October 23, 2018
by Steven Heinje
In the last two decades, the improved technologies of silicone, acrylic, and polyurethane have become better integrated into basic roofing practice, not only expanding the range of roofing applications but also competing successfully against the traditional asphalt coatings used for maintaining traditional built-up (BUR) roofs. Today, most manufacturers of entire roof assemblies also have a coatings line, including at least two of these four technologies.
In this article, roof coatings will be viewed as materials that may include reinforcement (“liquid membrane” applications) or not (“coating” applications). Most roof covers are manufactured as sheet goods or panels, which serve as the primary waterproofing in an assembly. Finally, a “coating” shall be viewed as functional rather than cosmetic, in contrast to many paints and finishes. This will be accomplished without delving deeply into important specifics of the chemistries and grades of coatings.
The “why” of coatings
The value propositions for coatings include maintenance, energy savings, tax incentives, leak repair, and less disruption than a total tear-off. In terms of sustainability, some of the advantages are urban heat island mitigation, reduction of landfill waste, solar energy efficiency, and roof runoff capture and reuse. Obviously, these benefits have climatic and regulatory implications. Nevertheless, many of these drivers have key conditions making them worthwhile.
Maintenance can mean protection from heat buildup, or reduction of heat history of the underlying roof cover, as well as protection from ultraviolet (UV) damage and moisture intrusion. Even in cold regions the impact of short, hot summers is significant because the effect of heat is exponential, making the average temperature a poor guide. In summer, a black roof in the north may age about eight to 16 times faster than in early spring. Degradation from heat, UV, and water are almost always combined and additive in nature. Since coatings address all of these, they can make an extremely effective weather barrier. A high percentage of the current building inventory was constructed with black roofs that are at least half way through their service life, and in many cases, a coating application can extend this service life five to 10 years. Roofing valleys and parapets on white single-plies often retro-reflect the sun’s rays, making coatings a simple mitigation to ensure a longer lasting roof in these areas. Finally, water shedding can be enhanced with a coating, dark- or light-colored, when the underlying roof has become rough and irregular or has exposed edges.
Energy savings in urban centers may be peak-demand energy based, which can tilt the economics of cool roofing in favor of white coatings. Utilizing white, reflective coatings on sun-exposed HVAC ducts and electrical conduits employed in solar power assemblies can improve energy efficiency. These opportunities exist even in very cold regions.
Best practice for leak repairs is to always use in-kind materials such as an asphaltic patch or sheet for an asphalt roof cover. However, reinforced coating patches can make sense over various dissimilar roofing materials. Most coatings are much more permeable than the roof covers they protect, allowing trapped water to escape over time. This is an important advantage when a full retrofit with new insulation is not justified. Following a small repair with a coating application over a larger area can prevent the adjacent areas of a repair from failing soon after the original leak is addressed.
Commercial owners often find a tax advantage in claiming the entire cost of a coating application in a single year when compared to the typical expensing of capital projects such as a complete reroof.
Sustainability drivers come in the form of government and corporate mandates, aspirational guidance from organizations like the U.S. Green Building Council (USGBC), and even the building owner’s input. Often underestimated, value-driven choices are the “who.” If the economics are close, values may become the tipping point issue for using a liquid coating. For instance, reroofing every 20 years—a common practice—is out of step with what one expects from most building materials. Houses more than 100 years old have been maintained by simply repainting them. Generally, one does not replace or re-side but instead maintain the siding with coatings. Roof coatings can provide an extension of this ideal, which some clients will gravitate toward.
The “when” of coatings
Two especially important cases for coatings are modified bitumen (mod-bit) and architectural metal roofing. Mod-bit comes in two forms—styrene butadiene styrene (SBS) and atactic polyproylene (APP) in either granulated or smooth finish. Often mod-bit is used as the “cap sheet” on a BUR assembly, but even when applied directly over asphalt, a coating application is much the same. Mod-bit is a robust, simple, repairable, and cold-weather-friendly product with needs—protection from weather and heat. Granulated mod-bit, commonly SBS in northern regions, is a material that ages both thermally and by a combination of UV light and moisture attack to the asphalt underneath. Eventually asphalt weakens and the granules release, at which point there is little protection left and soon the reinforcement gets exposed. However, an acrylic coating provides adhesive and encapsulating characteristics, even if applied lightly (1.5 to 2 m2/L [1.6 to 1.2 gal/100 sf]). The result is a cooler surface and less oxidation, postponing the eventual embrittlement of the roof cover. Subsequent reduction in UV and moisture significantly slow the aging process. Some manufactures now provide pre-coated mod-bit to combat these issues. Additional coating, or better yet, reinforcement over the seams and bleed lines, will slow the other main route of failure via ruptured seams.
Coating can be accomplished early or late in the membrane’s service life, but it is best done the second year. If the roof has begun to lose a significant amount of granules, a liquid membrane can still extend it another 15 years or more. The key is to treat the roof before it has significant degradation, making 10 years a good outside target date for coating. Leaks mean wet insulation, resulting in lost energy. In cold climes, the importance of efficient heating is such that investing in coating on a wet roof is throwing good money after bad. Granulated mod-bit and coatings work especially well together. A mod-bit/coating system
is also a good candidate for covering uninsulated concrete decks.
Metal roofs have the longest potential service life of any major roofing system, but they often leak. The issue is with expansion and contraction, which especially impacts the fasteners. Some metal roofs have hidden fastening, but all generally have horizontal seams that can experience ice damming or wind-driven rain, creating a route for moisture entry. Coatings combined with reinforcement at these points can easily add another 20 years of service to such a roof. Acrylics, urethanes, and silicones, as well as styrene ethylene/butylene styrene (SEBS), asphaltic, and silicone hybrid (SPUR) coatings are acceptable choices. Acrylics can be easily colored and tend to hold up well on metal, while silicones are best in cold weather applications. Urethanes excel at crack bridging and adhesion, and often perform best on a really problematic metal roof. In all cases, these coatings provide color selection, often a key reason why metal was specified in the first place. A typical metal roof coating project involves:
A light-color finish helps mitigate the severity of expansion and contraction cycles. Leaks should be repaired with a level of redundancy to ensure a longer service life than may have been initially provided, making proper detailing the key to this application. Proper coating of a moderately rusted galvanized metal roof illustrates a best-case situation. Even a dark-colored finish can reduce heat buildup as a result of improved thermal emissivity because a coating changes the rate at which heat is released from metal. White coating works in two ways—it is reflective, so less solar energy is absorbed in the first place, and it also allows heat to escape about three times faster. A white coating can drop peak temperatures by as much as 50 C (122 F). The reduced mechanical stress translates to a lower risk of leakage in the long run.
Although considered “cool roofing” materials, white single-plies (e.g. chlorosulfonated polyethylene [CSPE], polyvinyl chloride [PVC], ketone ethylene ester [KEE], and thermoplastic polyolefin [TPO]) may experience premature aging due to retro-reflectivity in valleys and parapets. Due to age or manufacturing variation, the reinforcing scrim may be too close to the surface, leading to localized maintenance needs. As previously mentioned, in-kind material repairs is best practice. This is especially true of these sheet goods. However, many of these repairs can be prevented. The key is to coat the roof before the degradation allows significant exposure of the scrim. The case here is not as strong as is the case for mod-bit—typically, maintenance coatings are applied to a five- to 10-year-old roof with the expectation it might add another five years to the service life. Since coatings and liquid membrane systems exist for use on all single-plies, it is possible to consider an appropriate coating application for premature weathering or a liquid membrane system for a roof with failing seams. Non-maintenance applications for color coatings include logos, graphics, or a simple change in original color. Changing color may alter the temperature of the underlying roof cover and impact its performance.
Complex roofs pose a unique set of challenges. This type of construction might be utilized for a laboratory, hospital, historic structure, large commercial, or high-rise residential building with numerous protrusions and surface-mounted equipment. Liquid membranes using poly methyl methacrylate (PMMA), SPUR, and urethanes work well on complex roofs for high-value buildings, as they are seamless, fully adhered, and completely adaptable to irregular shapes and complexities. In retrofits, the new membrane becomes an extension of the original roof cover, and is generally of higher performance and greater in thickness than most coatings. Typically, PMMA or urethane membranes are around 2 mm (2000 µ) thick compared to a typical two-layer coating application at about 0.6 mm (600 µ). Acrylic membrane systems are also employed in fully reinforced applications, using premium materials and thicknesses above 1.2 mm (1200 µ).
Sprayed polyurethane foam
Sprayed polyurethane foam (SPF) and coating is an entirely site-manufactured system requiring more skill and workmanship than many other roof covers. Being sensitive to temperature extremes and moisture during application and cure, it is a seasonal product that should be specified accordingly using an experienced professional. However, it is self-flashing, fully adhered, seamless, and offers the most effective insulation. It suits the complex roofs, and dry and tight roofs needing more insulation, since it can be applied over existing roofs. Foam is often coated with silicone when installed in more humid climates.
Figure 1 provides a list of preferred coating applications but is by no means comprehensive. Mod-bit and metal are maintenance applications enhancing the existing assembly and extending its life. Single-ply is a large and growing segment of roofing that can benefit from coating even if with a more limited scope of use. These three applications can be in-house for most roofers. The last two cases are complex roofs and SPF—these are cases where the unique attributes of liquids come into their own. These options fit roofs that would otherwise be arduous projects. Liquids and especially SPF may require subcontractors, but they also allow hybridization of a traditional approach with a liquid roofing membrane when it works best. These systems are designed to augment the strengths and minimize the limitations of traditional roofing within the parameters of contemporary construction.
Steven Heinje is the technical manager of liquid-applied systems for GAF. He has degrees in biology and chemistry, along with an MBA. Heinje has more than 30 years of experience in roof coatings, specializing in acrylic elastomers and urethane coatings. He is chairman of the Roof Coatings Manufacturers Association (RCMA) codes task force and leads several task groups in ASTM D08 roofing, as well as maintaining active memberships with American Society for Quality (ASQ), RCI, Reflective Roof Coatings Institute (RRCI), and the American Chemical Society (ACS). Heinje can be reached at firstname.lastname@example.org.
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