Application specifics
Roof coatings can be applied using spray, roller, or brush to all types of roof membranes and substrates. Coatings can be applied by a properly trained building owner or an experienced roof coating applicator. However, a roofing contractor is strongly recommended if there are any issues, such as existing leaks, wet insulation, damaged deck, or manufacturers’ warranties.

All roofs should be designed and built to have positive drainage and not allow ponding water. While some coatings are designed to withstand ponding conditions, this is not the case with others. One must check water swelling and absorption properties to see whether a coating can withstand ponding water conditions for extended periods.

As a coating naturally wears away, additional coating can be applied. Most coatings systems are re-coatable and provide a sustainable roofing system when properly maintained. The manufacturer should be consulted for specific requirements on re-coating applications.

Curing and drying time
For water-borne coatings, the ambient air temperature should be 10 C (50 F) and higher, with no rain for the next 24 hours. Solvent-borne coatings can be applied in colder weather, but as always, the coating manufacturer should be consulted.

While modern technology permits the application of some roof coatings at low temperatures, one can expect the cure time to be longer than on a warm summer day. A product that may cure overnight at a temperature of 21 to 23 C (70 to75 F) may take several days to cure at 4 C (40 F).

VOC issues
There is an environmentally based effort to continue to reduce the volatile organic compounds (VOCs) in roof coatings. Found in a wide variety of products and in naturally occurring processes, VOCs are compounds of carbon that participate in atmospheric photochemical reactions. Ground-level ozone is one of six ‘criteria’ pollutants regulated by the U.S. Environmental Protection Agency (EPA), because it has negative impacts on human health and can be damaging to vegetative growth. Ozone is formed when VOCs and nitrogen oxides (NOx) combine with sunlight. However, VOCs help protect roof coatings during shipment by providing freeze-thaw resistance. On the jobsite, VOCs help condition or soften the substrate and improve adhesion of the roof coating. They are integral to specific raw materials used in the roof coatings products. Roof coating manufacturers recognize the need to reduce total VOCs—over the last few decades, 90 percent of VOC content has been eliminated from roof coatings.

The most common replacement for VOCs is water, but water-based roof coatings have unintended consequences, including;

• hard settling, or phase separation, of the components during transportation and storage of the product before application;
• poor adhesion of the product to the roof substrate;
• the need for more coats of the product and heavier coverage rates, therefore possibly increasing total
  VOC emittance;
• reduction of product lifespan, requiring more frequent applications; and
• elimination of roof coating product lines that cannot be reformulated.

Conclusion
Roof coatings can provide many benefits, from aesthetics, to energy, longevity, and performance.¹ There are a wide variety of choices for roof coatings, many considerations when selecting and specifying roof coatings, and many reasons why coatings are used on new and existing roof systems. It is important to understand why a roofing coating is used so it can be selected properly.

James R. Kirby, AIA, is the codes and standards director for the Roof Coatings Manufacturers Association (RCMA). Kirby has more than two decades of roofing industry and association experience, and has a well-rounded knowledge of roofing materials and systems as well as standards development. He can be reached at jkirby@kellencompany.com.