OTHER HOT TOPICS
Hail Damage Thresholds: What do they really mean?
By Scott Morrison, PE, Haag Principal Engineer
Since 1963, Haag engineers have performed impact tests on and made field inspections of the myriad of roofing materials in order to determine their hail damage thresholds. These thresholds are often cited by industry professionals. Their use raises some important questions. What are hail damage thresholds, and what are they not? How are they determined? And when and how might they be useful to you during damage assessment?
What is a hail damage threshold, and what is it not?
A hail-damage threshold indicates the smallest size of hail which might cause functional hail damage to a particular type of roofing. Because a number of factors play a role in the causation of damage, a hail damage threshold is not a guarantee that hail of a certain size did or did not damage a particular roofing assembly. Thresholds are merely rules of thumb that ballpark the opportunity for damage to occur.
How are hail damage thresholds determined?
At Haag, tests for threshold damage follow specific parameters. The roofing materials are impacted:
Haag thresholds are the result of 52 years of impacting studies in the lab, as well as correlations of such research with many thousands of Haag field inspections. The table below lists established Haag thresholds for common roofing materials.
|Lightweight asphalt shingles||1”|
|Heavyweight asphalt shingles and wood shingles||1¼”|
|Medium wood shingles and clay tiles||1½”|
|Built-up roofing (smooth)||2”|
|Modified bitumen membranes||1½” to 2”|
|EPDM membranes (smooth)||2”|
|Other plastic singles-ply membranes (smooth)||1” to 2”|
|Membranes ballasted with gravel||2½”|
|Galvanized steel panels||2½”|
When and how might these thresholds be applied during damage assessment?
Consider this example for when hail damage thresholds might be particularly useful.
Suppose that hail has been reported as 1” and 2” in two areas of a city. Since the threshold for lightweight asphalt shingles is 1” hail, a valid hypothesis is that the greater opportunity for damage would be in the area where 2” hail had fallen. Here, the hail-caused damage should be more obvious in asphalt shingles than in the area where the size of hail is closer to the threshold size.
Scott Morrison, P.E., Haag Engineer, 01/2015
Scott Morrison specializes in Structural Evaluations, Foundations, Earthborne and Airborne Vibrations, Roofing Systems, Reconstruction Monitoring and Analysis, Research/Testing, and Architectural. He is a primary author of material in the Haag Certified Inspector courses and many of the Haag Damage Assessment Field Guides. See his profile here.