Focus On A Coating’s Overall Quality As Well As Specific Features
Aside from specific coating features like mildewcides and extra protection against water and foot traffic, a coating's overall quality also is a key consideration. "If you're going to be in the building a long time, get the best coating," Martucci says.
While many DIY home improvement stores sell roof coatings, the ones available generally won't meet the needs of a commercial roofing system, Gayle says, and typically these stores rarely stock the quantities a commercial roof requires.
In addition, the solids content of the coatings that are available in the DIY marketplace tends to be lower. The proportion of solids — what's left after any water in the formula evaporates — generally should be at least about 50 percent, Martucci says. A lower concentration may mean more coating applications are needed to get the desired thickness, driving up labor costs.
One way to determine a coating's quality is to see if it meets ASTM D6083, "Standard Specification for Liquid Applied Acrylic Coating Used in Roofing," Martucci says. (Note: According to the ASTM website, this standard was withdrawn in July 2014, pending an updating. However, it remains valid, Martucci says, adding that any changes probably will be insignificant.)
Martucci also points out that some coatings won't meet the standard for reasons that have nothing to do with quality. For example, some coatings with bleed-blocking properties become less flexible in colder temperatures, which means they fail this provision within the standard. But they still may be an appropriate choice for some roofs.
If a coating is being selected on the basis of its reflective properties, it's critical to check the initial reflectivity and the reflectivity remaining after three years, says Brian Villa, national sales manager with National Coatings. "You want the coating to hold its reflectivity as long as possible."
The quality of the material impacts the three-year number, says Micah Smith, technical director, also with National Coatings. Some resins yellow in the sun, while some bulk filler pigments erode quickly and discolor. Both yellowing and erosion can lower reflectivity over time. Checking the three-year reflectivity number upfront is key if energy efficiency is driving the coating purchase. "The payback on energy costs versus the installation costs comes from how long you can keep the roof white," Smith says.
Permeance, or the amount of moisture that can permeate a coating, is another attribute to consider for some facilities, Smith says.
Coatings generally fall into two types, according to Jeff Blank, vice president with SR Products: water-based and solvent-based. Solvent-based coatings can be more expensive, but often can work in a greater range of temperatures, he adds.
Acrylic coatings, which generally are water-based, are "the workhorse" of white roof coatings, Mellott says. They tend to be versatile and available at an attractive price. But because they're water-based, acrylic coatings may not perform as expected in roofs that regularly experience standing water, he adds.
Urethane coatings or coatings with urethane-like properties tend to be "more hardy and durable," Smith says. As a result, they're often used on roofs that need to be tough and walkable.
One available option with urethane is to use a urethane base with a silicone top coat, Gayle says. The urethane is physically strong, while the chemical make-up of the silicone withstands the sun. The two materials combined can protect against both ponding water, and sunlight. "It's a good marriage."
One emerging category of coatings is what is considered "high solids," says Blank. While these come in liquid form, they dry with little evaporative loss. That can mean fewer coats are required to obtain the desired coverage.