Roof coating on commercial flat and metal roofs

It’s a question we get asked from time to time: which roof coating performs best? The simplest answer we can give is—it depends. There are many variables to consider when it comes to the performance, longevity, and suitability of an elastomeric roof coating. Below, we provide a comprehensive overview of the benefits, chemistry types, and physical properties to look for in an effective coating solution.

The Benefits of Roof Coatings

Elastomeric roof coatings have been around for a long time. That being said, many facility owners and managers are still unaware of the numerous advantages they provide. Before we get into the various types of coating chemistries available in the industry, let’s take a closer look at some of the more significant benefits a coating system provides.

Costs Less

Roof replacement can be quite expensive. Depending on the type of roof you have, a full replacement can cost anywhere from $5 to $15 per sq. ft. Roof restoration, on the other hand, provides all the benefits of a new roof but for a fraction of the cost.

Because roof coatings are applied directly over existing surfaces, they require far less labor and material than traditional roof tear-offs and replacements. As a result, you can expect to pay 50-70% less by installing a roof coating system on your building.

Extends Life

Perhaps the best-known quality of an elastomeric coating system is its ability to extend roof life. The average commercial roof only lasts about 20 years, but the addition of a coating system can prolong a roof’s serviceable life by up to 10, 15, or even 20 years.

Elastomeric coatings provide aged roof membranes with a protective, waterproof barrier that stops leaks and improves performance. If properly installed and maintained, a coating system has the potential to be recoated numerous times. It also can significantly reduce the need for constant maintenance and repair.

Reduces Energy Costs

Coating your roof is a great way to improve the energy efficiency of your building. Unlike conventional dark-colored roofing materials, white roof coatings can reflect 80-90% of the sun’s harsh UV rays and diminish the flow of heat to your building. This reduces the workloads of HVAC systems during warm summer months and cuts interior cooling costs by up to 20% on average.

Additionally, the superior reflective and emissive qualities of a coating system can significantly cool roof surface temperatures and provide a more comfortable work environment for building occupants. Unconditioned spaces can experience a reduction in ambient temperatures, as well.

Improves Sustainability

In recent years, the demand for more sustainable roofing solutions has grown considerably due to more stringent environmental regulations and building codes as well as increased concern for the environment.

Commercial roof tear-offs contribute millions of tons of waste to landfills each year. In contrast, there is no tear-off required prior to the application of a commercial roof coating system, allowing you to reuse existing roofing materials with minimal impact on the environment. As mentioned above, the prospect of recoating also contributes to increased sustainability.

Environmentally Friendly

In addition to improved sustainability efforts, many of today’s elastomeric coatings are developed with little to no VOCs (volatile organic compounds), making them much safer for the environment. This also means buildings with a roof restoration system could qualify for the US Green Building Council’s LEED program.

Common Types of Roof Coating

Choosing the best coating for a particular roofing environment depends on a variety of factors such as roof type, age, weather conditions, environmental concerns, aesthetics, etc. Below, we take an in-depth look at some of the key strengths and weaknesses of today’s most common types of coating.

Acrylic

Acrylic roof coatings are among the most durable and inexpensive elastomeric coatings in the industry. Comprised of high solids emulsion polymers and potent biocides, these highly reflective coatings are ideally suited for extreme UV climates and provide superior resistance to mildew and dirt accumulation.

Traditionally used to restore metal roof surfaces, acrylic coatings have since evolved to work on a number of different roofing substrates including modified bitumen, single-ply, built-up roofs, and spray polyurethane foam. Acrylics are easy to work with and come in a variety of different standard, premium, and custom colors. These coatings can be spray or roll-applied at a rate of .75 to 1.5 gallons per 100 square feet, depending on the substrate.

Acrylics do not stand up well to ponding water and are only recommended for roofs with positive drainage. Cold temperature applications can also be a problem. Because acrylic is a water-based substance, temperatures at or below 50°F can negatively affect adhesion to the roof substrate and impede cure time.

Asphalt Emulsion

Another widely used roof restoration material is asphalt emulsion. This product consists of asphalt particles dispersed in water with clay used as a bonding element. Generally black or brown in color, asphalt emulsion contains organic and inorganic compounds, as well as other fillers to add strength and durability.

Asphalt emulsion is inexpensive and easy to work with. It is typically used to level “alligatoring” and other irregularities that commonly occur on modified bitumen and built-up roof surfaces.

While it can be installed as a stand-alone roofing material, an asphalt emulsion is most commonly used to provide a monolithic foundation membrane for other elastomeric roof coating products that have better reflectivity and UV stability. They generally require 3 to 4 gallons per 100 square feet depending on the substrate. Multiple passes may be required to achieve proper results.

Asphalt emulsions are a water-based product, which makes them susceptible to freezing temperatures. They can also be very difficult to work with during cold weather conditions.

Butyl Rubber

Butyl coatings are best known for their extreme elongation and tensile strength. This solvent-based polymer is highly reflective, fast-drying, and great for roof maintenance and repair. Due to their low permeability, butyl rubber coatings are most commonly used as a vapor-retardant barrier over spray polyurethane foam systems in cold storage/freezer applications.

A butyl coating system is suitable for a variety of commercial roof surfaces including metal, modified bitumen, single-ply, built-up roofs, and spray polyurethane foam. They are typically white, gray, or tan in color and can be sprayed or roll-applied at a rate of about 2 gallons per 100 square feet.

The primary disadvantages of butyl coatings are their low solids content and their lack of color stability. They require more material to achieve the desired dry film thickness, making them less cost-effective than other elastomeric coatings. They can also be challenging to spray and difficult on application equipment.

Silicone

Silicone roof coatings are the preferred choice when it comes to UV protection and resistance to ponding water. They erode much slower than other coatings without becoming brittle or hard. Modern silicone formulations have a high-solids content that typically exceeds 90 percent. This means less material is needed to achieve the required dry film thickness.

Silicone coatings are created by adding solvents and a catalyst to a silicone base. Available in white, gray, or tan, silicone can be installed on a variety of roof types including metal, modified bitumen, single-ply, EPDM, built-up roofs, and spray polyurethane foam. Depending on the substrate, most silicone roofing systems require multiple coats at 1.5 gallons per 100 square feet.

Silicones attract dirt rather easily. This means the system will gradually lose reflectivity over time if not cleaned periodically. They are also extremely slippery when moisture is present. Resistance to tearing is another inherent weakness. If pedestrian traffic is expected on the roof, we highly recommend incorporating roofing granules into the system.

VIDEO: Click here to learn some helpful best practices for working with silicone.

Urethane (Polyurethane)

When it comes to strength, durability, and adhesion, urethane roof coatings are second to none. Compared to other coatings, they are more impact-resistant and are better suited to handle pedestrian traffic. They are also strong enough to endure the natural expansion and contraction (thermal movement) of roof structures and the negative effects of ponding water.

There are two main types of urethane coatings: aliphatic and aromatic. Aliphatics are generally used as top coat, as they are more UV stable, resistant to dirt pick-up, and have superior color retention. Aromatics are less expensive but not quite as UV stable. They are primarily used as a foundation coating in most roof restoration systems. Both types can be spray or roll-applied, with most system specifications requiring multiple coats at a rate of 1.5 gallons per 100 square feet.

Being a xylene-based material, urethane coatings can be challenging to spray, harsh on installation equipment, and highly odorous. It’s also worth mentioning that urethanes are a moisture-cure coating, meaning they are constantly curing—even in the pail.

Notable Physical Properties to Consider

Every commercial roof is different. That’s why it’s important to evaluate certain physical properties of a coating to ensure its suitability in a particular roofing environment. Each property listed below, as well as other technical data, can be found in the manufacturer’s product data sheets.

Comparison of physical properties of today's most common roof coating chemistries

Elongation

Elongation is the percentage in which a material can be stretched before it breaks. Most elastomeric coatings typically measure anywhere between 250-900%. The higher the elongation value, the more suited a coating is to endure the normal expansion and contraction of a roof structure without cracking, thinning, or peeling off.

Tensile Strength

Tensile strength is the amount of pressure a material can withstand before it breaks. Depending on chemistry type, most elastomeric coatings rate anywhere from 250-1200 psi (pounds per square inch).

Elongation and tensile strength are closely related but in an inverse direction. As one increases, the other decreases. The best commercial roof coating formulas balance the two properties to provide a strong, durable membrane that can adequately stretch and move with roof surfaces.

Permeability (Moisture Resistance)

Permeability, or perm rating, is the state or quality of a material or membrane that causes it to allow liquids or gases to pass through it. This value varies greatly depending on the type of coating.

For example, acrylics have a high perm rating, meaning they are very breathable. Silicones, on the other hand, have a lower perm rating meaning they have superior resistance to moisture transmission. It’s important to note that higher permeability does not equate to inferior weather resistance.

Reflectivity

Reflectivity measures the amount in which a material can reflect light. It is typically represented as a percentage between 0-100%, with at least 85% being considered good reflectivity. Unlike conventional, dark-colored roofing materials, white roof coating systems reflect 80-90% of the sun’s energy back into the atmosphere—significantly reducing the risk of heat gain throughout the roof structure.

Emissivity

Emissivity refers to a material’s ability to release absorbed heat. This property is usually represented as a decimal between 0 and 1, with 0.65 being considered ideal for an elastomeric roof coating. This means 65% of the sun’s thermal energy is emitted away from the roof, thus reducing building heat loads and lowering cooling costs.

Solids by Volume

Solids by volume is the measure of how much solid material will remain after a substance has dried. Understanding this property is critical in order to properly achieve the required dry film thickness specified for certain coating systems. While it has no bearing on the performance of a coating, this particular property is certainly worth some consideration.

Though the physical properties highlighted above are the most notable in terms of performance, there are other data points to consider as well. For a complete overview of the physical properties for American WeatherStar products, click here to download our full roof coating comparison chart.

Conclusion

Ultimately, the performance of a commercial roof coating depends on a variety of factors. Each chemistry has its own unique set of strengths and weaknesses. Things like roof type, age, environmental concerns, aesthetics, and more must be taken into account before deciding on a suitable, long-term solution.

If you’re interested in one of our restoration systems, schedule a free roof inspection with an American WeatherStar Approved Contractor today.