Roof Coatings Considerations
By MS Editorial Staff March 2007 - Roofing
Coatings have been used to surface and restore roofing systems for many years. Many coatings used today have improved in performance compared to the first coatings applied 30 years ago. This improvement is especially true in terms of adhesion to a variety of roofing substrates, as well as long-term performance. Two important decisions that maintenance managers face well before any coating is applied are whether to first apply a primer to the roof surface and whether to apply an aluminum coating as the top coat.
Primers aid in the adhesion of the coating by providing a bond between the roof surface and the coating. Primers come in many different forms, so the type of primer employed depends on the substrate, weather conditions, and the type of coating being applied.
Managers should not make a decision to apply a primer without consulting the manufacturer’s requirements for the specific coating and substrate. A primer is never a substitute for proper roof-membrane preparation. The substrate should be clean and dry before the coating application. Workers should clean off dust, chalking film, bitumen exudate, greases and oil before application.
Surfaces that generally require primer include: metal flashings, gravel stops and other metal edging; concrete and masonry roof decks; masonry walls and floors; and gypsum and other porous surfaces. Asphalt primers should be used with asphalt materials only.
Primers are compatible with modified bitumen products, but managers should check manufacturer recommendations before using a primer. While primers generally work best on clean dry surfaces, asphalt emulsions primers can be applied to damp, not wet, surfaces. Primer and surface-coating application should occur in short order.
Matching Primers and Roofs
Managers can use these general guidelines regarding the application of primer to various types of roofs.
ASPHALT ROOFS. These roofs, including smooth-surfaced built-up or modified bitumen membranes, contain light oils called exudate, which can leave a membrane soon after its application. This process is normal, and the exudate generally will wash off the roof after rainstorms. Any exudate on a roof before coating application should be cleaned thoroughly. Special primers to prevent the exudate from bleeding through fresh coating might be required with application of acrylic coatings.
Glaze-coated built-up roofing also can release exudate. If the glaze coat is not reinforced, it will crack or alligator. One way to reduce this effect on new roofs, and to cover cracks in older roofs, is to prepare the roof with a layer of fiber-reinforced asphalt-emulsion coating before applying the reflective coating.
On old, weathered asphalt roofs, many coating manufacturers recommend using a primer after cleaning the membrane to prepare the surface for coating. When coating an existing aluminum roof with an acrylic coating, priming usually is needed. Managers should consult the manufacturer for recommendations.
METAL. Primers generally are recommended before applying a surface coating to a metal roof, regardless of the roof’s age. Special primers are made for both aluminum asphalt and elastomeric coating application over metal roofing. Certain primers, such as zinc-chromate-based primers, might be incompatible with some coatings. Using a primer is not a substitute for removing rust and scale on weathered metal roofing.
COAL TAR. Generally, coal tar roof coatings are considered self-priming and do not require the use of a primer before applying a coal-tar coating or resaturant.
ACRYLIC-COATED. On roofs with an existing layer of acrylic coatings, primer might be required prior to application of new coating. Whether to use a primer is largely dictated by the age and condition of the existing coating.
SPRAYED POLYURETHANE FOAM. Existing sprayed polyurethane foam roofs might have been coated with silicone-based coatings. The adhesion of acrylic coatings to silicone is difficult, so manufacturers often perform adhesion tests on a sample of the roof membrane and recommend the appropriate primer to help assure proper attachment of the new coating.
SINGLE PLY. If coating is an option, the manufacturer of the roof membrane generally sells specialty primers for these membranes.
Once managers decide whether to use a primer, they can turn their attention to specifying the coating. Aluminum coatings are increasingly popular options because they can offer energy-saving benefits to facilities.
Adhesion or bonding to the roof surface is critical to the performance of aluminum roof coatings. The surface must be properly prepared, and in some instances properly aged, before applying aluminum roof coating.
Surfaces such as asphalt glaze or flood coats and solvent based asphalt coatings typically require about three months of warm weather before coating to permit evaporation of the solvents. Asphalt emulsions generally can be coated a short time after curing — typically 5-14 days — without worry about staining, depending on application rates and ambient conditions.
Studies indicate that it might be better to coat many modified membranes before, rather than after, they age. Follow the membrane manufacturer’s recommendations when coating a modified-bitumen membrane with aluminum coating.
While an aluminum coating adds to moisture protection, it is not designed to stop leaks or repair seams or blisters. Workers should repair these problems before applying the coating.
Cold temperatures can cause dew or moisture to interfere with adhesion of a coating. Moisture under a solvent-based aluminum roof coating will result in a splotchy appearance when the coating cures. Low temperatures also can inhibit the leafing of the aluminum coating.
Occasionally, this problem resolves itself as the coating ages over a summer. High temperatures also might become a problem in application. While the aluminum flakes leaf out well when the roof is hot, extreme temperatures — typically, above 110 degrees — also might cause the coating to cure too quickly, resulting in streaks and highlights. Rain also might cause problems if the coating has not completely cured.
Most manufacturers do not recommend coating if rain is expected within 24 hours of application.
The surface must be free of loose debris, dirt, oil and other materials that could cause loss of adhesion. This might involve sweeping or vacuuming to remove loose dirt or other dry material, power-sweeping to remove light contaminants, power-scrubbing for heavier contaminants or, in the worst cases, cleaning by water blasting. This process can take off heavy exudate, loosely bound particles and old coatings. Managers need to consult with the membrane manufacturer for details on how to clean the roof.
Ponding water is never desirable on a roof because it can cause delamination of the aluminum coating from the surface and premature coating failure. Positive drainage is necessary to flush away accumulations of surface dirt from the roof and keep the reflective properties of the coating intact. Coatings in ponded areas have a much shorter performance life. A primer might be needed, dependant on the condition of the substrate.
Allow all primers to dry thoroughly before applying aluminum coatings. Solvent-based primers might activate the asphalt in some built-up or single-ply membranes, causing staining. On metal roofs, synthetic solvent-based primers can be used as recommended by the coating manufacturer. Certain types of primers, such as lead, have severe incompatibilities with aluminum coatings.
If applying by brush, workers should use either a three- or four-knot roofer’s brush or soft-bristled broom. A roller should have a medium nap roller cover. When roll or brush-applying an asphalt aluminum, it is important that they finish the strokes in basically the same direction to achieve the best aesthetics. If aluminum flakes orient in the same direction, they reflect light more uniformly.
Spray equipment is typically an airless sprayer, but applicators can use any unit capable of spraying the coating in an even pattern. Consult the coating or spray-equipment manufacturer for proper selection, as the viscosity of the material, the hose internal diameter size, and length of hose all help determine the type of equipment needed.
Workers should apply an aluminum coating according to the manufacturer’s recommended coverage rate. ASTM Standard D 2824 requires certain consistencies for the aluminum coatings, and the consistency allows for application in one coat.
Low and high coverage can result in premature failure. Type I non-fibered aluminums are typically applied at the rate of 0.5 to 1.5 gallons per square using a roller, brush or spray. Type II or III Fibered aluminums are usually applied at 1 to 2 gallons per square, and can be applied using a roller, brush or spray.
Underwriter’s Laboratories Class A, B or C ratings might require applying coatings at a heavier rate. Consult the manufacturer or the UL directory for proper coverage rates for the Class A, B or C rating. As with any coating, avoid walking on the asphalt aluminum coating until it is fully cured.
The Roof Coatings Manufacturers Association (RCMA) — is the national trade association representing the manufacturers of cold-applied coatings and cements used for roofing and waterproofing, as well as the suppliers of products, equipment, and services to and for the industry.