It is true that US Steel – the patent holder for Cor-ten(R) – doesn’t recommend using their product for “architectural applications,” which they clarify to mean roofing or siding.
Why not? There are actually several good reasons. Corten is a non-reflective material meaning pretty much the opposite of a sustainable sound “cool roof.” Plus, it’s important to keep this material well drained (and out of contact with standing snow, standing water or even a pile of wet leaves). And the very rusting weathering process that protects Corten steel can then drip rust-colored water onto adjacent surfaces like sidewalk or siding and stain them permanently.
That doesn’t mean that US Steel won’t stand behind their product or even that we architects can’t use it for anything. It just means we need to use it correctly – i.e. not as roofing or siding.
Corten can actually be used many purposes. It has become so popular as a feature in Landscape Architecture and sculpture that architecture jokes that “the temporal associations of COR-TEN® steel have become associated with a pointless gravitas in much the same way as Barber’s Adagio for Strings has become shorthand for ‘feel sad now.’”