How Slate Roofing Protects Against Hail and Storm Damage: What Homeowners Need to Know

It may be counterintuitive to worry about hail damaging a slate roof. After all, anyone who has ever thrown an ice ball at a rock wall knows that the wall remains perfectly intact, while the ice ball shatters to smithereens.

But when Mother Nature starts hurling two-inch diameter hailstones from the heavens onto your slate roof, stuff gets real in a hurry.

To test how well slate roofing resists hail, Factory Mutual Global, along with the American National Standards Institute (ANSI), have developed a test standard designated FM 4473 “Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls.” 

The 4473 standard outlines the specific requirements for every detail in the testing process. It specifies ice ball size and weight, the subject ‘roof assembly,’ as well as measuring the velocity and impact ‘energy’ of the ice balls.

Cooking Up a Storm: Manufactured Hail Strike Testing

While real hail varies greatly in terms of shape and density, the man-made spherical ice balls are the closest known approximation and are produced in a variety of thicknesses. These ice balls can be applied with sufficient force to simulate the effect of real hailstones. However, the man-made ice balls are harder and denser than real hail, so the results simulate the ‘worst case’ scenario.

Sample slate shingles are mounted on a test panel that simulates an actual roofing application and installed according to the manufacturer’s instructions. The sample test area is at least 3-ft. square and conditioned to temperatures ranging from 60 to 90°F (15.6° to 32.2°C).

An ice ball launcher powered by compressed air is used to ‘fire’ the ice balls at the ‘slate roof’ target. The launcher is placed no more than 5 feet (1.5 m) from the target, and each target location on the slate roof is impacted twice, with a maximum 0.5-inch (13 mm) distance between impacts. Target impact locations include edges, corners, overlaps, and joints on the sample roof. Once the testing is complete, the slate is visually inspected for damage on both the top and bottom surfaces.

North Country Slate Provides Top-Tier Protection from Hail Strikes

In an FM 4473 test conducted on behalf of the National Slate Association by Architectural Testing of Southlake, Texas, S-1 rated natural roof slate, including North Country Slate’s ‘Unfading Black’ tiles, were tested and evaluated. The results were impressive, to say the least.

North Country’s 1/4-inch-thick slate was subject to 1.75-inch diameter ice balls, fired from 36 inches at velocities ranging from 100.4 to 101.4 feet per second (30.60 to 30.91 m/s), or about 69 mph in a series of four tests. The results showed no visible cracking or breakage, earning the slate a Class 3 hail resistance rating. 

The test was repeated with 3/8-inch-thick roofing slate using 2-inch diameter ice balls fired at velocities ranging from 111.2 to 114.2 feet per second (33.89 to 34.81 m/s), or about 76 mph, with the same results – no visible cracking or breakage, good enough for a Class 4 hail impact resistance rating!

Class 3 and Class 4 Hail Impact Resistance scores are strong enough for some insurance companies to offer a reduction in residential insurance premiums due to the material’s extraordinary resistance to inclement weather.

Slate Remains a Great Roofing Option for All Weather Conditions and Climates

In addition to its resistance to hail strike impacts, slate roofing also performs extremely well in gale-force winds. North Country Slate shingles were tested against the ASTM D 3161-15: Standard Test Method for Wind-Resistance of Steep Slope Roofing Products (Fan-Induced Method) and received a Class F Designation, providing unflinching protection against winds as strong as 110MPH!

In the event of a catastrophic storm, slate damage from hailstones or wind is relatively easy to identify and repair. Hail damage will typically leave a visible hole in the slate tile. Replacing a few broken tiles on a roof is not an issue for any professional slate roofer or handyman. The rule of thumb is that if more than 20-30% of the slates are damaged, it’s better to replace the entire roof.

Weather the Storm with North Country Slate

When storm damage happens, readiness matters. North Country Slate is proud to maintain a wide inventory of all types of North American roofing slate to respond quickly to emergency situations. Our team is uniquely qualified to help identify and supply exactly the right slate for a quick response when timeliness matters most. Let us match your slate from our vast stock before a quick roof repair becomes an extensive roof replacement.

North Country Slate is 100% natural, and besides offering great resistance to hail, it is also highly resistant to moisture and chemicals, unaffected by freeze/thaw cycles, and non-combustible with a Class A Fire Resistance Rating. In short, North Country Slate can add beautiful aesthetic appeal and 75 years or more of life expectancy to any structure, all with superb protection against anything Mother Nature can dish out.