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.


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:

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


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.

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