A.Rob Haddock of the Metal Roof Advisory Council responds: If this roof is 120 years old, that would mean it was built around 1880. In those days, the options 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. The replacement material, manufactured by Follansbee Steel, is a zinc-tin alloy called Terne II (800/624-6906, http://www.follansbeeroofing.com/products/terne2).

Regarding this particular roof, it would not surprise me to see a copper, lead, or lead-coated copper roof last 150 or even 170 years. There are many examples in Europe, and a few here in the States, that are even older. But since you mention a roof painter, it seems likely that this is a terne roof, not copper or lead. Lead and copper roofs are rarely painted; terne steel roofs must be cleaned and painted periodically.

If they are kept up that way, however, terne roofs also 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 you might 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.