The Metals in Metal Roofing 2: What Is Galvanized Steel Roofing?

Our series begins with a common metal found in metal roofing: galvanized steel.

Because the traditional carbon/iron steel alloy is rust prone when exposed to the elements, steels used for the metal roofing industry are coated with a specific thickness of another metal or alloy on both sides of the base carbon/iron steel strip. The process used to accomplish this is called the hot-dip process, and involves running the steel through a molten bath of the metal to be applied. The hot-dipped process is basically a cheaper, more efficient alternative to a similar process electroplating.

Steels are classified and named according to the metal that is applied. Galvanized steel is base carbon/iron steel with a metallic coating of zinc. The coating metal offers two kinds of protection: galvanic or barrier. Galvanic protection is a self-sacrificial process by which the metal coating gives itself up rather than allow the base metal to corrode. Barrier protection is simply that the coating metal keeps the elements from reaching the base metal.

In the metal roofing industry, galvanized steel is used more often than any other metal and is available in most metal roofing profiles . It is lower cost than most other metals, strong and has a great affinity to hold paint. Because zinc provides galvanic protection, scratches on galvanized steel are somewhat self-protected or “band-aided,” preserving the steel from rust. Therefore, the life-span of galvanized steel depends largely on the thickness of its metallic coating since the more zinc that is present the longer the steel is able to remain protected and rust-free. G-90, the most common zinc thickness used in the metal roofing industry, means that 0.90 ounces of zinc are coated per square foot of steel surface. Lesser grade-galvanized steels are G-30 and G-60 and should usually be avoided for residential applications. Always check the manufacturer’s specifications to determine the thickness of the steel’s metallic coating.

It is important to note that G-90 only refers to the thickness of the zinc coating, not the thickness of the steel itself. That thickness is measured in gauge number (26 Gauge, 24 Gauge, etc.) and depends on the profile of the steel metal roofing product to be used. In regards to the overall metal thickness, the higher the gauge number, the thinner the metal is. With a few exceptions, such as mill-finished shingles or other metal roof styles, all galvanized steel systems are coated with a base paint coat of some sort. In addition, many of the higher quality galvanized steel metal roofing products, especially shake, shingle and tile systems that are used largely on residential applications, come with an added “post-forming” coat to help protect against corrosion in areas where the metal had been refashioned during the manufacturing process. This is discussed in further detail in the Coatings section.

Galvanized steel is not a good option for homeowners in coastal areas or areas with an above average amount of corrosive elements in the air. Salt spray and these other elements can speed up corrosion and shorten the life of galvanized steel. Overall, though, with consideration just for the metal itself, galvanized steel is very applicable for residential products.

Galvanized Steel Summary

  • Advantages: Strong, lower cost, comes in almost any look.
  • Disadvantages: Shorter life span than other metals, can rust prematurely if not used or installed properly, can be more difficult to work with, must be cut with a shearing action rather than saw-cut.
  • Thicknesses: 26-28 gauge (.018″ – .014″) is most common for shake, shingle, tile and slate profiles. 24 gauge (.024″) is most common for standing seam systems, with a good amount of 26 gauge as well.
  • Weight: Between 100 and 150 lbs. per square (100 sq. ft.).
  • Recycled Content: Usually around 35%.

The information above comes from Isaiah Industries, parent company of Classic Metal Roofing Systems. If you’d like to learn more about metal roofing for your North Carolina home, we’re ready to help you discover more and answer your questions. McCarthy Metal Roofing Systems is your North Carolina metal roofing expert.

Next up in the series: Galvalume Steel.