It is our goal to educate our customers about the positives and negatives of Corten roofing. We have supplied hundreds (if not thousands) of rusted roofing jobs and we have not had any complaints about the product. The main reason for this, is because, we do our best to educate our customers of when NOT to use rusted roofing. In most instances there will not be any problems. However, knowing the concerns or pitfalls of this product is just as important as knowing the positives. So, what do you need to know about weathering steel roofing products?
- Corten or bare steel should not be installed near salt spray environments, areas of high humidity, or subject to standing water or frequent rain locations as it impacts the lifespan of the roof.
- There are no engineering load charts or wind uplift information. Any engineering that was accurate at time of delivery would be different after many years of rusting.
- Corten or bare steel can not be used with dissimilar metals.
- Corten or bare steel roof panels can not be installed on a roof with a pitch less than 3/12.
- This material will rust and cause rust runoff, staining the surrounding areas.
Rusted Roofing Is Not Pre-Weathered
Corten steel does not arrive pre-rusted. When your roofing or siding shows up at the jobsite it will have a dull gray appearance and NOT a rusted appearance. You do not need to do anything other than expose it to the weather. There are different stages of weathering that the steel will go through. It will develop some patina immediately when it gets wet. It will continue to patina further as it is exposed to the weather. After about three months you will have a decent patina that continues to rust orange and eventually cinnamon brown.
There is No Warranty for Corten or Bare Steel
When you buy Corten steel you are buying a product that is designed to rust. That type of product is not warrantable. However, it is still an excellent product that will yield a long lifespan with the correct weather conditions. There is a standard disclaimer that is inserted in every order confirmation.
Corten or Bare Steel Disclaimer
Rust runoff will likely stain the surrounding surfaces. Western States Decking, Inc does not provide any warranties, express or implied on cold rolled bare (black steel) or A606 Type 4. This product is intended to naturally weather and rust.
Note: All warranties, express or implied, are disclaimed and excluded; including without limitation any implied warranty of merchantability or fitness for a particular purpose or US material is not pre-rusted load calculations and load tables do not apply to any A606 or bare steel product. Please contact a professional engineer for your specific application of the product.
A606-4 should not be used with dissimilar metals other than stainless steel. This will decrease the lifespan of the product. Screws to be stainless steel.
If you have a rusty roof or wall you will have rust run off. Rust stains are unavoidable. It would not be a good idea to have a rusted roof or wall next to white concrete.
It would also be a bad idea to have a corten wall in an area where people’s clothes would rub against it. This would lead to orange clothing. I would call these conditions bad architecture or design.
The only way to control the rust run off is installation of gutters and downspouts. These will allow you to divert the run off to a different location. The rust run off will be more prevalent when the steel is new and will decrease as the rust matures.
Incorrect Environmental Conditions for Corten Roofing
The idea behind rusted roofing is that the steel is allowed to rust and that rust forms a protective coating that slows the rate of future corrosion. Not all environmental conditions will allow this to happen correctly. Corten or bare steel is not recommended near salt spray environments, areas of high humidity, and frequent rain locations as it will shorten the life expectancy of the products.
These environments can create a condition in which the rust layer never reaches a stable point. Over time, the steel can become perforated and may need to be replaced.
There Are No Load Charts or Engineering for Rusted Roofing or Wall Panels
Corten or bare steel should be installed over a solid substrate. It is an architectural panel and not a structural panel. There is no engineering, load charts, or wind uplift information. Unless you are spanning from support to support than a load chart is NOT important. Any engineering that was accurate at time of delivery would be different after many years of the product rusting.
Incorrect Roofing or Wall Design for Corten or Bare Steel
Here are the main considerations when designing your project:
Your roof should have a pitch of a minimum 3” in 12”. We have sold jobs in which the pitch is less than 3/12, but our recommendation is 3/12.
Eliminate Standing Water
Rusted roofing must go through wet and dry cycles.
Areas that have standing water will rust at a much faster rate. Eliminate moisture and debris entrapment. Corten and bare steel are not rust proof materials. If water accumulates in pockets, those areas will corrode quicker so provision for drainage must be made.
The Correct Roofing Underlayment for Corten
Use a high temperature underlayment.
Heavy Gauge Steel
Do not use a light gauge steel on your corten roof or wall. Any company that tries and sell you 24 gauge or 26 gauge steel for this application is not experienced in selling this product. Buy from someone else as they are simply trying to maximize their profit at your expense. You are buying a product that is designed to rust. Some of the main factors that will determine the longevity of the steel are thickness of the steel and the alloys within the steel. We recommend using a 22 gauge or a 20 gauge corten steel. The only exception would be an indoor application or a fencing application. In those instances a lighter gauge and the use of bare steel would make sense.
Do NOT use dissimilar metals with Corten.
Galvanized, Galvalume®, and zinc should NOT come into contact with Corten.
If you have a wall project with galvanized studs than we recommend putting something between the galvanized and the Corten.
Stainless steel is compatible with Corten.
Acid Washing Corten Steel
Do not acid wash the steel. Lots of people want the steel to arrive pre-weathered. There are chemicals that make this happen faster than nature. Just realize that the cost of doing this is a rusted roof that will not last as long. There is no definite answer as to how much you are degrading the steel. Just know it decreases the longevity of the product.
If any of these problems concern you there is a different solution. Painted panels that are designed to look like a real rusted roof. Painted Rusted Roofing is a Kynar®/PVDF paint system that is designed to look old and rusty. This product has a paint warranty, arrives looking weathered, and there is no rust run off. There are many different rusted color options. It will increase the cost of your job by about $.75/cents per square foot. However, you will avoid all of the problems associated with a real rusted roof.
About The Author
Paul Rubio is the Vice President and one of the owners of Western States Metal Roofing. He has been in the metal roofing industry for 25 years. WSMR is a manufacturer of metal roofing, wall panels, and metal siding. We have colors and finishes that nobody else has with nearly 100 different colors in stock. Our selection of color options and finishes is likely the largest in the industry.