A variety of coating system choices can be applied to metal roofing, including acrylic, epoxy/polyurethane, and fluoropolymer systems. Specifications frequently call for fluoropolymer coatings, which are considered to offer the best performance in long-term weathering resistance.
Roof panels are fabricated from sheet-metal substrates such as steel, galvanized steel, aluminum, and other metals. Galvanized steel (a zinc layer on steel) is probably the most common sheet material, although a common variation is produced through a similar process with the application of a zinc-aluminum layer to the steel.
This type of sheet roofing is often referred to as a “coil” material. The term “coil” refers to the large coil, or roll, on which the sheet metal is stored. Coatings are most often applied to the metal in highly automated coil-coating operations in factory settings. Application is completed in this manner to ensure the highest quality results. It is also important to verify that the shop applicator is experienced in applying high-performance coil coating materials. Coating manufacturers typically approve or certify applicators to apply their materials.
When fluoropolymer coatings perform as advertised, they provide many years of nearly maintenance-free performance – and coating manufacturers often provide warranties of 20 years or more. These coatings, however, have a low tolerance for application and installation errors. When problems occur, the long service life that is expected can be dramatically shortened. This appeared to be the case for a large church building located in the U.S. midwest.
The Failing Church Roof
Coating problems on the church’s roof were observed just a few years after installation and the owner contacted a coatings consultant to investigate. The project specification called for “G90” galvanized steel sheet with a fluoropolymer coating system consisting of a primer and a finish coat with a total minimum dry film thickness (DFT) of 0.9 mils. The specified color was dark green.
A field investigation was conducted to evaluate the reported problems. A visual examination, as well as dry film thickness measurements and adhesion testing, were performed. The consultant observed that the green-coated roof panels had small areas of scattered rusting distributed over the roof’s surface. A typical rusted area was less than one-square-inch, with some larger areas measuring several square inches. Although the rusted areas were estimated to be approximately five percent or less of the total surface area of the roof panels, the rusted areas were scattered throughout and were readily visible from ground level.
In addition to the rust spots that were large enough to be clearly visible, many small pinpoints of rust were also observed. There were also tiny spots where under-film corrosion appeared to be starting; however, in these places, the coating’s film was not yet broken and rusting was not visible. When these spots were scraped with a knife, rusting was clearly evident under the coating. There was no sign of damage to the surrounding coating at these spots of pinpoint rust and under-film corrosion.
Some of the rusted areas appeared to have been initiated by mechanical damage, such as from a cut in the coating. However, there was not any apparent damage overall to the coating surrounding many of the rusted areas.
The total DFT, which included the galvanized zinc layer, was measured over the roof surface using an electronic gage. Numerous areas over the roof were measured and the resulting DFT range was 1.41 to 2.04 mils. The thickness of the coating alone was estimated by subtracting the galvanized layer. The “G90” designation for the galvanized steel sheet meant that the weight of zinc on both sides of the sheet was a minimum of 0.90 ounces per square foot – or 0.45 ounces per side. This mass of zinc corresponded to a minimum thickness layer of 0.76 mils on each side of the sheet. Subtracting the zinc layer from the DFT measurements for total thickness resulted in a coating thickness ranging from 0.65 to 1.28 mils. Overall, a majority of the areas measured showed a coating thickness below the minimum specified DFT of 0.9 mils.
Tape adhesion testing was performed following ASTM D3359 (method B, cross-hatch test). The results showed good adhesion where the coating was intact. Coating samples were also collected for further analysis in the laboratory.
The laboratory performed microscopic examination to determine the coating thickness of the field samples collected. This examination found that the coatings consisted of a primer ranging from 0.2 to 0.4 mils and a green finish coat ranging from 0.3 to 0.6 mils. For the majority of samples, the total thickness was below the minimum specified DFT of 0.9 mils.
Overall, the field data and laboratory measurements indicated that deficient coating thickness was the primary factor in leading to premature failure. The thin film likely reduced the weathering performance of the coating system and rendered it more susceptible to mechanical damage. Mechanical damage to the coating, which probably occurred during installation, was a minor contributing factor in the problem. Although some of the rusting areas appeared to have been caused by mechanical damage (cuts) in the coating, this was not generally the case, and no damage to the coating was apparent at many rust pinpoints.
A Redeemed Roof
A variety of coating repair options were considered to restore the roof to an acceptable condition. A major consideration was the fact that the coatings used to repair the roof panels would have to be compatible with the existing fluoropolymer coating system. Generally, fluoropolymer coatings are difficult to recoat due to their hard, slick surface. Depending on the repair coatings chosen, additional surface preparation or primers are often required. Repair systems should include a spot primer for rusted areas, a full primer coat, and full finish coat applied to the entire roof deck. The typical options for a repair coating system include acrylic, polyurethane, and fluoropolymer coatings.
A typical acrylic coating system consists of an acrylic primer and acrylic finish coat. These materials are generally comprised of single-component, water-based materials that are relatively easy to apply, and have good weathering resistance. An acrylic coating system would likely extend the service life of the roof deck for up to 10 years before significant coating maintenance or recoating are needed. Acrylic coatings are the lowest cost option.
A typical fluoropolymer coating system consists of a specialized primer (as specified by the coating manufacturer) and a fluoropolymer finish coat. Although field-applied fluoropolymers also provide excellent weathering resistance, a “factory” finish is not reproduced in field application. These coatings also carry a significantly higher cost and exhibit little tolerance for application error. A fluoropolymer coating system would likely extend the service life of the roof deck for up to 20 years before significant coating maintenance or recoating are needed.A typical polyurethane coating system consists of an acrylic or epoxy primer and an aliphatic polyurethane finish coat. Polyurethane coatings are characterized by excellent chemical resistance, and aliphatic polyurethane formulations exhibit excellent resistance to weathering. A polyurethane coating system would likely extend the service life of the roof deck for up to 15 years before significant coating maintenance or recoating are needed. The cost of a polyurethane system is somewhat higher than an acrylic system.
Ultimately, an acrylic coating system was chosen and repairs were successfully carried out. Surface preparation prior to recoating the roof was accomplished using low-pressure water cleaning supplemented by power tool cleaning to remove all loose rust, loose coatings, and any surface dirt and debris.
Jay Helsel is a Senior Coatings Consultant with KTA where he has been employed for over 10 years. He is a registered Professional Engineer in numerous states, and he is also a NACE Certified Coatings Inspector Level 3 (Peer Review). Jay has extensive marine and shipboard experience having served 11 years in the Coast Guard, most recently as a Lieutenant Commander in the area of Marine Vessel Inspection. At KTA, he has managed the field investigations and laboratory analyses of coating failure cases, including those of a major automobile manufacturer and luxury home developer.