A roof coating is a monolithic, fully adhered, fluid applied roofing membrane. It has elastic properties that allows it to stretch and return to their original shape without damage.
Typical roof coating dry film thickness vary from paint film thickness (plus or minus 3 dry mils) to more than 40 dry mils. This means a roof coating actually becomes the top layer of a composite roof membrane and underlying system. As such, the roof coating is the topmost layer of protection for the membrane, receiving the impact of sunlight (both infrared and ultraviolet (UV), rain, hail and physical damage.
Acrylic Roof Coatings
Acrylic is a water-based, cost-effective solution for a variety of roofs. They are good for most climates and provide the best value.
- Strengths: Acrylic coatings offer an excellent balance of cost and performance. They are highly reflective, UV resistant, and easy to work with.
- Weaknesses: Acrylics will lose mil thickness with weathering, need to be applied at 50°F or above, and typically do not perform well in ponding-water situations.
Polyurethane Roof Coatings
Polyurethanes are more impact resistant and handle traffic the best of all coatings. There are two main types of polyurethane roof coatings: aromatic and aliphatic. Aromatic coatings are less expensive and not UV stable, so they are usually used as a base coating. Aliphatic coatings are more expensive, but are UV stable, hold color well, and stay cleaner than most other coatings.
- Strengths: A urethane roof coating system with an aromatic base and an aliphatic top coat is durable, stays cleaner, and is more resistant to ponding water than an acrylic coating. They also do well as cool roofs because they can be white and stay clean.
- Weaknesses: They are more expensive than acrylics and can have a stronger odor compared to most other coatings.
Silicone Roof Coatings
Silicones are moisture cured and humidity can even promote their cure. In some applications, the use of silicone can eliminate the need for a primer.
- Strengths: Silicones weather better than other coatings with almost no erosion and they don’t get hard or brittle. Silicone is also good in ponding water situations.
- Weaknesses: Silicones hold dirt and rain acts as a cleaner; therefore the material loses reflectivity over time. It is also hard to adhere to silicone, so when a roof needs re-coating, you must either remove the coating completely or re-coat with a compatible silicone. Resistance to tearing is a mechanical weakness and silicone coatings should be used with fabric as a common practice.
Roof Coatings should not be confused with deck coatings. Deck coatings are traffic bearing – designed for waterproofing areas where pedestrian traffic is expected. Roof coatings will only waterproof the substrates but will not withstand any kind of on going use by people (such as walkways, patios, sundecks, restaurants, etc.).
The National Roofing Contractors Association’s (NRCA) director of technical services has stated “The roofing industry is aware of a number of issues that could have negative consequences for field application of coatings. Anyone considering this type of application should be aware of the concerns so they can weigh them against the benefits claimed in coating product materials.
Most flat roofs are not perfectly sloped, causing water to puddle in some areas, in particular around the drains and scuppers. Many roof coatings are not rated or warranted for “ponding water”, and thus might fail pre-maturely if exposed to standing water over long periods of time. Serious consideration should be given to the performance and limitations of the coating being selected. Some manufacturers will warranty their coatings including ponding water. Typically, manufacturers for acrylic roof coatings do not include warranties for ponding water, while some polyurethane roof coatings manufacturers will.