We choose the materials we use for building structures based on availability. Barns are usually no exception. Throughout history there have been stone barns, dried mud barns, sod barns, log barns, timber frame barns with plank siding, barns built from fired brick, and in modern times, barns built with various polymers and metals.
The most common barn siding metal today is steel, and for good reason. Steel is strong, it can be galvanized and paint to the point where rust is rarely a problem, and it requires little maintenance. Steel can be used as a siding on post frame structures, timber frame structures, and steel frame structures.
Before you start to wonder if wood is no longer a viable barn building material, give a little thought to this: With advances in treated wood, wood sealers, and paint, wood can still hold it's own in modern barn building. Wooden barns may require a little more maintenance than their steel counterparts, but if you are trying to get that rustic look to blend with the scenery, wood may be the perfect barn building material for your project.