Tag Archives: galvalume steel

Minimum Steel Substrate Coating

Minimum Steel Substrate Coating for Residential Steel Roof Panels

When it comes to residential buildings – whether your barndominium or shouse (shop/house) is stick frame (stud walls), post frame, PEMB (pre-engineered metal building) or some other structural system with a steel roof, there is one import aspect of this roofing material frequently overlooked.

Substrates

Most popular metal roofing steel substrates are Galvanized and Galvalume. Galvanized substrates are coated in zinc, while Galvalume substrates are coated with an alloy of zinc plus aluminum. Both Galvalume and Galvanized are good substrates for metal panels. 

With Galvanized substrates, greater zinc amounts mean 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 contained on both sides of 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. This means a G100 substrate contains 66% more protective zinc than a G60 substrate. This difference will have a direct correlation to panel longevity and long-term aesthetics of your barndominium.

For most applications, a Galvalume substrate offers best long-term solutions, and Galvalume’s superior performance has been field proven. Over four decades of testing has shown Galvalume delivers superior corrosion resistance compared to Galvanized panels. In fact, Galvalume’s construction industry performance  has been so superior, steel mills warranty it against rupture, perforation, or failure due to corrosion. Similar warranties do not exist for Galvanized.

In one test, unpainted Galvalume and Galvanized panels were placed in four different types of outdoor environments, including normal marine and a rural setting. After 23 years of side-by-side comparison in all areas, Galvalume was two to four times more durable than Galvanized.

For extended reading on Galvalume: https://www.hansenpolebuildings.com/2013/04/galvalume/

SO what does all of this have to do with your new barndominium?

Plenty

Building Codes (IRC – International Residential Code and IBC – International Building Code) address substrate requirements for steel roofing.

IRC Table R905.10.3(2) requires a minimum of AZ 50 for 55% aluminum-zinc-alloy-coated steel (Galvalume) or G-90 for Galvanized steel. These same requirements can be found in IBC Table 1507.4.3(2).

What is absolutely amazing is – America’s largest roll former (at least in terms of quantity of machinery) has this on their website for their 29 gauge panels, “…..perfect for residential roofing….”, however when one digs deeper on this website, this panel has only G-60 galvanization, meaning it cannot be used for roofing on anything other than “U” buildings by Code. Only an upgrade to this manufacturer’s 28 gauge “Pro” product will meet residential Code requirements!

Be an educated buyer – know what you are investing in and be certain it is indeed Code conforming. Chances are your contractor, kit provider or maybe even your roll former is unaware.

Best Barndominium Steel Roofing and Siding in Coastal Areas

If you are one of many looking to install steel roofing and/or siding on your new barndominium, shouse (shop/house) or post frame home, understanding differences between galvanized and galvalume is essential to getting top performance you expect from your new steel roofing or siding..

In most residential steel roofing applications including near-coastal areas — beach homes located near 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.

Read more about Galvalume at https://www.hansenpolebuildings.com/2013/04/galvalume/

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

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

Now, because aluminum, one of 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.

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. (For extended reading on Kynar: https://www.hansenpolebuildings.com/2014/05/kynar/ )

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.

Galvalume is similar in investment to galvanized steel.

In the early 1800s galvanized steel was invented and developed for commercial use, 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 telltale “spangles,” a crystalline surface pattern found on galvanized steel.

Bethlehem Steel developed galvalume introducing 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, this dip is 55% aluminum, 43.5% zinc and 1.5% silicon.

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

How Corrosion Occurs in Each

Unpainted Galvalume vs. galvanized steel exposure over time.

Death of metal roofing, as we all know, is corrosion. 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. One exception is when coating is penetrated – scratched or chipped by falling or blowing debris, for example.

This exposed sheet metal beneath the coating will quickly corrode. However, aluminum coating will prevent corrosion from spreading; it will be contained.

Galvanized steel: More than just coating 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 spread.

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

To illustrate differences in performance between galvanized and Galvalume steel, let’s consider how these 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 galvanized steel often holds its rust-free good looks longer than Galvalume thanks to self-healing properties of zinc.

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

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

20 years: Changes in Galvalume roofing are slow and imperceptible, though if you compared a picture of the roof when new to its current state, nicks and scratches would be visible. You might also notice a slight patina common to ageing aluminum.

Galvanized roof, depending on climatic factors, might show a light rust hue. This is a result of the zinc layer wearing away, leaving steel substrate exposed.

Beyond 20 years: 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 any steel roofing substrate, steel integrity will suffer and your steel roof will begin to fall apart.

For maintaining good looks and longevity in coastal applications Kynar paint over galvalume is a winning combination!

Spray Foam Insulation, Steel Roofing and Corrosion

Hansen Pole Buildings’ Designer Rachel recently had an interesting discussion with a client. The gist of the discussion was the client had heard spray foam insulation will corrode the steel and void the warranty of the steel.

Rachel did some research and found this article: https://www.greenhomeguide.com/askapro/question/can-i-apply-spray-foam-insulation-directly-to-the-underside-of-a-metal-roof.

When I added the external elevator shaft to the rear of our steel covered post frame home, my choice of insulation was closed cell spray foam. Although I knew it was going to be more expensive than other choices of insulation, I was (and remain) convinced of it being a superior R-value, as well as completely sealing the system. In the case of our addition, the steel roofing was applied directly over the wood roof purlins, without any solid sheathing or other barrier.

So, will spray foam insulation actually corrode the steel?

Highly unlikely, as from the research I have been doing there appear to be no chemicals in the spray foam which would react with the steel or the galvanized or galvalume protective layer over the bare material. Most steel roofing is factory finish painted, which adds yet another barrier surface in the interior primer paint coat which further isolates the steel from the spray foam.

There are some cases where I could see some challenges.

One would be if someone went on the cheap and used open cell spray foam, rather than closed cell. In this case moisture could get through the open cells and be in contact with the underside of the roof steel.

The other could occur if there was a leak in the roofing or the ridge cap which would allow moisture to get trapped between the roof steel and closed cell foam.

As to the warranty discussions – steel warranties primarily cover fade and chalking of the exterior finish of the steel. Personally I am hard pressed to see how it is the application of closed cell spray foam insulation on the interior of the steel roofing, would influence the life of coatings on the exterior.

Of course everyone looks for an “out” when it comes to warranties, and the reality is a good warranty protects the seller/manufacturer far more than it protects the consumer.

If I had it to do all over again, I would still closed cell spray foam my own steel roofed building. Check back with me in another thirty or forty years and see if my opinion is yet the same.