Technical Details: Proper Application of Roof Coatings
There has been a significant increase in the use of roof coatings in the past decade. The increase in the use of roof coatings on commercial roof applications can be linked to environmental concerns and technical advancements. Coatings have been used to restore existing roof systems, to reflect ultraviolet radiation, and to shield the membrane from erosion caused by rain, snow, sleet and hail. When properly applied, coatings can provide significant advantages to a roof system. Coatings have been documented to expand the service life of existing roofs, improve a building’s energy efficiency, resist degradation from chemicals and ultraviolet radiation and eliminate the formation of small cracks associated with these degenerative conditions. Improper coating application leads to premature coating failures, which could lead to the demise of the roof system.
Based on recent projections, the increase in coating applications will continue for the next several decades. Codes limiting the use of aggregate as roof surfacing will further enhance coating applications. It is in the best interest of roofing contractors to become familiar with best practices of coating applications. The Roof Coating Manufacturers Association (RCMA) recommends the following application procedures to achieve a successful coating application.
General ConditionsThe most important criteria of coating application is adhesion to the substrate. To achieve full bonding of the coating, the surface must be properly prepared. Preparation requirements are contingent upon the type of surface that the coating is being applied over and the age of the applied surface. For the best results, check with the coating manufacturer to determine what the age requirements are for application on different surfaces.
Typically, aluminum roof coatings should not be applied over asphalt glaze or flood coat surfaces for three months to allow for the evaporation of solvents. Asphalt emulsions can be coated within five to 14 days of application depending on how the ambient temperatures affect the curing. Recent studies have indicated that modified bitumen surfaces should be coated prior to aging. Waterborne coatings should be applied in accordance with the manufacturers’ requirements.
Weather ConditionsThe ambient weather conditions also play a critical role in the coating application process. Most coatings should not be applied when the ambient weather temperature is below 40 degrees Fahrenheit. This is primarily true of waterborne coatings. Some solvent-based coatings can be applied in lower temperatures if they are properly stored in a heated environment prior to application. The concern in applying coatings in lower temperatures is that dew or moisture can accumulate over the surface and interfere with the bonding of the coating. Temperatures over 110 degrees F can also be detrimental to the coating application. Application in extreme temperatures could result in the coating curing too quickly, contributing to streaks. Coatings should not be applied if rain or precipitation is forecasted within 24 hours of application.
Surface ConditionPreparation of the surface is the most critical component of the application process. The surface must be clean and free of all moisture, contaminants, debris, oils and loose particles. Surface contaminants create impediments to the coating adhesion, resulting in disbondment or peeling of the coating. The surface can be cleaned of all contaminants by power washing, brooming or vacuuming. Power washing is typically used to remove heavier debris or remnants of existing coatings.
If contaminants on the surface are extensive, a primer may be required prior to the coating application. The primer must be dry (fully cured) prior to the application of the coating. It is also important to note that not all primers are suitable for all coatings; some primers are incompatible with specific coatings. The coating manufacturer should be consulted prior to surface cleaning to determine what cleaning methods are acceptable, if a primer is required, and which primers are acceptable.
Coatings should only be applied on roof surfaces with positive drainage. Ponded water typically reduces the service life of the coating and contributes to a reduction of the coating’s mil or film thickness, peeling and delamination. Standing water should be eliminated from the roof surface through additional drains or tapered insulation prior to the coating application. Some manufacturers have developed products that are specifically made for application in ponded areas. These types of products can be considered if there is no other means of eliminating ponded water.
Application MethodsThe initial step in the coating application process is to properly mix the coating prior to - and during - the coating application. Pigments used in the manufacturing of the coating settle during storage of the coating. Properly mixing the coating allows for uniform color and optimum reflectivity. Most manufacturers recommend the use of mechanical mixers to properly mix the coating while it is still in the pail. The mixer should have a blade that is designed for use with fibered products, not regular paint blades.
Another important criterion prior to the application phase is that the contractor should use the coating in its manufactured state. In other words, do not mix solvents or water into the coating to thin it out. The coatings are manufactured with special formulations to achieve optimum performance. The addition of thinners can contribute to premature failures or shortened service life. Thinning the coating may decrease the final film thickness, leading to improper weathering.
There are three accepted methods of coating application: brush, roller or spray equipment. The brush should be a three- or four-knot roofers brush or a soft bristled broom. If a roller is used, it should have a medium nap roller cover. The application methods with brushes and rollers are similar in that the strokes should be finished in the same direction to achieve cohesive aesthetics. If more than one coat is required, some manufacturers recommend that the coats be replied at right angles to one another.
The spray equipment used can be any type that is capable of spraying the coating in an even application. In roof coating applications, airless sprayers are used most often. The viscosity of the coating material, the internal diameter size of the hose and the length of the hose are important determining factors for the proper selection of the spray equipment. For guidance with the spray equipment selection process, consult the material manufacturer for the type of sprayer that provides the best performance with their materials.