Mastering Roof Inspections: Asphalt Composition Shingles, Part 45

by Kenton Shepard and Nick Gromicko, CMI®



The purpose of the series “Mastering Roof Inspections” is to teach home inspectors, as well as insurance and roofing professionals, how to recognize proper and improper conditions while inspecting steep-slope, residential roofs. This series covers roof framing, roofing materials, the attic, and the conditions that affect the roofing materials and components, including wind and hail. 



Although the mat is only about 2% of the shingle's weight, it is a crucial component in determining the impact resistance of asphalt shingles. Thicker, heavier mats resist damage from impact more effectively than thinner ones. Mat thickness varies among manufacturers and among shingle types produced by the same manufacturer.

Again, a fractured mat is functional damage.

Asphalt Composition

Asphalt used for asphalt shingles varies in both quality and thickness, and these two properties can affect the severity of hail damage.

Poor-quality asphalt can be the result of poor-quality or improper ingredients used to manufacture the asphalt, or the ingredients may be used in the wrong proportions. Quality is also affected by the manufacturing methods.

Asphalt used in the manufacture of shingles may be blended by a shingle manufacturer or bought already blended from a supplier. The methods for mixing asphalt are proprietary and manufacturers do not publish this information. As an inspector, there’s no way for you to judge the quality of asphalt; you just need to be aware that the quality can vary.

The thickness of the asphalt that is applied to the mat varies among shingles produced by different manufacturers. Obviously, a thicker asphalt layer resists fracture more effectively than a shingle with thinner asphalt of similar properties.

A crack in the asphalt layer does not constitute functional damage. The crack must extend through the mat.

Lack of Studies

One of the key questions about the point at which functional damage occurs in asphalt shingles is the degree to which exposure of the asphalt surface causes premature failure. Determining this accurately would require long-term studies by a neutral third party on a large number of shingles. No such studies are publicly available and probably don’t exist. There is no easy answer.

UV Radiation

The primary force that deteriorates exposed asphalt is ultraviolet or UV radiation. The degree to which it affects shingles depends on several things:

  • asphalt quality. Low-quality asphalt is less resistant to deterioration;
  • the percentage of granules missing. Due to the lack of available studies, disagreement exists concerning the amount of exposed asphalt that will significantly shorten the long-term service life of asphalt shingles. Some shingle manufacturers have issued technical bulletins stating that any granule loss will cause premature failure, but they haven’t published studies to support this claim; and
  • the climate zone where the home is located. Asphalt shingles of similar design and quality are installed in widely differing climate zones across North America. The rate at which exposed asphalt deteriorates depends on the intensity of the UV radiation in each climate zone.

Roofs in areas that have a high number of sunny days each year are exposed to more UV radiation than homes in areas with fewer annual sunny days. Roofs at higher elevations are also exposed to more UV radiation. The angle of the roof's slope and the direction it faces also affect the amount of average, annual UV radiation received.


Does granule loss that exposes asphalt cause asphalt shingles to fail prematurely?

There is no industry-wide consensus. No credible, publicly available, long-term studies exist to support claims made by supporters of either position, so it’s currently impossible to supply a definitive answer.

We know that UV radiation from sunlight deteriorates exposed asphalt. As a greater amount of asphalt is exposed, the chances of premature failure due to UV-ray deterioration increases. The chances for premature failure are not the same everywhere, but are influenced by:

  • the amount of asphalt exposed;
  • the quality of the shingle; and
  • the average annual level of exposure to UV rays.
In most climates, the length of time it takes for the asphalt to actually leak as a result of granule loss from a hailstrike is close to the service life of the shingle anyway. And since shingles overlap, beneath each area of exposed asphalt is another shingle.


Learn how to master a roof inspection from beginning to end by reading the entire InterNACHI series: Mastering Roof Inspections.

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