Impact Resistance Rating (UL 2218 and FM 4473)

Impact resistance ratings are laboratory classifications that indicate how well a roof covering resists damage from simulated hail. In North America, the two most widely cited methods are UL 2218 and FM 4473. Both rank products from Class 1 (lowest) to Class 4 (highest), and Class 4 products may qualify for insurance premium discounts in some US states.

UL 2218

UL 2218, Standard for Impact Resistance of Prepared Roof Covering Materials, covers asphalt, wood, tile, fiber-cement, plastic and metal shingles and panels. A 0.9 by 0.9 m (3 by 3 ft) specimen of new product is struck twice in the same spot at several vulnerable locations by steel balls dropped from set heights:

• Class 1: 32 mm (1.25 in) ball
• Class 2: 38 mm (1.5 in) ball dropped about 4.6 m (15 ft)
• Class 3: 44 mm (1.75 in) ball dropped about 5.2 m (17 ft)
• Class 4: 51 mm (2 in) ball dropped about 6.1 m (20 ft)

A product passes a class if there is no tearing, cracking, splitting, rupture or opening on the exposed surface, back surface or underlying layers. Cosmetic marks alone do not count as failure.

FM 4473

FM 4473 is used for rigid materials such as clay and concrete tile and slate. Instead of steel balls, it fires pre-molded ice balls of 32 to 51 mm (1.25 to 2 in) at the specimen. A pass requires no visible cracking or breaking.

Criticisms and newer methods

Experts have noted several limits. Tests use new, unweathered products, but hail usually strikes aged roofs; outdoor exposure reduces the impact resistance of many materials. UL 2218 is run at room temperature, while real hail often falls during sudden cooling. Steel balls do not behave like natural hailstones, and pass/fail classes give little way to compare products, since many shingles carry the same rating.

The Insurance Institute for Business & Home Safety (IBHS) developed an alternative protocol for asphalt shingles, published in 2021, that uses manufactured ice spheres matched to field-measured hail strength and scores damage modes such as dents and granule loss on a continuous scale. IBHS publishes shingle ratings from this testing, and the FORTIFIED Roof hail supplement accepts shingles rated Good or Excellent by IBHS.

Terminology

Because ratings are tied to specific test methods, technical writers recommend citing the method and class (for example, "UL 2218 Class 4") rather than describing a product as hail-proof.