Asphalt roofing remains an important part of today’s construction industry, and in terms of installed roof inventory, remains the largest segment. Asphalt is a classic roof material, offering an economical solution with a long service life and good water resistance. Asphalt roofing has changed in ways consistent with the trends in any technology: less reliance on bulk material and a move toward higher technology.
Built-up roof (BUR), multi-ply, and mod-bit systems can all include coatings as part of their initial design or as a maintenance item. White reflective coatings represent one of the latest additions to the long evolution of asphalt roofing. Asphalt cutbacks and aluminized coatings were already well established as part of standard roofing practice when acrylic and other coatings came into regular use. In particular, acrylic coatings have proven very useful in adapting asphalt membranes (commonly granulated modified bitumen) into cool roof systems. These hybrid mod-bit/coating systems combine asphalt’s proven field-worthiness with some of the most reflective materials found in any roofing system.
How Asphalt Ages
The service life of asphalt is related to its oxidation. This is regulated mostly by heat acting throughout the membrane and by ultraviolet (UV) exposure acting on the surface. Oxygen, working in direct relationship with heat, will eventually harden and embrittle the material from within. UV, acting alone and in combination with water, will act to break down the surface. Oils exuded on the surface will weather and eventually wash away. These forces all work together to turn a flexible and tough membrane into a progressively weaker and less flexible material.
Crucial to protecting asphalt roofs is some sort of surfacing. It may be rock or gravel, a granulated sheet, a foil facing, or liquid-applied coating. This top layer acts as a UV screen and may also reduce heat through reflectivity or mass effect.
Protecting the Asphalt
Due to the normal changes brought about by aging and weathering of exposed asphalt roofing, cutbacks (aka black solvent-borne), and aluminized coatings were adopted as integral parts of these systems. More recently, acrylic coatings have played a key role as evidenced by coating standard ASTM D6083: Standard Specification for Liquid Applied Acrylic Coating Used in Roofing. The standard appeared in the first International Building Code, IBC-2000, section 1507.10 Built-Up Roofs alongside the asphalt coatings D1227, D2823, and D4479.
Acrylics are typically white or light colored, and they are the most reflective of these code-approved surfacings. As such, their primary benefit is the reduction of heat. Where surface temperatures might be as high as 180° F (82° C) on a black roof, a white coating can reduce the surface temperature to about 110° F (43° C). They also screen UV and inhibit the combined action of oxygen and water at the surface, as well as reduce heat-mediated degradation through the entire membrane.
Among asphalt systems, there is no better case for an acrylic coating than on granulated mod-bit systems. Here, the coating can augment the roof membrane by preventing the loss of granules, and simultaneously protect seams and bleed lines from UV and water attack. As little as 2–3 gal./square foot (81.5–122.2 L/m²) of a typical ASTM D6083 coating can lock down granules and help protect the roof for a decade or more. By blocking UV, water, heat, and oxygen from the asphalt, overall degradation is effectively delayed. The modified bitumen can then perform the same way as other cool roof systems by fully leveraging the benefits of an acrylic coating.
The Best Coating Option
There is a well-established array of asphaltic roof products on the market and a range of coatings designed for those various surfaces. How does the type of asphalt membrane impact coating selection? There are three main issues:
1. A cosmetic problem known as “tobacco juicing”
2. Asphalt’s limited low-temperature flexibility
3. Asphaltic bleed
First, the problem of tobacco juicing is a temporary reaction best managed through proper surface preparation. Tobacco juicing may appear on asphaltic roofs and surrounding areas under certain climatic conditions. It is the result of UV and water acting on the asphalt to form a brown or yellowish sticky substance during the normal aging process of the surface. Power washing will normally remove this, and even if it persists and causes staining, this should fade over time.
Second, the fact that asphalt is often stiff and immobile when cold means that firmer and less flexible coatings and primers can be safely used over them — and may even offer some advantages. As a result, a variety of coatings have been designed specifically to cover asphalt.
Third is asphalt bleed, which is the migration of oils from the asphalt into the coating. This can be a serious and sometimes permanent problem. It may show up weeks after installation, so it is important to understand the potential for bleed prior to application. The effect that asphalt bleed has on a coating varies according to the type of coating and the type and age of the asphaltic roof.
Coatings for Asphalt
Asphalt-based coatings have always benefited from a “like goes with like” factor, providing excellent chemical compatibility and eliminating any problems with bleed resistance. They include black cutbacks, emulsions, and aluminized products, in both solvent- and water-borne formats.
Currently, many polymeric coating materials, including styrene ethylene butyl styrene (SEBS), urethanes, silicones, and acrylics, are growing in use. All of these are subject to some degree of asphalt bleed and may require a primer, or must be made in a grade specifically intended for asphalt. There are two key concerns:
1. the effect of the coating’s solvent, and
2. the coating’s resistance to asphalt bleed through.
SEBS, conventional silicone, and conventional urethanes can all cut into asphalt and become discolored immediately. While the “bite” from a solvent-borne coating may not hurt performance, the resulting discoloration is often deemed unacceptable and will lower reflectivity as well. With silicone and urethane coatings, a common solution is the use of a bleed-blocking epoxy primer. Even high-solids polyurethane, polyurea, and silicone products that may not contain solvents can benefit from primers in terms of both improved adhesion and ultimately higher reflectivity.
In the case of acrylics, there are two distinct classes:
1. The ASTM D6083 coating that has excellent low-temperature flexibility down to -15° F (-26° C) and good long-term weathering, and
2. The non-ASTM D6083 coating that provides good asphalt bleed blocking but is typically only rated down to 14° F (-10° C).
Most coatings, even if not designed for asphalt, will work well over “slight” or “low-bleed” substrates, such as granulated SBS. For substrates with moderate bleed, an asphalt-specific product or system should be used. In the more extreme case of newly installed smooth atactic polypropylene (APP), it is best to confirm compatibility specifically for that application, since the wrong coating may not last more than a few years. The asphalt bleed reaction can result in embrittlement, splitting, and shrinking of the coating. Often, these are combined into a system of bleed-blocking basecoat or primer and code-approved topcoat designed for long-term performance. Manufacturers have responded with products and specifications to match the full range of available asphalt membranes. It’s worth noting that if a coating works on smooth APP, it can be assumed to work on all asphaltic membranes.
Coatings have long been a part of asphalt roofing technology, and the important role they play in extending the service life of roofs has been well documented over many decades. More recently, acrylic coatings in particular have expanded on this trend through compliance with cool roof mandates. With proper formulation, polymeric coatings provide asphalt systems with longer service lives that meet stringent customer expectations.
About the Author:
Steven Heinje is the technical manager for GAF’s Liquid-Applied Systems, headquartered in Parsippany, N.J. With a B.S. in biology and chemistry and, more recently, an MBA, he has 32 years of experience in roof coatings, specializing in acrylic elastomers and urethane coatings. He is chairman of the Roof Coatings Manufacturers Association (RCMA) Codes Task Force and leads several task groups in ASTM D08, as well as maintaining active memberships with various other organizations. For more information, contact: GAF, www.gaf.com