Building Materials: Corten or Weathering Steel


Last week’s post featuring the recently completed Jordano Photography Studio also showed off a favorite exterior materials – Corten steel – in both its front facade and alley-side patio.  We’ve used it before at the Crew Bar + Grill and also have it on the books for several up and coming designs.  What is this intentionally weathering material and why do we like it so much?

What is Cor-Ten

Corten – or weathering steel – Most of what makes Corten great – or not – is that pretty rusting effect.  The cool trick is that the thin coating of rust over the surface actually protects the solid steel inside leading to minimal maintenance. To get the protective (and beautiful) rust coating, weathering steel requires an alternating cycle of wet and dry conditions which means it isn’t great for the deep south or the coasts but it works just GREAT in the midwest.  If you are a Chicagoan, you’ll recognize the material from the Picasso sculpture in Daley Plaza – calmly oxidizing since 1967.

corten picasso daley plaza

We really like having corten as part of our material pallet of possibilities.  The qualities of low maintenance, long lifespan and reusability fit in with our general material choices.  Corten is not only made from pre and post consumer recycled steel … it is 100% recyclable itself.  Plus the warm rusty coloring – and changing color tone over time – are very compatible with other building materials we favor: clean bare concrete, reclaimed wood, exposed brick structure, etc.

The Nay Sayers

While researching this post, I was amused to come across this rant on Treehugger complaining about “all this architect love for COR_TEN steel.”  The author practically pulls out ‘kids-these-days’ line in his grumping about the material’s “recent” popularity and then he goes on to confuse the intentional weathering process with “deteriorat[ing] before your very eyes.”

In fact, Corten is hardly a new kid on the block.  Eero Saarinen used it in his design for the John Deere Corporate HQ  (Moline, IL), in 1964.  The choice appears to have worked out well.

corten at john deere hq

A 20-years-on review by the New York Times described it as “giving the the facade a delicate and subtle texture” and pointing out the benefits of Corten and underlining the way it was used for both the building frame but also for the sun louvers and screens on the glass wall.

Corten – What not to do

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.

Using Corten the RIGHT way

Corten can actually be used many purposes.  It has become so popular as a feature in Landscape Architecture and sculpture that Misfits 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.’”

crew bar and grill corten facade

At moss, we like to use Corten as a rain screen material.  That means it is used as an exterior wall panelling but set off from the structure with an air gap, allowing air to circulate completely around it.  The outer material protects the (hidden) weatherproof layer of the building from rain and impact damage, but remains separate from the building structure. We’ve done that effectively at both Crew Bar + Grill (above) and at Jordano Photography Studio.

jordano corten fence

We’ve also used it as a visual screen – fencing between the back patio and the alley – at Jordano where it serves as both structure and cladding material of a fence that creates a tiny private niche out of an otherwise exposed area.  The corten neatly bridges the divide between natural area concept of a patio and the slightly harsh industrial nature of a Chicago alley.

jordano corten facade

So there’s our nod to Corten steel.  Once you start looking, you’ll see it all around you.