What is the difference between galvanized and galvalume metal roofing?
Saturday, July 28, 2018
Both types of metal roof panel are a sheet of steel with a rust-resistant coating. For many years the standard coating technique was galvanization, which is a hot dip in molten zinc. The zinc bonds with the steel and forms a distinct crystalline surface pattern after exposure to air, called “spangles.”
In the early 1970s, Bethlehem Steel introduced an alternate rust-resistant coating that they trademarked as Galvalume®, which is a combination of zinc and aluminum, with a trace amount of silicon. The vast majority of metal roofs in our area have been galvalume since the mid-1990s because of its longer lifespan.
But both materials have pros and cons, and galvanized is still the predominant choice for applications other than roofing, such as agricultural and livestock structures:
- Coating more resistant to damage during bending or cutting
- Less expensive
- Has distinct spangle pattern and bright metal color when new, like in the close-up photo above.
- Better paint adhesion
- Lasts only a little more than half as long as galvalume
- Less resistant to corrosion due to rain and coastal salt air
- Lasts almost twice as long
- More resistant to corrosion due to rain and coastal salt air
- Coating more easily damaged during bending or cutting
- More expensive
- Muted spangle pattern and softer, gray metal color when new, like in the photo at the top of the page
- Not as good paint adhesion
Because any installation work that disturbs the rust-resistant coating will open up an area for early corrosion, it is always better to cut metal roofing with shears rather than a saw, which tears away the coating near the saw cut. Also, screws have replaced nailing as a method to secure the panels to the roof deck. A screw holds the panel snug and a neoprene washer under the screw head covers and seals the small area of coating damage at the screw penetration. Nails had the annoying tendency to loosen and gradually pop up over the years, making numerous tiny openings in the roof, like in the photo below.
The next two photos show rust at galvanized metal roofing at the bottom panels due to damage of coating when cutting panel, and corrosion behind chimney due to acidity of wet leaves accumulated behind chimney deteriorating the coating.
Also see our blog post Can metal roofing be used on a low slope/pitch roof?
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