Silicone roof mastic on seams of commercial metal roof
Silicone roof mastic on a commercial metal roof.

What Is a Roof Mastic?

Roof mastics (also known as roof repair mastic, roof mastic sealant, or roof mastic coating) are used to seal any areas of a commercial or industrial roof that are especially prone to leaks. Mastics are thicker than typical elastomeric roof coatings, so they are ideal for waterproofing because they allow for the natural thermal movement (expansion and contraction) of the roof surface.

Often described as “elastomeric,” mastics are highly flexible. After the surface preparation phase of a coating system application is complete, roof mastics are usually the first product to be applied to a roof. Mastics can be applied on multiple commercial and industrial roofing substrates, including metal, modified bitumen, EPDM, single-ply, smooth built-up roof, concrete, asphalt, and spray polyurethane foam.

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Types of Roof Mastic

Mastics are made in a variety of materials, such as acrylic, urethane, butyl, or silicone. American WeatherStar mastics also come in different types of grades, such as brush-grade (best when applied with a brush), butter-grade (whipped and thicker than brush-grade), and flashing-grade (formulated with an even higher viscosity for use on vertical surfaces).

Sealants, though technically in a category of their own, are similar to mastics in that they also have high viscosity and adhesion. Sometimes, roof mastics will be labeled as sealants. The difference is that mastics are typically a part of a larger roofer system and perform best when paired with a coating top coat, whereas sealants are standalone repair products used for small patch sections of a roof.

In some cases, however, mastics can be used as a sealant/repair product. The easiest method of remembering the difference is that all mastics are sealants, but not all sealants are mastics.

What Purpose Do They Serve?

Mastics allow the roof surface to expand and contract as temperatures fluctuate. Due to their high viscosity (thickness) that acts as a barrier to moisture, mastics are used to seal and waterproof certain roofing components (seams, penetrations, fasteners, drains, etc.) that are more vulnerable to leaks. They can be applied to both flat and metal commercial roofs. In this video, we demonstrate why a mastic makes a better alternative to seam tape.

Sidenote: Want to learn more about how roof restoration systems work? Check out our comprehensive guide to fluid-applied commercial roof restoration.

How to Apply a Mastic

Roof mastics can be applied using a roller, brush, caulking gun, bulk gun, or trowel. Unlike roof coatings, mastics are too viscous to be applied using a hydraulic sprayer. It is because of their meticulous application method that mastics are often referred to as part of the “details” stage of a roof restoration system.

Mastics should be applied on all seams, penetrations, drains, skylights, vents, fasteners, and any other flashing details on a commercial roof. Since these areas are the most likely spots for leaks to occur, it’s imperative they be properly sealed with a roof mastic.

The specific conditions, temperature, coverage rates, and roofing substrate in which the mastic should be applied should be outlined by the manufacturer. American WeatherStar, for example, provides this information in our Application Guidelines and Mastic Coverage Rates.

American WeatherStar also provides Detail Drawings (pictured below) which outline how a mastic should be applied to specific roofing structures such as drains, seams, pipes, skylights, etc. We offer detail drawings for both flat roofs and metal roofs.

Roof Mastics We Provide

We provide a variety of high-performance roof mastics designed to seal and waterproof seams, penetrations, drains, fasteners, and other roof flashing areas. Whether for use in full system applications or isolated roof repairs, these roof mastics provide a durable, flexible, and watertight membrane capable of withstanding the natural expansion and contraction (thermal movement) of commercial and industrial roof structures.


A roof mastic is an integral part of any fluid-applied roof restoration system because it serves as the baseline of defense for a commercial roof. The high viscosity of a mastic is critical to ensuring water cannot penetrate through a roof and cause a leak.

Do you think your commercial roof may have a need for a roof mastic, roof sealant, or a full roof coating system? We recommend scheduling a free roof inspection with an American WeatherStar Approved Contractor to assess your roof.

And if you are a roofing contractor with quality workmanship and are interested in using our roof mastics or any other of our commercial roofing products, consider becoming an American WeatherStar Approved Contractor today. Click here to get the process started.