Mastering Roof Inspections: Asphalt Composition Shingles, Part 3

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.

Asphalt Composition

Asphalt which is used to manufacture shingles may be blended by the shingle manufacturer, or it may be bought from an asphalt supplier already blended. The methods for mixing and applying asphalt are proprietary and manufacturers do not provide information about either process.

The performance characteristics of asphalt vary according to the type and proportion of ingredients added when the asphalt is mixed.

Crude Oil

One of the ingredients of asphalt that can affect asphalt quality is the crude oil from which it’s made. Crude oil may be heavy or light. Light crude flows more easily than heavy crude. Crude oil may also be sweet or sour. Sour crude has higher a level of sulfur. Heavy, sour crude is considered to be of lower quality.

Asphalt quality is affected by both the chemical composition of the crude and the process used to refine it.

Asphalt is one of the lower-value products produced from crude. As the price of oil continues to rise, efforts to extract a larger percentage of higher-value products from each barrel of oil will continue to increase. The price of asphalt will continue to increase, and manufacturers will always look for ways to control costs. Some solutions may result in a reduction of the quality of asphalt by some manufacturers. An example is the use of excessive amounts of filler.


Asphalt alone is not durable enough for use as shingles. Finely-ground minerals are added to the asphalt mix as filler, sometimes called a “stabilizer.” These fillers toughen the asphalt and improve its resistance to cracking. Common fillers are limestone and dolomite.

High-performance fillers maximize heat-transfer, helping to keep shingles cooler and extend their lifespan.

Adding filler also reduces the cost of manufacturing. Up to 70% of filler may be added, depending on various factors. Too much filler reduces shingle quality.

Quality Control 

In the past, ingredients used in the wrong proportions have been a concern. This problem no longer exists, since the measurement, addition to the asphalt mix, and the blending process that used to be controlled by people are now all controlled by computer.

Asphalt Standards

The ASTM provides standards for asphalt used in shingles, but compliance is not mandatory, and you will not be able to tell by simply looking at shingles installed on a roof whether they comply with any standards. The 2010 standards address the softening point of the asphalt, and the minimum amount of filler used in manufacturing each shingle.


The thickness of the asphalt layer affects shingle performance. Shingles with a thicker asphalt layer resist fracture better than similar shingles with a thinner layer.

Thickness varies among manufacturers, and even among different products by the same manufacturer.

ASTM standards require a minimum of 15 pounds of asphalt for every 100 square feet of shingles. This is not per square of installed shingle, but of actual shingle material.

There’s really no way for you to judge the quality of asphalt.  You just need to be aware that quality can vary.


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

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Inspecting Underlayment on Roofs
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