The Class 4 rating means that a sample of the product did not crack when hit twice in the same place by a 2-inch steel ball. Consumers should keep this in mind when using the UL standard to gauge quality. IBHS further cautions that while some materials may test well, they may perform poorly in practice. For example, some roof coverings may resist cracking, the measurement criteria of the UL 2218 standard, but they can dimple or be dented. Finally, IBHS also recommends that homeowners be aware of a roofing material's wind and fire resistance features as well.
Good advice is available from design professionals, insurance companies and roofing installers.