Mastering Roof Inspections: Asphalt Composition Shingles, Part 44

Hail is one of the most common forms of environmental damage to roofing materials. Hail damage is so widespread that in the U.S. alone, it costs close to $1 billion each year in insurance claims.  In developing the articles on wind and hail damage in this series on mastering roof inspections, we've consulted with major insurance companies for their expertise.

If inspectors from the insurance, roofing and home inspection industries use the same criteria in identifying damage, then we stand a good chance of reducing disagreement among the experts, as well as confusion on the part of homeowners.

Impact Resistance

Underwriters Laboratories have developed standards and provide testing for asphalt shingles' resistance to impact. Shingles are rated Class 1 through 4, with Class 4 shingles having the highest resistance.

You will not be able to determine the impact rating of shingles by looking at them, although the impact rating is usually marked on the cellophane on the underside of the shingle.

Resistance may be provided in several ways.

An older method was to attach a layer of reinforcing fabric, called a scrim, to the back of the shingle, as you see in the photos above.

There are several methods commonly used today:

• One method is to increase the weight of the mat used.
• Another method is to make the shingle thicker by increasing the thickness of the asphalt layer, or bonding layers of shingles together.
• A third method is to modify the asphalt mix by adding polymers. Polymers are chains of molecules that link to each other, often called cross-linked polymers. They can be used to improve the physical characteristics of a wide variety of materials. In asphalt shingles, they’re added to the asphalt to improve shingle strength and impact resistance.

Let’s look closely at what happens when a hailstone strikes an asphalt shingle.  We’ll assume that the hailstone hits at 90° to the surface.

Characteristics of Damage 

The effects of hailstrikes on asphalt shingles vary according to the conditions at the time of the hailstorm. If hail is accompanied by rain, as is common in the U.S. in the Midwest and on the East Coast, the rain will cool the roof, making the asphalt harder.  Hailstones striking rain-cooled, hard asphalt are more likely to loosen or displace granules.

When the hailstone strikes, the shingle flexes downward. The top surface of the shingle is in vertical compression and the bottom is in horizontal tension.  The bottom surface of the shingle has to expand in order to absorb the impact of the hailstone. This expansion creates tension, which is relieved by cracking. This crack is referred to as a fracture.

A shingle fracture starts at the bottom surface of the shingle and spreads toward the top surface. If the stress is great enough, the shingle will fracture clear through, from bottom to top. If the fracture extends through the mat, the shingle has suffered functional damage.

The amount of indentation in the image above has been exaggerated to illustrate the point.

Indentations are usually slight unless the shingle is fractured clear through, as you see here.

The hailstrike may not be severe enough to cause a fracture extending clear through the shingle. The fracture may extend through the mat but not reach the upper surface. This condition is called a bruise because it creates a soft spot on the shingle, which you can feel with your finger. It feels like the bruise on an apple.  Bruises are functional damage.