Weathering steel, more commonly referred to by the U.S. Steel tradename Cor-Ten™, is a group of steel alloys that develop a stable oxidation requiring no additional coating. Upon exposure to normal environmental stresses, it acquires a dense, dark brown colored barrier layer that protects it from further oxidation.

Many people like the natural appearance and simple maintenance that weathering steel fabrications provide. In addition to being used in structural framing, roofing and siding panels, weathering steel is often used for decorative fabrications, including signage and sculptures.

There are many things an exterior wall should do.  Among the most significant performance requirements is environmental separation; "it needs to keep the outside out and the inside in," according to Joseph Lstiburek, Ph.D., P.Eng. It also must be safe and structurally sound, and it has to look good. Structural engineers do a good job of keeping structures from falling down.  Building codes do a good job of ensuring that buildings are safe.  Architects do a good job of making buildings look good.  Where we most often fail is in the environmental separation.