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Why metal roofs with AZ35 coatings won’t cut it

The metal roofing industry has seen a massive expansion. And for good reason. With long-term protection and durability – not to mention strong esthetics – many homeowners and contractors are catching onto the appeal of metal roofs. As an added bonus, they are energy efficient and may qualify for homeowner’s insurance discounts, which offers the potential for more money in homeowners’ pockets.

With metal panels, there are plenty of options available, which is great. At least in theory. Contractors love having the choice of style, cost and installation method. The problem arises with how each option affects the roof’s performance, which is more than a contractor might think. As is often the case, the cheapest solutions available on the market happen to also be the riskiest. Unfortunately, what professionals may not realize is that cheaply made metal roofing products could be more of a headache than they’re worth – costing much more in the long run.

Why might contractors not realize they’re sacrificing performance? Because often the most popular manufacturers either miscommunicate or misunderstand what’s needed for effective quality and performance.

The reality is, for maximum performance and protection from corrosion, the steel core of every metal panel must be coated with a substrate layer of aluminum-zinc alloy. More to the point, panels must be coated with a specific thickness of this alloy.

When manufacturers go light on this protection layer, they put roofing jobs at risk. The biggest culprit? AZ35 substrates. AZ35 coatings are too thin. In fact, they’re so thin that AZ35 coatings do not meet building codes for residential roofing in most counties. And yet, tons of metal roof manufacturers sell it.

Misconceptions of metal roof coatings, standards and warranties not only leave projects underprotected, but also put businesses' reputations at risk, leaving them liable for damages resulting from roof failure.

To meet today’s codes and standards, metal roofing panels need a minimum AZ50 anti-corrosion coating.

Knowing the difference between AZ35 and AZ50 coatings is critical for today’s jobs. The best place for contractors and roofers to start is to understand what’s beneath the surface of their roofing materials.

Getting Under the Surface

Most people think metal is metal. But it’s not. Across different products, there is a wide variety of paint types and thicknesses, rust-blocking coatings and steel hardnesses and thicknesses that are specially formulated to protect roofs from fading, rusting and denting. When you start to break down the layers of metal roofing materials, it’s easy to see where cutting corners can be detrimental to roof performance.

Metal roofing panels are made up of three distinct layers:

  1. Steel – This layer, first and foremost, is what gives the roof its strength. Variations at this layer relate to the thickness of the panels, indicated by the gauge. Each gauge number represents a range of thickness. For example, a 29-gauge panel could be as thin as .0115” or as thick as .0155”. Industry-standard 29-gauge metal for roofing is .0142”.
  2. Substrate– In order to protect the steel core from rusting, roofing metal has an aluminum-zinc alloy coating. Thickness of the coating is measured in weight over 100 square feet (AZ55 = 55 ounces of aluminum-zinc alloy per 100 square feet). Coating thickness is the critical factor in the effective application of the protective top paint layer to the substrate. Quality metal roofing has AZ50 and AZ55 substrates to provide a thicker coat of anti-rust protection. When AZ35 is applied, it provides less long-term protection for the steel layer and can also impact the performance of the top paint layer that follows.
  3. Paint – On top of providing the look and esthetics of the panels, this outer layer should also provide optimal protection from the elements. Typically, this would be a .9 to 1 millimeter acrylic or paint layer (composed of pigment, resin and solvent), which is designed to protect the substrate and steel from weather exposure, while providing fade-resistant color and gloss.

When manufacturers give the proper care and attention to all three distinct layers, the result is high-quality metal roofing material. But, when manufacturers cut corners with the middle layer of protection – arguably the most critical component – by using sub-par AZ35 coatings, it leaves the roof susceptible to premature rust and corrosion, which can lead to roof failure and early replacement.

Building Standards Point to the Better Way

As we know, building standards exist to ensure projects are completed properly, every time. It’s why almost every state has adopted a version of the IBC / IRC for their state codes. Those that don’t adopt statewide standards often have local codes that use IBC / IRC standards. When it comes to selecting metal roofing materials, the codes are very clear: only metals coated with AZ50 coatings and higher are acceptable.

When panels don’t meet that minimum standard? It can get expensive. Mark Barnett, director of Strategic Projects at Reed’s Metals Inc., has seen what happens when metal roofs aren’t adequately protected.

“Far too many times, we’ve seen what happens when contractors purchase panels without the proper coating protection,” shared Mark. “Cheaper AZ35 products may seem like a good deal, but they are not up to code and put you at risk of losing your license and being sued. Even if you avoid those consequences, oftentimes contractors are left having to replace a roof once the materials start to show their true colors from harmful damage and corrosion. The initial savings of using an AZ35 product now could put you at risk of replacing the roof at significant dollar expense to your company, as well as poor referrals.”

It’s not just that building codes call for AZ50 or greater; it’s also worth knowing that any coating under AZ50 is not warranted by the steel mills. Many products with an AZ35 substrate will offer warranties with a “40-year” guarantee, but those warranties only cover the paint. They don’t cover the substrate itself and the protection it must provide from corrosion. This is a major risk that ultimately leaves the metal roofing materials without any rust or corrosion protection at all. Be sure to read any manufacturer warranties carefully to understand coverage terms.

What the Research Has to Say

Of course, following building codes is important, and in this case, it is for a very good reason. When comparing AZ35 and AZ50 substrates, recent research proves the detrimental effects of using the sub-par alternatives.

To understand the differences, an independent testing lab performed accelerated ASTM B117 weathering testing (salt fog) of a panel, protected with industry-leading AZ50 Galvalume® substrate with a corrosion warranty. They did the same test on a 40-Year Standard panel that uses a non-compliant AZ35 substrate. This test is one of the methods used to assess performance of pre-painted steel coils as referenced in ASTM A755.

Through this testing, the research showed edge corrosion was 15 times worse for panels with the non-compliant AZ35 substrate versus AZ50-protected panels. Harmful edge corrosion not only affects the quality and longevity of the metal, but also the look and performance of the paint. Therefore, as expected, the AZ35 panel also experienced significant paint failure, with 1.5 times more fading than the AZ50 panel.

The research in this case was clear: When compared to AZ50 substrates and higher, AZ35 coatings are not able to deliver what’s needed for metal roof rust prevention and protection.

Start by Making Informed Decisions

When looking for metal roofing products, it’s important to start by asking the right questions. Assuming that products meet building codes and standards is risky, so make sure you know the facts at every level:

  1. Steel for strength.
    Know what you’re buying. Ask for the manufacturer’s specifications on metal thickness. A lower gauge number indicates heavier steel. Heavier steel resists dents and wind uplift. It also can span larger framing for pole barns and metal buildings. Lighter, more economical 29-gauge steel may be preferred for residential roofing with solid decking. No matter the project, it’s important to confirm the material gauge requirements in the construction drawings to ensure the roof meets design expectations.
  2. Substrate for rust prevention.
    Of course, make sure to use a product with substrate classification of AZ50 or greater. Ask your supplier to show you their written substrate warranty. If they cannot provide one, then you may be purchasing a material that does not meet international building codes and standards. For a simple solution, look for products that feature Galvalume®. For a product to use the Galvalume® trademark, it must use an AZ50 or greater coating.

    On top of meeting the critical industry standards, AZ50 Galvalume® products are superior in quality, esthetics and performance over AZ35 products.

    As Reed’s Metals’ Mark Barnett explains, “There’s a reason we only offer AZ50 Galvalume®-protected products and higher. Anything less than that leaves jobs, homeowners and our customers’ reputations at risk. Choosing Galvalume®-protected products from the start is the best way to ensure roof quality and performance, long term.”
  3. Paint for color and protection.
    Economy or non-warranted panels tend to have less coverage and quality when compared to better-performing panels. Be sure you are getting a superior paint layer. For example, using a strong silicone-modified polyester (SMP) coil coating system is an ideal coverage solution. A system like this can offer benefits in gloss retention, resistance to color fade, formulation and durability.

Reed’s Metals’ Galvalume®-protected products have many advantages:

  • Superior corrosion resistance
  • 1.5x slower paint fade than the competitors
  • Brightness retainment for a long period (long service life)
  • More than 50 years of use in virtually all climatic conditions has proven the product's overall superiority

Protect Your Work and Reputation

On top of knowing your panel make-up, in many cases it comes down to really knowing your supplier. Make sure you use a reputable supplier who can provide you with open, honest information about your purchases and give you the warranties and support to get the job done right.