Posted by McElroy Metal ● December 05, 2019

Do Steel Roofs Rust?

So, do steel roofs rust? The short and sweet answer is yes. Steel rusts, especially when exposed to weather conditions over a long period of time.

The question metal roofing buyers should ask instead is - how to reduce the risk or minimize the potential for rust. The answer is actually pretty simple: become an educated consumer and put an emphasis on choosing the right metal substrate before starting any metal roof project.

Choosing the Right Substrate

It's not unusual for homeowners to spend more time picking out the color for a new metal roof than they spend thinking about the substrate of the steel. And admittedly, it isn't nearly as much fun!  But the reality is that the substrate is essential to the life expectancy and long-term aesthetic performance of any roof. Consequently, homeowners focusing more on color than substrate choices are often faced with premature rusting and find themselves stuck with a roof that needs replacement much quicker than expected.

metal-roof-substrate

The most popular steel substrates in the metal roofing industry are galvanized and Galvalume®. Galvanized substrates are coated in zinc, while Galvalume substrates are coated with a mixture of zinc and aluminum.

For residential roofing applications, most manufacturers recommend Galvalume substrates due to actual field performance over many decades. In fact, the Metal Construction Association recently published a paper indicating Galvalume substrates have an expected service life in excess of 60 years when properly manufactured and maintained. (That’s a big number and one that shingle products just can’t touch!)

But even with Galvalume life-expectancy potentially spanning six decades, some homeowners still consider galvanized substrates instead. For these folks, it's important to understand that higher amounts of zinc offer greater protection against panel corrosion. Two commonly used Galvanized substrates are designated as G100 and G60. These designations relate to the total amount of zinc on both sides of the panel surfaces. G100 contains 1.0 ounce of zinc for every one square foot of panel, while G60 substrates contain 0.6 ounces of zinc. It's important to realize that a G100 substrate contains 66% more protective zinc than a G60 substrate. This difference directly impacts how long the roof will last. As a result, G60 substrates simply do not provide the longevity and performance expected by most homeowners. Yet, G60 substrates are widely available. For the uneducated consumer, it’s an easy mistake to make and one that likely leads to a metal roof with accelerated rusting and failure.

That's why for most applications, Galvalume substrates offer the best long-term solution, and its superior performance has been proven time and again in the field. Over four plus decades of testing and field performance have shown that Galvalume delivers greater corrosion resistance compared to galvanized panels. In fact, the performance of Galvalume in the construction industry has been so superior that steel mills warranty it against rupture or perforation due to corrosion. Similar steel mill warranties simply do not exist for galvanized substrates, regardless of the amount of zinc.

metal-roof-substrate-testing

Metal Roof Substrate Testing

But a picture really is worth 1,000 words. The images above are actual photos from a test where unpainted Galvalume and galvanized panels were placed in different types of outdoor environments, including a normal marine and a rural setting. The harsher the surroundings, the better Galvalume performed. After 23 years of side-by-side comparison in all areas, Galvalume was two to four times more durable than galvanized. Needless to say, it’s critically important homeowners ask their contractor what type of substrate will be used. Who doesn't like to save a few bucks and admittedly choosing a G60 substrate can do that. But the initial savings come at a high cost with the reduced life expectancy of the material. It’s a trade off that most homeowners simply aren't willing to make once they understand the differences.

Which Coating Systems are Best?

So clearly, there is a lot to consider about what’s under the paint (the substrate). But it’s also important to consider what’s “in” the paint.

When considering both long-term performance and curb appeal, the coating system, or paint, for metal roof panels is an equally important consideration to the substrate. The coating/paint is not only the visible component of the roof, but it’s also a critical part of the overall metal roofing system. For the life of a metal roof (remember that could be up to 60 years) ), the coating will be exposed to Mother Nature’s elements: ultraviolet sun rays, acid rain, dirt, etc. Over time, these environmental conditions can break down the coating, resulting in what is commonly referred to as panel fade and chalk. Like rust, fade and chalk aren't attractive. The degree of fade, and chalk are directly related to the coating system.

The most popular coatings for metal panels are Polyester, Silicone-Modified Polyester (SMP) and Kynar 500®(PVDF). Polyester paint systems represent the lowest quality of the three available paint systems and experience the highest degrees of fade and chalk. Consequently, they simply aren't appropriate for residential applications. Kynar 500®, or PVDF, paint systems on the other hand were developed in the 1960's and quickly became the go-to product for commercial and high-end architectural applications. During this same era, SMP systems were the go to product for everyday projects, including residential applications. In the mid 1990s, Kynar 500® paint became more competitively priced and McElroy Metal made the decision to adopt it as our standard paint offering. That decision was based on the fact that it offered our clients unmatched performance at a marginally higher price than SMPs. Today, PVDF (Kynar 500®) paint systems still represent the best performance available for today’s metal panels. The photo below is a real world example of how the two different paint systems stand up against mother nature. Be sure to read the capture below the photo for more details.

Kynar 500 vs SMP

Quality Coatings Made in the USA by Sherwin-Williams®

McElroy Metal’s paint supplier is Sherwin-Williams®. While McElroy Metal offers Polyester, Silicone-Modified Polyester and Kynar 500®/PVDF coatings. Based on performance like that featured in the picture above, we strongly recommend PVDF coatings for residential applications where long-term aesthetics and performance are the top considerations for homeowners.

Hiring an Expert Contractor

For homeowners intrigued by all the benefits that metal roofing offers, finding a contractor who specializes in metal roofing is the next step. The "specialty" part is important because contractors that only dabble or occasionally install metal roofing often steer homeowners toward shingles because it’s more in the contractors comfort zone.

But we're noticing a new trend. More and more savvy consumers are pushing back and refusing to let someone else's comfort zone push them to accept a shorter-term solution for one of their biggest investments, their home. That's why the “Find a Contractor” feature was developed on our website. Visitors simply enter the zip code, adjust the desired distance range, and select “Residential Roofing” from the drop-down menu on the right side of the page. A list of installers with metal roofing experience meeting the criteria is instantly returned. If the search results come up empty, just contact us and we're happy to help as not all of our contractors are listed on the site.

For another approach, check out some of the tips below to find a qualified contractor with metal roofing experience:

  • ask friends/neighbors with a metal roof who they used as a contractor
  • look online for reviews and comments made by former metal roofing customers
  • review contractors' websites and look for examples of local metal roof installations. And don't forget to look for reviews and customer feedback
  • check out our earlier post "How to choose a residential metal roofing contractor"

Conclusion

Let's face it, the roof is a key element of every home, both in how it looks and functions. The right combination of substrate and coating for metal roofing can give homeowners peace of mind for decades. But cutting corners on substrates and paint systems to save a little bit of money today, can also result in premature failure for the total roofing system.

Selecting the right roofing contractor is also an important part of the decision making process. It's important homeowners ask potential contractors what substrate and coating they recommend. When contractors either don’t know the material specifics, or push consumers to lower quality materials, that may be a red flag. Some contractors and even manufacturers like to state that SMP coatings are “almost as good as PVDF”. Trust us, we sell both types of paint and from our 56 year experience, there is simply no comparison between SMP and Kynar 500 painted metal products. Kynar 500 paint wins that one all day, every day!

Download the Residential Metal Roofing Guide

Learn everything you need to know about Residential Metal Roofing. Check out our metal roofing resources for inspiration and answers to your questions.

About McElroy Metal

Since 1963, McElroy Metal has served the construction industry with quality products and excellent customer service. The family-owned components manufacturer is headquartered in Bossier City, La., and has 13 manufacturing facilities across the United States. Quality, service and performance have been the cornerstone of McElroy Metal’s business philosophy and have contributed to the success of the company through the years. As a preferred service provider, these values will continue to be at the forefront of McElroy Metal’s model along with a strong focus on the customer.

Topics: FAQ, Residential

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