Selecting and specifying roof coatings

by Katie Daniel | July 31, 2015 9:46 am

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By James R. Kirby, RCMA
Coatings have been successfully used as part of roof assemblies for more than 50 years in the United States. However, in other parts of the world, coatings have been employed for millennia. The development of sophisticated protective coatings occurred in the latter half of the 17^th century, when a German chemist found a way to use the stickiness of coal tar as protection on wood and ropes. During the 19^th and early 20^th centuries, scientists developed or discovered various new organic, inorganic, and synthetic materials for use in coatings, binders, and solvents. Perhaps the most dramatic advance in coating properties has come in the past 40 years, with the development of polymers.

Roof coatings are designed to protect and extend the service life of roof assemblies for new construction and, more commonly, for existing coverings, such as built-up (BURs), metal, modified-bitumen (mod-bit), single-ply, and sprayed polyurethane foam (SPF) systems. Coatings protect roof membranes from ultraviolet (UV) degradation and weathering, assist and enhance a system’s fire resistance, and provide a watertight layer on an existing roof. These coatings can enhance reflectivity and improve the aesthetics of the surface, lessening the building’s cooling load and heat island effect because the heat strain on the roof has reduced. Roof coatings also help decrease expansion and contraction of the membrane, by minimizing the membrane’s temperature swings.

Roof coatings are thicker than traditional paint, and have higher solids content; they are also applied in greater volumes. They are formulated to resist weather for many years and are also extremely elastic because they typically contain a level of high-quality resins that are introduced during manufacturing allowing the material to maintain its elastic property. Once cured, these resins form an elastomeric and durable film, which provides an additional measure of waterproofing and allows the roof coating to bridge small cracks and membrane seams.

Types of roof coatings
The most common types of roof coatings are acrylic, asphalt, polyurethane, and silicone.

Acrylic coatings
Acrylic water-based coatings are suitable for high UV environments where a reflective roof is desired. They can be colored, but generally are sold in white, tan, and grey. Many specialized versions are compatible with specific substrates.

Asphalt coatings
Asphalt coatings are either solvent-based ‘cut backs’ or emulsions; they can be either black or aluminized. These coatings are often most effective when applied over other asphalt substrates. Able to be used in cold and inclement weather, aluminized coatings are employed when a reflective and UV-stable asphalt coating is needed.

Polyurethane coatings
Polyurethane coatings are typically solvent-based and come in two main types: aromatic (less UV-stable) and aliphatic (very UV-stable). Urethanes have good mechanical properties and high abrasion resistance. They are best used in hail-prone regions and where a roof is exposed to heavy foot traffic.

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Silicone coatings
Silicone coatings, like acrylics, are suitable for high-UV environments where a reflective roof is desired. Silicone is often used in locations where rain is a daily occurrence, or if the roof is often wet and experiences a lot of ponded water.

Other types of roof coatings
Roof coatings can be made with other resins or combinations of resins. These include:

• styrene ethylbutylene styrene (SEBS);
• fluoropolymer;
• styrene-acrylics;
• polymethyl methacrylate (PMMA);
• polyester;
• silyl-modified polyurethanes (SPUR);
• polyuria; and
• butyl resins.

Codes and standards
The 2015 edition of the International Building Code (IBC) does not have specific requirements for roof coatings, except when used in combination with SPF systems. There are IBC requirements for liquid-applied roofing, but this should not be confused with coatings. Liquid-applied roofing is classified as a roof covering in the model codes, whereas a roof coating is not.

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From a building code perspective, a coating can be applied on new and existing roofs as a maintenance item; application of a roof coating is not considered to be the application
of a new roof. Therefore, an existing roof can be coated or re-coated at any time—for example, a building owner can add a highly reflective roof coating to reduce energy costs. Additionally, applicators of a roof coating on an existing roof should consider any in-place warranties so they are not voided or violated.

The 2015 IBC lists acrylic, silicone, and moisture-cured polyurethane coatings for use on SPF roofs. Each has an ASTM material standard that each type of coating must meet.

• ASTM D 6083-05e01, Standard Specification for Liquid-applied Acrylic Coating Used in Roofing;
• ASTM D 6694-08, Standard Specification for Liquid-applied Silicone Coating Used in Spray Polyurethane Foam Roofing Systems; and
• ASTM D 6947-07, Standard Specification for Liquid-applied Moisture-cured Polyurethane Coating Used in Spray Polyurethane Foam Roofing System.

Figure 1 lists film physical property requirements for each coating type.

To date, there are numerous coating types that do not have consensus standards (e.g. ASTM) for minimum properties. Specifying coatings that do not have a consensus standard should be based on physical properties provided by manufacturers. Some of the coating types that do not have an associated ASTM standard include polyureas, aliphatic polyurethanes, aromatic urethanes, butyl, and SEBS coatings.

Application considerations
Coating application includes considerations such as;

• storage;
• surface and substrate preparation;
• primer use;
• application specifics; and
• curing and drying time.

Storage
One should always keep roof coatings stored as close as possible to room temperature. In cold weather, a heated warehouse is generally the preferred type of storage area. Water-based products susceptible to freezing may be damaged or rendered useless if subject to temperatures at or below 0 C (32 F).

Surface and substrate preparation
The roof must be thoroughly cleaned, and allowed to dry completely. This is a critical step to ensure a successful coating application. Dust, chalking film, bitumen exudate, grease, oil, and other loose debris should be cleaned off the roof surface before the coating application. Care should be taken when pressure-washing to not disturb the integrity of the underlying roof membrane, particularly where there are adhered seams. Roof coatings should never be applied to a frost-, snow-, or ice-covered surface. In addition, for most solvent-based roof coatings, the surface must be dry unless the coating has been specifically formulated for adhesion to wet surfaces. It is advisable to apply coatings only when there is a minimum of 24 hours before forecasted rain. Coating manufacturers have specific requirements for roof preparation.

Primer use
Primers are designed to prepare the roof surface for accepting a coating and improving the overall adhesion. However, a primer is never a substitute for proper surface preparation. Typically, these products exhibit low viscosity and are formulated to quickly wet the substrate in a thin layer. When cured, it will leave behind a thin film acceptable for receiving a coating. Primers are available in various binder and solvent combinations, and the type used in a particular application will be based on the substrate, weather conditions, and coating type.

Any required roof or flashing repairs should be completed and allowed to adequately cure as necessary. Some water-based primers may be applied to damp, but not wet, surfaces. Coating manufacturers can be consulted about non-dry surface application. Applying the surface coating should occur soon after the primer has been spread and cured to ensure the desired results.

Surfaces that generally require primer include metal flashings, gravel stops, and other metal edging. Certain roof substrates may require priming not only to promote adhesion, but also to prevent the substrate from negatively impacting the coating performance. For example, rust-inhibitive primers are often used to prevent rust/corrosion on metal flashings from transferring through to the surface of a water-based coating.

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Primers for asphalt roofs
Asphalt roofs, such as smooth-surfaced BUR or mod-bit membranes, and granulated cap sheet, contain light oils called exudate, which can leave a membrane soon after its application. This process, called, ‘tobacco juicing,’ is normal and the exudate will generally wash off the roof after rainstorms. Any exudate on a roof prior to coating application should be thoroughly removed by cleaning. Special primers, called bleed-blocking primers, are designed to resist the exudate from bleeding through to the fresh coating and may be required before the application of light-colored elastomeric coatings.

Glaze-coated built-up roofs can also release exudate. In addition, the glaze coat, if not reinforced, will crack or ‘alligator.’ One method to reduce this effect on new roofs, or to cover existing cracks in the glaze-coat of older roofs, is to prepare the roof with a layer of fiber-reinforced asphalt emulsion coating before the application of a reflective or protective coating. On old, weathered asphalt roofs, many coating manufacturers recommend use of an asphalt primer after cleaning the membrane to prepare the surface for coating.

Primers for metal roofs
A number of different types of factory-finished, metal roof panels are available. These include galvanized, fluoropolymer-based (PVDF) and siliconized polyester-based prefinished aluminum and steel, and granule-surfaced metal shingles. Depending on the factory-finish of the metal panel and the weathered condition of the panel, a primer may or may not be required before coating.

Primers are generally recommended before applying a coating to a galvanized metal assembly, regardless of whether the roof is new or old. Special primers are made for both aluminum asphalt and elastomeric coating applications over galvanized metal roof panels. Certain rust-inhibitive primers, such as zinc-chromate-based primers, may be incompatible with some coatings. Therefore, guidance from the coating manufacturer should be sought.

As previously mentioned, the use of primer is not a substitute for proper surface preparation. Rusted metal roofs can be coated after the rust conditions have been addressed. If the metal roof includes white rust (i.e. on zinc or aluminum), it can be coated after cleaning. If the metal roof includes red rust, it must be removed or treated with a corrosion-inhibiting primer. All corroded fasteners should similarily be treated or replaced.

Primers for sprayed polyurethane foam roofs
SPF roofing is coated immediately after installation to protect the material from UV degradation. Therefore, primers are not generally required in this application. However, existing SPF roofs may be coated with silicone or acrylic coatings. Sticking to silicone is difficult, so performing adhesion tests on a sample of the existing roof coating or membrane before recommending the appropriate primer is useful to help ensure the proper attachment of the new coating.

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Primers for single-ply roofs
A variety of single-ply roof membranes are used in the industry, including ethylene propylene diene monomer (EPDM), polyvinyl chloride (PVC), and thermoplastic polyolefin (TPO). Each of these membranes exhibit unique coating challenges, so membrane-specific primers or primer and washes may be required before coating.

Primers for previously coated roofs
Recoating over existing coatings is a key benefit when using field-applied membranes as it leads directly to reducing the life cycle costs of a roofing assembly. Coatings have a finite service life, meaning recoating will be necessary after the existing coating has weathered and is no longer providing useful protection to the surface. Depending on the type of existing coating and the extent of weathering, it may be necessary to prime the surface before applying a fresh application of roof coating.

When coating an existing aluminum-coated roof with a water-based coating (including acrylics), priming may be necessary, because the chemical treatment used on the aluminum pigment can interfere with adhesion. Therefore,
a test application should be performed and evaluated prior to coating an existing aluminum-coated roof.

On roofs with an existing layer of acrylic coating, primer may be required before application of the new coating. The use of primer, as well as the type of product selected, will be largely dictated by the age and condition of the existing acrylic coating.

In general, only silicone-based primers and coatings should be used over existing silicone coatings because adhesion to silicone is difficult. Whether a primer is required before recoating will depend on the condition of the existing silicone-based coating.

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Application specifics
Roof coatings can be applied using spray, roller, or brush to all types of roof membranes and substrates. Coatings can be applied by a properly trained building owner or an experienced roof coating applicator. However, a roofing contractor is strongly recommended if there are any issues, such as existing leaks, wet insulation, damaged deck, or manufacturers’ warranties.

All roofs should be designed and built to have positive drainage and not allow ponding water. While some coatings are designed to withstand ponding conditions, this is not the case with others. One must check water swelling and absorption properties to see whether a coating can withstand ponding water conditions for extended periods.

As a coating naturally wears away, additional coating can be applied. Most coatings systems are re-coatable and provide a sustainable roofing system when properly maintained. The manufacturer should be consulted for specific requirements on re-coating applications.

Curing and drying time
For water-borne coatings, the ambient air temperature should be 10 C (50 F) and higher, with no rain for the next 24 hours. Solvent-borne coatings can be applied in colder weather, but as always, the coating manufacturer should be consulted.

While modern technology permits the application of some roof coatings at low temperatures, one can expect the cure time to be longer than on a warm summer day. A product that may cure overnight at a temperature of 21 to 23 C (70 to75 F) may take several days to cure at 4 C (40 F).

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VOC issues
There is an environmentally based effort to continue to reduce the volatile organic compounds (VOCs) in roof coatings. Found in a wide variety of products and in naturally occurring processes, VOCs are compounds of carbon that participate in atmospheric photochemical reactions. Ground-level ozone is one of six ‘criteria’ pollutants regulated by the U.S. Environmental Protection Agency (EPA), because it has negative impacts on human health and can be damaging to vegetative growth. Ozone is formed when VOCs and nitrogen oxides (NOx) combine with sunlight. However, VOCs help protect roof coatings during shipment by providing freeze-thaw resistance. On the jobsite, VOCs help condition or soften the substrate and improve adhesion of the roof coating. They are integral to specific raw materials used in the roof coatings products. Roof coating manufacturers recognize the need to reduce total VOCs—over the last few decades, 90 percent of VOC content has been eliminated from roof coatings.

The most common replacement for VOCs is water, but water-based roof coatings have unintended consequences, including;

• hard settling, or phase separation, of the components during transportation and storage of the product before application;
• poor adhesion of the product to the roof substrate;
• the need for more coats of the product and heavier coverage rates, therefore possibly increasing total
  VOC emittance;
• reduction of product lifespan, requiring more frequent applications; and
• elimination of roof coating product lines that cannot be reformulated.

Conclusion
Roof coatings can provide many benefits, from aesthetics, to energy, longevity, and performance.¹ There are a wide variety of choices for roof coatings, many considerations when selecting and specifying roof coatings, and many reasons why coatings are used on new and existing roof systems. It is important to understand why a roofing coating is used so it can be selected properly.

James R. Kirby, AIA, is the codes and standards director for the Roof Coatings Manufacturers Association (RCMA). Kirby has more than two decades of roofing industry and association experience, and has a well-rounded knowledge of roofing materials and systems as well as standards development. He can be reached at jkirby@kellencompany.com[8].

Source URL: https://www.constructionspecifier.com/selecting-and-specifying-roof-coatings/

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