170 years might be optimistic, but it's not out of the question if the roof has been well maintained over the years--especially if it's located in a benign climate like that in some of the drier western states.
Recognize too that the modern materials that one might use to replace this historic roof with would likely not be as durable as the original material. The more popular metals used today are coated carbon steel and aluminum. You can generally expect 40 to 60 years out of those if they are installed properly. However, no painted finish on those materials will last that long. Today's premium factory paint options will go 35 years at best.
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keywords: preserving old house, historic house, building, heritage building, historic buidling, heritage buildings, dyi, do it yurself, home improvement
keywords: old house, terne, metal roof, paint, metal roofs, heritage building, preservation