Reflecting on Today’s Roof Coatings
Changes in Coatings Can be Linked to Environmental Concerns and Technical Advacements
There has been a significant change in the use of roof coatings on commercial roof applications over the years. The material formulations and uses of roof coatings have changed gradually in the past two decades.
The initial roof coating applications were typically completed with aluminum coatings and their primary purpose was to prevent oxidation and ultra-violet degradation in smooth-surfaced built-up roofs and modified bitumen membranes.
Aluminum roof coatings are comprised of asphalt petroleum solvents, aluminum pigments and chemical additives to ensure proper pigment distribution. The coatings can also be manufactured with oxidized asphalt cutbacks. These types of coatings fall under ASTM D2824-85 and were initially manufactured in three classifications:
- Type I: Non-fibered
- Type II: Fibered, contains asbestos fiber
- Type III: Fibered, but contains no asbestos fiber
These types of coatings are typically silver in color. Aluminum coatings are economical coatings that can be applied over smooth surface BUR or modified bitumen roof systems. Higher-grade materials can restore new life to older systems, particularly dried-out asphalt roof surfaces. Reflection rates of these products helped reduce thermal shock by resistance of the ultraviolet rays.
In the mid-1990s the Energy Star Program was developed, which impacted roof materials by requiring all low-slope roof systems to have a reflectivity rate of 65% to 70% after three years of application. The only way for most membranes to achieve this rating was with the application of a top coating. Coating colors were primarily white or tan, as studies had indicated lighter colors provided better reflectivity than darker colors. The coatings were applied at a minimal rate to provide a film surface to act as a barrier against the sun’s ultra-violet rays. Selections of coatings were made based on their “Cool Roof Rating.”
Recent advancements in coating material technology has increased the uses for roof coatings in the commercial market. Today, coatings are manufactured as monolithic, fully-adhered, elastomeric materials. Coatings are now used to restore existing roof systems by acting as a surfacing which shields the membrane from erosion caused by rain, snow, sleet and hail and to reflect ultra-violet radiation. 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 chemical attack and ultra-violet radiation, and eliminate the formation of small cracks associated with these degenerative conditions.
It is important to point out that although coatings can extend the service life of the roof system and provide repairs to specific conditions, most coatings do not provide long-term, stand-alone waterproofing protection. The waterproofing is still completed by the existing membrane.
There are several types of coatings available on the commercial roofing market. The material formulations, uses and application methods are all different so the roofing contractor should examine the manufacturer’s material data sheets to determine which product is suitable for their specific project. Not all coatings are acceptable or compatible on all roof surfaces. Also, proper preparation should be completed in accordance with the roof coating manufacturer’s requirements prior to application. As with all liquids and adhesives used in the roofing industry, proper on-site material storage is important and all of these products have a specified shelf life.
Acrylic coatings were originally developed to provide ultra-violet protection for sprayed urethane foam applications. These types of coatings are now used across a variety of roof membrane surfaces, including single-ply membranes and metal systems. Acrylic based materials provide excellent resistance to radiation and hail damage and have the inherent flexibility required to withstand dimensional instability of most roof membrane surfaces. Some studies have indicated that properly formulated acrylic coatings can reduce surface temperature as much as 20 degrees F when exposed to direct sunlight at 85 degrees F.
Acrylic coatings are manufactured of 100 percent acrylic and can be applied in one or more coats. Most manufacturers provide warranties based on the coverage rates; extended warranties require additional coverage. Acrylic coatings should not be applied in freezing temperatures or when precipitation (rain, snow, ice, dew) may occur within a specified period from application. Cure time of these products is highly weather dependent and cold weather or high humidity will impede curing. Ideal curing takes place with warm weather and low humidity.
Acrylic coatings are economical, provide excellent reflectivity, and are easy to work with. However, they lose mil-thickness from weathering, require application in ambient temperature of above 50 degrees F, and they cannot withstand ponding water.
Several acrylic coating manufacturers now offer instant-set materials. This reduces the cure time of the acrylic coating to a few minutes, eliminating the possibility of coating run-off if there is precipitation directly after application.
Silicone coatings are manufactured with a high solids dispersion of 100 percent silicone. It is a highly elastic material that provides excellent adhesion to the existing surface. Silicones provide excellent weather resistance and rarely become hard or brittle. In the last decade silicone coatings have gained market share on the commercial roof market. The rise in applications is primarily due to the material’s ability to withstand long-term exposure to ponding water. Most of the other coatings cannot withstand ponding water. Silicone coatings also provide excellent ultra-violet protection in extreme temperatures, harsh environments and resistance of oxidation. Silicone is available in a variety of colors and provides high reflectivity and emissivity ratings.
Polyurethane coatings were initially developed to be applied over sprayed-in-place foam roofs. They are now used as coating applications over a variety of existing roof membranes. Polyurethane coatings provide the best rates of all coatings for impact resistance and for foot traffic. There are two types of polyurethane roof coatings: aromatic and aliphatic. These types of materials are typically used in combination of base coat and top coat. The aromatic coating is used as a base coat material because it durable but does not offer high UV resistance. The aliphatic coating is used as a top surface because it is UV stable, stays clean and holds color longer than other coatings.
When considering the type of coating to apply, the contractor should review each material’s advantages and disadvantages as they relate to the specific project they are looking at. Each material has different material formulations and application methods that may impact requirements on a project.
Since cost is always a determining factor in material selection, the contractor should review not only materials costs, but also labor costs associated with the application. Some of the more expensive materials may provide significant labor savings. The specific material requirements to take into account should be coverage per gallon, cure time, whether multiple coats are required, whether primer is required, the mil thickness required for warranties, and loss of mil thickness over time.
Standard Coating Application Requirements
No matter which type of coating is applied, there are standard best practices for proper applications. The contractor should review the coating manufacturer’s specific requirements for preparation of the substrate and the material, temperature constraints and coverage rates.
The most important criteria of a 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 over which the coating is being applied and the age of the applied surface.
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 coating manufacturer should be consulted prior to surface preparation to determine what cleaning methods are acceptable, whether a primer is required, and which primers are acceptable.
The initial step in the coating application process is to properly mix the coating prior to—and during—the coating application. Pigments and chemicals used in manufacturing of the coating settle during storage of the coating. Properly mixing the coating allows for uniform color and optimum reflectivity.
Another important criterion prior to the application phase is that the contractor should use the coating in its manufactured state. In other words, the contractor should 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 also decrease the final film thickness, leading to improper weathering.
The coatings should be applied in accordance with the manufacturer’s temperature constraints and coverage rates. The manufacturers have tested their products and know the optimum performance rates. Too much coating or too little coating will result in poor performance or premature failure.