Traditional coatings rely on basic materials that have been used for decades and are designed to be chemically compatible with the existing roof. By protecting the roof from direct exposure to UV light, water and other weather elements, coatings extend the life of the roof. They also serve a secondary purpose of sealing minor imperfections in the roof. Traditional coatings include coal-tar, asphalt emulsions and solvent-based asphalt applications.
Reflective coatings also protect the roof from exposure to sunlight and weather processes, but with the added benefit of reflecting infrared heat. Even modest reductions in roof temperature can significantly extend roof life. This is particularly true for well-insulated roof systems, which tend to be hotter because they cannot shed heat into the building. Well-insulated black roofs are generally seen to have a shorter life than the identical system placed over lower insulation.
Reflective coatings come in two predominant types: water-based white acrylic roof coatings and reflective aluminum asphalt coatings.
Acrylic coatings reduce infrared heat absorption in the roof membrane. These elastomeric coatings help reduce the internal temperature of uninsulated buildings, saving cooling costs. Because they generally contain low VOCs, water-based coatings are more environmentally acceptable than solvent-based coatings and last as long as other coatings.
Water-based white acrylics must be selected carefully. For example, to use a white coating on an asphalt roof, you must specify a coating formulated specifically for asphalt.
More so than other coatings, water-based white acrylics are not intended for use in standing water. If a roof has a tendency to pond, maintenance staff should first fill the depressions where ponded water accumulates before applying an acrylic coating. Their application also is limited to emulsion surfaces, and they should not be applied at temperatures below 45 degrees. For a gravel roof, the recommendations of the manufacturer should be followed. Finally, the curing time should not be underestimated: Water-based coatings can require from six to 48 hours of cure time before the roof can be exposed to rain or cold temperatures. It is crucial that the instructions on the label are followed.
Reflective aluminum-asphalt coatings use aluminum flakes in an asphalt matrix. While they retain slightly more infrared heat than white coatings, they can be applied on a variety of substrates, including metal, single-ply and built-up roofs. Either type of coating can be used on unpainted metal roofs, but only the aluminum-asphalt roof coatings will allow the roof to retain a metallic appearance.
Maintenance membranes use a combination of coating and reinforcing fabric. The membranes are used either as a short-term effort to stabilize a roof that might be compromised and eventually will need to be replaced, or as a longer-term solution that can extend the life of the roof five to 15 years. But simply applying coatings is not the same as applying a complete maintenance system.
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