Galvalume vs. Galvanized Steel Roofing – What’s the Difference?

If you are one of the many homeowners looking to install a steel roof on your home, understanding the difference between galvanized steel vs. Galvalume is essential to getting the top performance you expect from your new metal roof.

Right off the bat:

In most residential steel roofing applications including near-coastal areas — beach homes located near the shore, and even homes located in the middle of heavy salt-spray — severe marine environments, Galvalume steel will be a better and more corrosion-resistant option than galvanized steel.

The one exception when Galvalume should not be used:

Galvalume steel should not be used on, in, or around concrete or mortar. Concrete and mortar are highly alkaline environments. Bare Galvalume and painted Galvalume sheets suffer rapid corrosion when in contact with mortar and concrete.

Bare Galvanized steel and painted Galvanized perform better in this type of environment.

Now, because aluminum, one of the two metals in Galvalume coating, provides a barrier protection for steel, instead of galvanic or self-healing protection in galvanized steel, scratches and cut edges in Galvalume are less protected.

Galvalume steel is best for use in prefabricated metal wall panels and standing seam metal roof applications with concealed fasteners.

We would not recommend using Galvalume panels with exposed-fastener steel roofing systems such as corrugated or ribbed steel roofs.

Normally, Galvalume is offered in both bare and pre-coated (pre-painted) versions. Most residential-grade Galvalume metal roofing products – like galvanized steel – are coated with Kynar 500 or Hylar 5000 paint finishes.

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Galvalume has an excellent performance lifespan in bare exposures (unpainted panels) as well.  Both galvanized steel and Galvalume weigh 100 to 150 pounds per 100 square feet and contain about 35% recycled steel post-consumer content.

The cost of Galvalume and Galvalume Plus are about the same as that of galvanized steel.

A product called Galvalume Plus features an extra coating of acrylic. One advantage of Galvalume Plus is that it can be roll-formed dry, without vanishing oil. Thus, Galvalume Plus is very easy to form and install safely in the field, using portable roll-formers.

What is galvanized steel:

Galvanized steel was invented and developed for commercial use in the first half of the 19th Century, so it has nearly 200 years of proven track record. Carbon sheet steel is dipped in molten zinc. It’s more than a coating, however. A chemical bond occurs and produces the telltale “spangles,” the crystalline surface pattern found on galvanized steel.

Tip: Look for G-90 galvanized steel for residential applications not G-60

What is Galvalume:

Bethlehem Steel developed the product and introduced it to the world in 1972, so it’s been in use nearly 60 years. Similar to galvanized steel, Galvalume is produced by a hot-dip process. Instead of 100% zinc, the dip is 55% aluminum, 43.5% zinc and 1.5% silicon.

  • Zinc bonds with the steel surface to create a barrier to corrosion-causing moisture
  • Aluminum is a metal that naturally resists corrosion and reflects heat too
  • Silicon enhances the adhesion of the coating, keeping it in place when the steel is rolled, stamped or bent

A product called Galvalume Plus features an extra coating of acrylic. One advantage of Galvalume Plus is that it can be roll-formed dry, without vanishing oil. Thus, Galvalume Plus is very easy to form and install safely.

How Corrosion Occurs in Each

Unpainted galvalume vs. galvanized steel exposure over time.

via ABC Metal Roofing

Corrosion is the death of metal roofing, as we all know. Galvalume and galvanized steel roofing are affected differently by corrosion.

Galvalume: Aluminum has tremendous corrosion resistance, so it will generally corrode more slowly than galvanized steel. The one exception is when the coating is penetrated – scratched or chipped by falling or blowing debris, for example. The exposed sheet metal beneath the coating will quickly corrode. However, the aluminum coating will prevent the corrosion from spreading; it will be contained.

Galvanized steel: More than just coating the steel, galvanizing steel produces a chemical bond resistant to corrosion, scratches and nicks. Galvanized steel will self-heal for small scratches and along cut-edges.

Over-time, when galvanization layer in galvanized steel panels wears down or is penetrated, corrosion will begin to spreads.

Spreading corrosion will affect steel beneath the galvanized, bonded layer, and the layer galvanization will be shed. The front line of the corrosion spreads like a storm front across a radar screen. Slowly, perhaps, but steadily.

Uncoated/Unpainted Galvanized Steel vs. Galvalume Wear: 10, 15, 20 Years and Beyond

To illustrate the difference in performance between galvanized and Galvalume steel, let’s consider how the two kinds of steel would perform in an uncoated/unpainted steel roofing application.

Note: With a quality paint finish such as Kynar 500, both G-90 galvanized steel and Galvalume steel should provide consistent, rust-free performance for 30 plus years when used in accordance with manufacturers’ specifications.

With unpainted steel (normally you cannot buy unpainted galvanized steel panels), in the short-term, galvanized steel often holds its rust-free good looks longer than Galvalume thanks to self-healing properties of zinc.

5 to 10 Years: The galvanized roof will look “perfect” accept for some corrosion beginning where fasteners penetrated steel during the installation. The Galvalume roofing may show corrosion at nicks and scratches and around field-installed fastener holes.

10 to 15 years: The Galvalume roofing will look about the same, but with a few more nicks that produce isolated spots and lines of corrosion. The galvanized steel roofing will start showing its age. Corrosion has continued to spread outward from its starting point.

20 years: The changes in the Galvalume roofing are slow and imperceptible, though if you compared a picture of the new roof to its current state, the nicks and scratches would be visible. You might also notice a slight patina common to ageing aluminum. The galvanized roof, depending on climatic factors, might show a light rust hue. This is the result of the zinc layer wearing away, leaving the steel substrate exposed.

Beyond 20 years: The lifespan for unpainted galvanized roofing is 15-25 years depending on climate, less where oceanic salt spray is common. Unpainted Galvalume has a lifespan up to 40 years. Once corrosion has penetrated the sheet steel substrate of any steel roofing, the integrity of the steel will suffer and your steel roof will begin to fall apart.

Selecting the Right Paint Finish for Your Steel Roof

The coating or paint finish used on Galvalume is very important because it is critical to the length and conditions of the entire warranty of the steel panel product.

Over the years, coated metal roof panels have progressed from a single coat straight polyester paint system in the early years to Poly vinyl idene fluoride (PVDF) acknowledged as the premium resin for coil coatings.  It is a kind of fluoride, a family that includes such well known products as Teflon and Halar.

PVDF resin has superior chalk resistance and gloss retention as well as stain and chemical resistance. It is softer than polyester, making it highly formable without risk of cracking. Current pretreatments and primers along with superior paint coating have increased product warranties dramatically.

Did you know? Most homeowners don’t check to make sure that they are not getting the inferior straight polyester system. Straight polyester should really not be used for metal roofing and metal wall panel in today’s market.

If you see there is a dramatic difference in the price of two different painted Galvalume or galvanized steel panels, then the less expensive, less effective  kind of coating should be suspected in a cheaper panel.

Tip: Always look for Kynar 500/Hylar 5000 or equivalent paint finish for residential applications.

What about horse barns and agricultural uses?

Neither Galvalume or galvanized substrate are warranted to be used, in or around animal confinement or agriculture. Bare Galvalume and coated Galvalume panels will very rapidly deteriorate when exposed to the corrosive agents of animal confinement.

But bare Galvalume and coated (painted) Galvalume panels have an outstanding corrosion resistance in a wide variety of general environments, including industrial, marine and even severe marine environments.

And Galvalume (as well as Kynar 500 coated and CoolRoof rated G-90 steel) offers exceptional heat reflectivity, resulting in lower energy load on homes and HVAC equipment, resulting in improved interior comfort and energy savings for homeowners.

Painting and Maintaining Aging Galvanized and Galvalume Roofing

Both types of steel can be painted to improve durability. Paint adheres better to galvanized steel than to Galvalume, so it requires repainting less often.

Note: With Kynar 500 or Hylar 5000 Paint finish or its equivalent, you will not have to paint over your steel roof for at least 30 years. — That’s the normal length of warranty on Kynar 500 paint finish.

When corrosion is spot-treated to stop its spread and the roof is painted when needed, galvanized steel roofing can last 50 years or more. While Galvalume isn’t yet 60 years old, the product should easily last 50+ years when painted and maintained.

Closing Thoughts on Steel Roofing

Galvanized steel and Galvalume offer excellent value – cost over their service lifespan. Unlike other roofing materials, such as asphalt, with Kynar 500 steel roofing there is no need for ongoing maintenance to ensure best long-term appearance, durability, and longevity.

The quality of the roofing installation itself plays a major role in how long a steel roof will last. Interview several qualified, experienced metal roofing contractors to discuss your options and costs for galvanized steel vs. Galvalume.

References:

http://www.englertinc.com/blog/architects-notes-the-differences-between-galvalume-and-galvanized-in-metal-roofing-projects/


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