Photo courtesy of Mule-Hide Products Co. Inc., Beloit, Wis.
Photo courtesy of United Coatings
Photo courtesy of Black Jack® Coatings, Tampa, Fla.
Photo courtesy of United Coatings
Photo courtesy of United Coatings, Spokane Valley, Wash.
Photo courtesy of Black Jack Coatings
Because there are more roof coatings available to roofing contractors than ever before, it can be difficult to choose a proper coating. And choosing a coating becomes even more complicated given the myriad of roof systems currently on the market. However, before you can determine what coating to use, it is important you understand some basic information about coating technology.
Coating technology
Roof coatings commonly are classified according to binder type and carrier type. Application methods and compatibility with existing roof surfaces stem from the binder type and carrier type.
A coating's binder is its basic matrix material that adheres to a roof surface. Different binder chemistries are used to meet price and performance criteria, as well as to match specific roof substrates. The binder type generally dictates most of a coating's primary physical properties, including elongation, tensile strength, adhesion to a particular substrate material, permeance, water swelling, low-temperature flexibility, and resistance to fungi and dirt. Binders commonly used in roof coatings range from asphalts to a wide variety of elastomers (acrylics, urethanes, silicones, thermoset rubbers and various block polymers), as well as blends of these. Binder chemistries constantly are evolving, and new coating formulas are being developed for special applications.
A coating's carrier is a liquid that is combined with the binder to reduce the coating's viscosity to a workable level. The carrier generally evaporates during curing of the coating. The carrier also generally dictates a coating's installation process and cure time.
Water can be used as a carrier, in which case the binder generally is suspended in the water as an emulsion. But traditionally, most coating binders use solvent as the carrier.
Environmental regulations tend to favor water-based emulsions over solvent-based coatings, which are high in volatile organic compounds (VOCs). It generally is easier to clean a work site when a water-based coating is used. However, in many applications, solvent-based coatings wet surfaces better. ("Wetting" refers to the contact between a liquid and surface. A liquid with high surface energy—or strong internal bonds—tends to form droplets; a liquid with low surface energy tends to spread out on a surface.) Also, solvent-based coatings cure in a broader range of weather conditions and provide more water-absorption resistance than water-based coatings.
A coating's cure rate can vary greatly because it is affected by ambient humidity, temperature and sunlight. This is especially true of water-based coatings, which can dry slowly and even freeze in winter and can dry faster than conventional coatings in summer. If a coating is not properly cured, its performance can be compromised.
A coating's permeance and thickness also affect its curing process because a thick coating with low permeance can "skin over" with the top layer curing first and preventing the remainder of the carrier from evaporating.
Pigments and fibers can be added to binders to control physical properties and increase reflectivity. Because reflectivity and emittance are defining properties of cool roofs, organizations such as the Cool Roof Rating Council have been formed to develop standardized methods for measuring these coating properties in real applications.
Understanding the options
Coatings suppliers generally offer two types of coatings: all-purpose coatings that are suitable for use on a wide range of substrates and application-specific coatings that are tailored to one type or a limited range of substrates.
Different substrates have different coating requirements. A coating's adhesion may depend as much on the substrate's characteristics as the coating type itself. For example, it is difficult to adhere certain coating types to certain types of roof membranes—most notably TPO. In general, adhesion suffers on hard, smooth, chemically inert surfaces and is more successful on rough, irregular, chemically active surfaces. Adhesion also depends on cleanliness and preparation of a roof surface, which will serve as the substrate for a coating.
A coating's adhesion to a substrate often can be improved by using a primer or base coat. These products generally are required by roof coatings manufacturers for matching a specific top coat to a specific substrate. You should only use a base coat or primer specified by the coating's manufacturer.
Coatings can be factory-applied to roofing materials under highly controlled conditions. For example, metal roofs and modified bitumen cap sheets often are coated at the factory. However, roof system sustainability typically requires recoating existing roof systems to extend roof system service life. Therefore, most coatings are field-applied and depend on real-world conditions and workers' practical skills. Field application requires careful attention to consistent application and depends on weather conditions, including temperature, humidity, sun and wind for proper curing.
Coating use
Understanding coating technology is only one part of proper coating selection. Establishing what a coating's intended purpose or function will be for a roof system is equally—if not more—important. Roof systems are coated primarily for the following reasons:
These various objectives have become part of an industrywide movement aimed at energy conservation and sustainability. Roof system restoration and life cycle extension reduce the volume of solid waste in landfills. Longer roof system life cycles also reduce the energy consumed in tearing off failed roof systems and transporting waste materials to landfills, as well as the energy consumed in manufacturing new roofing materials, transporting them to job sites and installing them.
Longer roof system life cycles also can decrease downtime and lost income a building owner may experience during a roof system replacement.
California Title 24 legislation is especially pertinent to consider when selecting coatings for roof systems in California. Title 24 requires detailed energy audits on new buildings to ensure solar heat is dissipated or removed in an energy-efficient manner, such as through insulation and climate-control systems, unless a building has a highly reflective roof system with reflectance values above 0.70. The legislation has motivated manufacturers of all roof system types to develop products that meet this minimum reflectance value.
Reducing solar heat gain directly affects peak electricity requirements. Peak electric loads often are the basis for electricity prices because they establish a region's energy-generating needs.
When initially installed, some white reflective coatings can reflect 80 percent or more of the sun's rays to reduce air conditioning costs. A highly reflective cool roof often can reduce energy consumption by 10 to 20 percent or more and can reduce urban heat island effects. When used as part of an ongoing roof system maintenance program, coatings typically can be treated as an expense, not a capital investment (as with reroofing).
The next step
After you establish what function you want a coating to serve, you must consider the type of roof system being coated.
There are various coating options for the different types of low-slope roof systems, and the roofing industry still is learning which coatings work best. Existing formulas will continue to be tweaked, and innovative new formulas will be introduced into the marketplace. Popular coating options may soon be displaced by materials that have not yet been introduced.
Next month's issue of Professional Roofing will discuss coating requirements for specific roof system types.
Reed Hitchcock is the Roof Coatings Manufacturers Assocation's executive director.
COMMENTS
Be the first to comment. Please log in to leave a comment.