With the growth of large sprawling urban and suburban areas in hail-prone regions, the U.S. crossed the threshold of billion-dollar hailstorms during the early 1990s. As a result, manufacturers, insurers and testing laboratories have developed test methods to evaluate the impact resistances of roof coverings.
The first test method to gain widespread use was UL 2218, "Standard for Impact Resistance of Prepared Roof Covering Materials," which uses the impact of a steel ball falling from prescribed heights to establish performances of flexible roof coverings.
Although the UL 2218 test method has proved to be a good measuring stick for the relative hail resistances of flexible roof coverings such as asphalt shingles, the intense impact forces imparted by the steel ball usually are too great for rigid roofing materials, such as tile.
In response to this observation, in July 2005, FM 4473, "Specification Test Protocol for Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls," which uses an ice ball shot from an air cannon, was modified. With the completion of that test standard, manufacturers now have two options for testing the impact resistances of roof coverings though it is expected UL 2218 primarily will be used for flexible roof coverings and FM 4473 primarily will be used for rigid roof coverings.