Base flashing, also known as weatherproofing, is used to create a water-tight seal around roof protrusions such as vents, skylights, chimneys or walls in order to protect a structure from water leaks and the elements.

Asbestos felt, or asbestos roofing felt, was often used under base flashing to provide homes with thermal insulation and further protection from water damage.

Asbestos in Base Flashing & Felt

Now typically made of metals such as copper, aluminum, lead or steel, base flashing was once made with asbestos fibers or asbestos felt. Asbestos fibers in roofing materials offered a low-cost means to provide thermal insulation, high tensile strength, flexibility and fire resistance to roofs in the mid-20th century. Asphalt shingle roofs, flat roofs and built-up roofs were often constructed using asbestos-containing base flashing and felt materials. The adhesives used to attach the base flashing and felt to roof shingles and roof sheathing also frequently contained asbestos fibers.

Asbestos felt was a type of tar paper that was applied to roof surfaces in the form of a large roll. The material was commonly used to provide a waterproof barrier between sheathing and roofing shingles or other roofing surfaces, and the material could often be found on built-up roofs. Asbestos fibers in roofing felt offered additional reinforcement, as the strong fibers could resist the pulling force of the roofing material. Asbestos felt from the 1950s to the 1970s was known to contain an average of 10 to 15 percent chrysotile asbestos fibers.

Dangers of Base Flashing & Asbestos Felt

The U.S. Environmental Protection Agency (EPA) has since eliminated the use of asphalt-saturated asbestos felts used as base flashing. Considered a "nonfriable," Category I ACM (asbestos-containing material) by the EPA, asbestos base flashing and felt may pose a health hazard only if the material is significantly aged, damaged or weathered. If severely worn, the material can be considered "friable," which means that asbestos fibers may break free and become airborne. If airborne asbestos fibers are inhaled or ingested, the fibers can lodge within the body and increase the risk of serious health conditions such as lung inflammation, lung cancer, asbestosis or mesothelioma.

Asbestos-containing base flashing or felt should never be ripped, sanded, drilled, cut, scraped or otherwise disturbed in order to avoid releasing asbestos fibers into the air.
Individuals at risk for exposure to asbestos roofing materials such as base flashing or asbestos felt include:

Roofers or slaters
Construction workers
Remodelers
Demolition crews
Homeowners

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