Mastering Roof Inspections: Asphalt Composition Shingles, Part 47

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. 


Area of Impact 

The edges and cutouts of shingle tabs are more vulnerable to damage because the edges lack the support of a surrounding shingle. A few small, random, damaged areas near a shingle's edges are not a problem. Some insurance companies don’t consider a shingle to be functionally damaged unless the damage reaches ½-inch in from the edge.

Regardless of whether it's at the edge or in the field of the shingle tabs, all significant damage to shingles within a test square should be considered in making your report.

Angle of Impact

Hailstones that hit shingles with a glancing blow do less damage than hailstones that hit shingles more directly.


Shingles are more brittle and less able to absorb impact when they’re cold. Low temperatures, especially below 50º F, leave shingles much more vulnerable to hail damage.

Substrate Support

The best substrate for asphalt shingles is a solid deck. Many manufacturers require a solid deck for their warranty to remain valid. Still, you’ll see many homes with newer asphalt shingles installed over older asphalt shingles, and even over wood shingles or shakes.

Shingles installed over an existing layer of roofing are more likely to suffer hail damage because of poor underlying support. Areas where newer shingles bridge the butts of underlying shingles typically have gaps. These areas will be more easily damaged by hailstones.

Cap shingles on ridges and hips are also more vulnerable to hail damage because they aren’t supported as well as shingles installed directly on the roof deck.

Asphalt shingles installed over existing roofing materials have uneven support. The butts of existing shingles create voids beneath the new shingles. The parts of the newer shingles bridging the voids are more vulnerable to damage from hail than the parts of the shingles that have full support of a solid substrate beneath them.

Hail Damage vs. Blisters

Occasionally, damage from hail is mistaken for damage from blisters.

Blisters are caused by the expansion of volatile gasses escaping from the asphalt layer. Gas migrates to the shingle's surface where it forms bubbles. Eventually, the tops will erode away.

This illustration shows the difference in profile between a blister and a hailstrike. Blisters leave steep-sided pits or pockmarks that are usually missing some asphalt material. Sometimes, enough asphalt is missing that the mat is exposed. The surface of the shingle may be slightly raised around the perimeter of the pits.

Hailstrikes usually leave very shallow indentations with very lightly sloped sides.

These photos show details of blisters in 3-tab fiberglass shingles. In the two photos above, you can see that the blisters are widespread.

The photo directly above includes a penny for perspective.  You can clearly see the steep sides and the exposed mat.  Because these shingles are 20 years old, this condition will not be covered by the manufacturer’s warranty.

The two photos above show the obvious difference between hail damage and blister damage. Note the shallow depth of the indentation and the granules remaining in the area of hailstrike damage.


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

 Take InterNACHI’s free, online 
Roofing Inspection Course
Mastering Roof Inspections
Roofing Underlayment Types
Inspecting Underlayment on Roofs
Fall-Arrest Systems
Roofing (consumer-targeted)
More inspection articles like this