How Do I Select The Right Roof Coating for My Flat Roof?
What Coatings Are There?
ASPHALT COATINGS: Asphalt cutbacks are bituminous products thinned with a solvent to aid application. Manufacturers add aluminum pigments to the topcoats for reflectivity, while other modifiers can enhance strength, longevity and thickness. Fiberglass fibers or strands can provide additional reinforcement.
Asphalt emulsions are similar to cutbacks but use water as a carrier instead of solvents. Some emulsions comply with minimum standards for volatile organic compounds (VOC), and emulsions have many of the same modifiers and additives as asphalt cutbacks. Aluminum pigments added to asphalt cutbacks and emulsions give coatings solar reflectance index (SRI) values of 21-30 on a scale of 0-100. The SRI index measures the ability of surfaces to reflect solar radiation and to emit stored radiation from the surface. This index uses zero as a standard black surface, with a reflectance of 0.05 and an emittance of 0.90, and 100 as a standard white surface, with a reflectance of 0.80 and an emittance of 0.90.
ACRYLIC ELASTOMERS: These new-generation coatings have a highly reflective surface, often with an SRI greater than 100. Most highly reflective acrylic elastomers are white, and workers can install them over existing bituminous or non-bituminous roofing. Acrylic elastomers typically are specified at 12 mils for five-year warranties and at 20 mils for 10-year warranties. Some manufacturers specify up to 40 mil applications.
POLYUREAS: These polymer-modified products are relatively new to the market. Some polyureas are specified at similar application rates to acrylic elastomers, and some are specified at much thicker dry mil thicknesses of up to 60 mils, which is similar to urethane coatings. Polymer-modified products and silicone coatings not already mentioned are less common. But manufacturers are always modifying formulations, and new products regularly enter the market.
URETHANES: These polymer-modified products are most commonly installed on concrete substrates, such as parking decks or on wood substrates. Urethanes can be one- or two-component formulations and typically are not applied over other roofing products.
SPF: This product is really an insulating system, but it deserves mentioning with coatings. SPF roofing is spray-applied foam that forms to any roof curb or penetration and provides insulating value. The surface of SPF insulation typically is coated with a urethane for protection and waterproofing.
What Surfaces and System Do I Have?
Manufacturers initially developed roof coatings for application over bituminous roofing products, such as traditional built-up roofing and modified bitumen roofing. Both of these roof types are excellent candidates for a roof coating, provided the surface is smooth and accessible.
Gravel surfacing is extremely difficult to coat, although it is sometimes done with limited success. The age of a bituminous roofing system also is important because aged membranes often exhibit surface degradation and irregularities that workers must address before applying a coating.
Ethylene propylene diene monomer (EPDM) roofing also can be coated, but product choices are limited. Workers can apply acrylic polymers and urethanes, and some manufacturers will extend their present warranties if this work is done. Managers should check with the manufacturer of existing EPDM roofing for compatible coating products.
Other types of single-ply roofing, including polyvinyl chloride (PVC), are unlikely to benefit from coating applications because most have integrated solar protection and reflectivity. Steep roofing tiles and sheet metal roofing also can be coated, and a new generation of engineered pigments for polymeric products offer a range of colors other than white for application.
Finally, concrete roof decks are excellent candidates for roof coatings. Parking decks usually feature waterproof coatings designed to withstand pedestrian and vehicular traffic, road salt and oil. A number of polymers and epoxy products are available for concrete decks, but urethane coatings are the most cost-effective method for waterproofing traffic areas.