The Case for Corten Steel Cladding: Material Case Study

Attending architecture school in the mid-2000's, I was always enamored with the material Corten steel. Corten is a great material that has not only a nice warm hue and texture, but also is authentic - real metal that is really rusting. The sentimental architect in me lauds the poetry of the building's age being seen in the facade as it continuously rusts, like an active ruin of a bygone era. I know, I know - you didn't come here for that - but it does have a beautiful aesthetic that makes any building stand out in a crowded market. We believe that it should be used more often in our cities and especially on our affordable housing projects. We will use our recently completed project 17 Mississippi Apartments as a case study.

Why Corten?

​COR-TEN (CORrosion resistance, TENsile strength) steel as it is officially referred to was a steel that was developed in the 1900's for outdoor applications, as the steel would form a natural protective coating (rust) as it is exposed to the elements.  As the panels rust, a beautiful patina develops, starting at an orange color and ending at a brown color. This process can take up to ten years and is constantly changing day by day. The steel can be made into a variety of different forms and shapes - including flat sheets for cladding. The product has been used around the world on infrastructure, public art, and buildings both as roofing and cladding.  There are many benefits in using Corten cladding on buildings, including:

• Aesthetic - Corten has long been a sought out material by designers. It has a rich orange tone and a robust texture that really punctuates whatever building employs it. 
• Authenticity - Many cladding materials available today try to imitate real materials such as wood and stone. Corten is real metal with real rust. No artificial product can truly replicate its unique qualities.
• Durability - As mentioned above - a protective rust coat develops over the Corten that protects it from corrosion.  This can mean up to  100 years + lifespan.
• Sustainability - The steel is made from pre and post consumer steel and is 100% recyclable. 
• Cost - When metal panels are mentioned, especially in the affordable housing world, most of the time they are value engineered out of the project.  However, because Corten is an untreated metal, it has a much more reasonable price and compares favorably with fiber cement panels. 
• Maintenance - The panel is allowed to rust into perpetuity, although there are some challenges we will discuss below.

Our Process

17 Mississippi Apartments is a recently completed 41 unit affordable housing project that utilized Corten cladding.  Below is our step-by-step process for success with Corten:

1. Design and price
2. Mock-up wall testing 
3. Install 
4. Apply rust accelerant product on panels with a paint roller and reach the patina of choice over 2 weeks.
5. Apply rust neutralizer to clean the accelerator off with a paint roller.
6. Apply sealant product with a paint roller to ensure aesthetic is kept.

Design and Price

​RustWall Concealed Fastener Panel

The building had a number of units set aside for artists in the neighborhood, so we wanted to with a palette of unique and tactile building materials.  We reviewed a few options, but came across the Rustwall product from Western Metal Roofing.  The concealed fastener panel is easy to install and only needs a 1/2” z channel to work in a rain screen system.  The astonishing thing was the price for the material - I recently re-checked this and the material is under $4 a sf.  As we went through the pricing and value engineering process, we were able to keep the product in the budget.  

 Mock-Up Wall Testing

As we discussed the material with the client, it came up that the client wanted the patina finish to not only be achieved by groundbreaking, but also for it to remain into perpetuity. The panel, when it arrives on site, is a battleship gray.  Our renderings relied on the orange patina and the client expected  this on day one.   Luckily, there are rust accelerants to speed up the process, as well as rust coatings that would stop the process all together. 
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Testing was performed on a mock-up wall prior to implementation on the building.  We tested all sorts of application styles (brush, roller, spray etc…) and number of coatings.  The rust accelerant was particularly impressive.   In a matter of minutes, what would take a year to create magically appears in front of your eyes. The mock-up phase was necessary to get ownership buy-in and also for us to get the correct coloring for the building.  We were now ready for the full order and install.

Mock-Up wall prior to accelerant

Mock-Up wall with one coating (left and two coatings (right) of rust accelerant

Installation

At 17 Mississippi, we used a vertical alignment for the panels and applied directly to the Zip sheathing backup.  This allowed water to flow down the weather barrier without any pooling. We installed all panels without any coatings applied and then washed the panels with a hose to remove any oils or adhesives that may have been used during shipping.  The client was a bit concerned about runoff staining, so we installed a 3” wide gravel strip on grade below the panel. This process was complete within a week and is not any different from a traditional cladding install.

Vertical panel install

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Coating 1 - Rust Accelerant

We applied a rust accelerant from Bromoco (XL-08) (really mostly water and vinegar) that was applied to the building after install of the panels with a simple paint roller.  We tested the number of coatings and application styles during the mock-up phase and settled on two coatings rolled on by a paint roller. XL-08 prices for roughly $55 a liter, with a liter covering roughly 400 sf.

Coating 2 - Rust Neutralizer

After we were happy with the rust level, we applied Bromoco's Rust Neutralizer (ERW-BD) which helps to clean the rust accelerant off and is necessary before the final coating. This was also applied using a roller brush. The steel will continue to rust at its natural pace after this point.  ERW-BD prices for roughly $50 a liter, with a liter covering roughly 1000 sf.

Coating 3 - Rust Sealant

Finally, the client wanted to ensure that no staining from runoff occurred in the future. We added Bromoco's Sealant (CT-7571-W) to stop the rusting process and preserve the aesthetic for the next five years.  The sealant does add a bit of darkness to the panel, so the mock-up phase again is crucial to developing the aesthetic you are going for.  CT-7571 prices for roughly $300 a liter, with a liter covering roughly 400 sf.  To keep the coating fresh, this will need to be reapplied every 5 years.

Midway through the application process

Final Thoughts

The pictures below speak for themselves - we achieved the proposed aesthetic and the building really pops.  We are also proud to announce that  a year after install the product is still in good condition and no staining/runoff is seen at any of the windows or at the ground.  The panels even look better than they did a year ago. 

I also want to emphasize that the three coatings are not needed - this product could be installed and left to rust naturally over time.  It is really up to the building owner’s preference.
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Although there is some extra care that is needed with this material, it is not overly expensive or hard to work with and maintain. This should open the door for more builders and designers to add a new look to their palette and further beautify our world in the housing they provide. 

​1 year later and no runoff staining seen