ANSI/FM 4473

ANSI/FM 4473, Specification Test Standard for Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls, was developed by FM Approvals and is an American National Standard. It is the main alternative to UL 2218 for rating hail resistance, and it is used mostly for rigid products such as clay and concrete tile, slate, and some metal and composite products.

How the test works

Ice balls are made by freezing water in controlled moulds and then fired from an air cannon at a 3 by 3 ft (0.9 by 0.9 m) specimen of new material. The test is run at a temperature between 60 and 90 °F (16–32 °C). Each target location receives two coincident impacts, and at least two locations are tested.

The four classes correspond to nominal ice ball diameters:

• Class 1 – 1.25 in (32 mm)
• Class 2 – 1.5 in (38 mm)
• Class 3 – 1.75 in (44 mm)
• Class 4 – 2.0 in (51 mm)

A 2 in ice ball weighing about 0.135 lb (61 g) travels at roughly 117 ft/s (about 80 mph, 129 km/h) and carries around 29.5 ft-lb (40 J) of energy, according to one technical summary. To pass a class, the sample must show no visible cracking, breakage or perforation.

Why ice rather than steel

Real hail can shatter or flatten when it strikes. Ice balls spread their energy differently than solid steel balls, which deliver a sharp point load. Supporters of FM 4473 argue that steel-ball tests unfairly penalize brittle materials like tile and slate. Because the methods differ, a product can pass one and fail the other, so ratings are not interchangeable.

Where it is required

The IBHS FORTIFIED Roof program's hail supplement accepts tile rated Class 4 under FM 4473, while metal and low-slope systems are expected to meet UL 2218 Class 4. FM Approvals also uses impact testing within its roofing approval standards, and FM Approvals publishes a separate standard, 4475, for roof components in hail-prone regions.

Limitations

Like any lab test, FM 4473 uses new material at mild temperatures. Ageing, cold brittleness, and wind-driven hail at an angle can all make real damage worse than test results suggest.