"Class 4" gets said a lot in Calgary, usually right after a hailstorm and usually by someone selling a roof. It's a real and meaningful rating. It's also narrower than most people think.
This guide explains exactly what the two main impact tests do, what a pass tells you, what it leaves out, and how insurance credits for impact-rated roofs work in Alberta. If you've already decided you're interested in a Class 4 roof and want help choosing, our Class 4 impact shingles guide picks up from there.
UL 2218: the steel ball test
IBHS calls UL 2218 the recognized test standard for asphalt shingles, and describes it as a steel ball drop test: a steel ball is dropped from the height needed to match the energy a hailstone of the same size would carry in a storm. Each class uses a bigger ball.
The ball hits the same spot twice, and the targets are the most vulnerable parts of the assembly, like edges, corners and joints. Then the tester turns the shingle over. According to IBHS, any tear, fracture, crack or rupture visible on the back is a failure. Crushed or dislodged granules, dents or marks visible only on the top are not failures under UL 2218's criteria.
| Class | Projectile diameter | UL 2218 (steel ball) | FM 4473 target energy (ice ball) |
|---|---|---|---|
| Class 1 | 1.25 in (31.8 mm) | Steel ball drop | 5.0 J |
| Class 2 | 1.50 in (38.1 mm) | Steel ball drop | 10.4 J |
| Class 3 | 1.75 in (44.5 mm) | Steel ball drop | 20.3 J |
| Class 4 | 2.00 in (50.8 mm) | Dropped from 20 ft | 36.4 J |
FM 4473: the ice ball test
FM 4473 is published by FM Approvals, and it was written for rigid roofing: tiles and planks made of materials like slate, concrete or clay. Some shingle manufacturers list it alongside UL 2218 too.
Instead of dropping steel, it uses ice balls frozen in moulds at about -22°C and fired from a launcher at the speed that gives each size its target energy. Each impact spot, chosen at vulnerable areas like edges, corners and overlaps, is hit twice within half an inch. To pass, the specimen can show no visible cracking or breakage. FM's standard notes that ice balls are generally harder and denser than hailstones, so they simulate a worst-case stone, while real hail varies much more in shape and density.
What an impact rating doesn't prove
A rating is a pass on a specific lab test. Here's what sits outside it.
- Age. Both tests use new material. FM 4473 states that long exposure to the elements has the potential to significantly lower a roof's hail resistance. InterNACHI makes the same point about shingles: as they age they become more brittle and less able to absorb a hailstrike.
- Surface damage. Crushed or dislodged granules and dents that show only on the top don't fail UL 2218. Granules are what shield the asphalt from UV, so losing them can still matter over the life of the roof.
- Real hail. Hailstones vary in shape, density and hardness, and they fall with wind. IBHS says standardized tests don't exactly replicate natural hail, though they're useful for comparing products. In its 2014 lab series on 22 shingle products, none of the products, basic or impact-rated, got past Class 2 impacts without at least one impact location failing.
- Conditions. FM 4473 conditions its samples at 15.6 to 32.2°C, and InterNACHI notes shingles are more brittle and less able to absorb impact when they're cold.
- Warranty coverage. An impact rating isn't a hail warranty. IKO's Canadian shingle warranty excludes hail, and IKO's product pages state that the impact rating is there to help owners obtain an insurance reduction, if one is available, and doesn't mean hail damage is covered.
Why the rating still matters
None of that makes impact ratings meaningless. The rating tells you a sample of the product resisted cracking through under a hard, repeatable impact. Between two otherwise similar shingles, the Class 4 one passed a test the other didn't.
IBHS has also developed its own hail impact test protocol using laboratory-made hailstones, for a more realistic evaluation. Ratings give you a floor. They don't rank products above that floor.
Insurance credits in Alberta: ask, don't assume
Hail is expensive in Alberta. The August 2024 Calgary hailstorm alone caused $3.35 billion in insured damage (CatIQ final estimate, August 2026), and the Insurance Bureau of Canada (IBC) has reported that Alberta has had five of the ten costliest disasters in Canadian history, all since 2016.
IBC notes that some insurers offer a discount for a newer roof or a specific type of roof, and says it's critical for homeowners to speak with their insurer about what's covered and what options are available. Whether a credit exists, how big it is and what proof it needs is up to each insurer, so nobody but your insurer can tell you what you'd get.
- Call your insurer or broker before you choose a roof, not after.
- Ask whether they give a credit for a UL 2218 Class 4 roof, and whether FM 4473 is accepted.
- Ask what proof they need, such as an invoice or product documentation showing the manufacturer, the product and the rating.
- Ask whether the credit applies to your whole policy or only part of it, and whether it continues at renewal.
- Ask whether roof age or rating affects your deductible or how roof claims are settled.
What does Class 4 impact rating mean?
Under UL 2218, it means a sample of the product took two hits in the same spot from a 2-inch steel ball dropped from 20 feet and showed no crack or opening on the back. Under FM 4473, the same class uses a 2-inch ice ball fired at a set energy. It's the highest class in both tests.
Are Class 4 shingles hail-proof?
No. The rating is based on a test of new material, and dents and granule loss don't count as failures. Large or wind-driven hail can still damage a Class 4 roof, and shingle warranties such as IKO's exclude hail.
Do I get an insurance discount for a Class 4 roof in Alberta?
Maybe. IBC notes that some insurers offer a discount for a specific type of roof. It isn't automatic, and terms vary by insurer. Ask your own insurer or broker before you buy.
What's the difference between UL 2218 and FM 4473?
UL 2218 drops steel balls and is the recognized test for asphalt shingles. FM 4473 fires ice balls and was written for rigid products like tile and slate. Both use four classes with the same projectile diameters.
Is a Class 3 shingle worth anything?
It means a sample passed with a 1.75-inch projectile instead of 2 inches. Mainstream architectural shingles can carry it: IKO lists its Cambridge and Dynasty lines as Class 3, for example. Neither Class 3 nor Class 4 makes a roof hail-proof.
Where the facts on this page come from. If a source has changed since we last checked, tell us and we'll fix the page.
- Relative Impact Resistance of Asphalt Shingles: Summary of UL 2218 Impact Tests (2014) · Insurance Institute for Business & Home Safety (IBHS)
- Relative Hail Impact Resistance of Asphalt Shingles · Insurance Institute for Business & Home Safety (IBHS)
- Specification Test Standard for Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls (Class 4473) · FM Approvals
- RICOWI Roof Guide: Cedar Shingles and Shakes (UL 2218 Class 4 test description) · Insurance Institute for Business & Home Safety (IBHS)
- Inspecting Asphalt Shingle Roofs: Student Book · International Association of Certified Home Inspectors
- Limited Warranty Information for Asphalt Shingles (Canada) · IKO Industries
- Cambridge Architectural Shingles (UL 2218 Class 3) · IKO
- Dynasty Performance Shingles (UL 2218 Class 3) · IKO
- Nordic Performance Shingles (UL 2218 Class 4) · IKO
- Crowne Slate Designer Shingles (impact rating disclaimer) · IKO
- Alberta continues to see large-scale impacts from hailstorms · Insurance Bureau of Canada
- August hailstorm in Calgary results in nearly $2.8 billion in insured damage · Insurance Bureau of Canada
- Final industry loss estimate, 2024 Calgary hailstorm · CatIQ