Whereas high end natural metals like zinc and copper do not need any coatings, there are four basic coating options available for steel and aluminum roofs. Three of those four are paint finishes which I will review here in a bit. The other type is a stone coating.
Stone coatings incorporate granules similar to those on asphalt and fiberglass shingles but they are glued to steel. These finishes are beautiful but they are prone to two of the common asphalt shingle problems that we often hear folks want to avoid. They lose granules over time, much like standard shingles, and they also are quite prone to black streaks of algae and mildew growth.
The painted products do not have granules to wear off and they are far more resistant to streaking but there are quality options available with painted finishes as well. At the very low end of finishes are clear coatings. Commonly applied to galvalume steel for a natural metal look, these acrylic clear coatings are primarily to provide lubricity and protection for the metal while it is being formed, transported to the jobsite, and installed. These coatings wash away over time – usually in less than 10 years.
The next step up in terms of paint finishes are coatings that use polyester as the resin to hold the paint to the metal and also to bond the pigment – or coloring – in place. There are several grades of polyester coatings and you will hear words like siliconized, modified, and super polyester. While the high end polyesters out perform the lower end products, these are still mid-level coatings in terms of longevity and fade and chalk resistance. The polyester resin will break down over time, allowing the pigment to be released through chalking. The breakdown of the resin also allows UV rays to begin to cause the pigments to change color.
The highest quality architectural coatings today are known by the chemistry name polyvinylidenefluoride or PVDF for short. They are sold under the trade names of Kynar and Hylar. These unique coatings use the mineral fluorite for their resin, creating a very tight molecular bond that is similar to Teflon. They resist dirt and hold their integrity for the long term, allowing the metal roofing industry to give the best fade and chalk warranties for these finishes. There is no better finish even on the horizon at this point. For this reason, we strongly encourage these PVDF coatings for residential metal roofs.
Kynar 500® is a registered trademark belonging to Arkema Inc.
Hylar 5000® is a registered trademark belonging to Solvay