Where on a metal chimney is rust most commonly found?

Rust on a metal chimney is most commonly found above the roof line due to exposure to moisture and elements. When a chimney extends beyond the roof, it is more susceptible to rain, snow, and other weather conditions that can lead to oxidation and rust formation, especially if the chimney lacks proper protective coatings, is improperly flashed, or if there's an absence of regular maintenance.

While rust can occur in various areas of a metal chimney, the upper portion, or the part above the roof line, experiences a greater accumulation of moisture that can lead to corrosion over time. Areas like the base, inside the flue, and beneath the cap can also harbor moisture; however, the exposure of the upper section to environmental factors makes it more prone to rust development. Additionally, it's common for the upper areas to have less protective features compared to other parts, particularly if they are not regularly inspected or maintained, leading to deterioration more rapidly in those locations.