Impact Resistant Roof Coverings
Clearly, the most effective way to minimize hail damage is to use roofing materials that are resistant to hail impacts. In 1996, the first test standard was developed to assess the impact resistance of roof coverings – UL 2218 Impact Resistance of Prepared Roof Covering Materials. Subsequently, an additional test standard, FM 4473, Specification Test Protocol for Impact Resistant Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls, was developed. UL 2218 is intended primarily for testing flexible roof coverings, but has also been used to test rigid roofing materials. FM 4473 was developed specifically for testing rigid roof coverings. FM 4473 defines rigid roofing materials as those manufactured as tiles or planks from slate, concrete, or clay materials.
The UL 2218 test standard uses steel balls ranging from 1.25 inches to 2.0 inches in diameter. The steel balls are dropped from heights of 12 feet for the 1.25 inch ball to 20 feet for the 2 inch ball. Although this apparatus tests for impact resistance, not hail resistance, the impact of the steel ball simulates the impact energy of free-falling hailstones. The test assembly is struck with the steel ball twice in the same location on the assembly. To meet the acceptance criteria of UL 2218, the roof covering material exposed surface, back surface and underneath layers must show no evidence of tearing, fracturing, cracking, splitting, rupture, crazing or other evidence of opening of the roof covering layer. Qualifying assemblies are given a class rating depending upon successful performance of the assembly under impacts from the varying sized steel balls. The class rating is outlined in Table 1.
Table 1 - UL 2218 Impact Resistant Roof Covering Classes
The impact-resistance of rigid roofing materials such as tile and slate is assessed by the test standard FM 4473. This test is much like the UL 2218 test with the exception that freezer-made ice balls are used instead of steel balls. Additionally, in FM 4473, ice ball missiles are shot at the test assembly to achieve the equivalent impact energy of a free-falling hailstone. FM 4473 also provides four class ratings for a product depending on the performance of the assembly when subjected to the ice ball missiles.
Even though most of the common roofing systems used today can be altered or modified for increased impact resistance, the features that make a roofing product impact resistant vary depending upon its material type. Wood shingles and shakes can be made more impact resistant by increasing their thickness and density. Such alterations make them less prone to splitting, which is the primary mode of failure after an impact. Metal roofing can be made more impact resistant by increasing the thickness or the stiffness of the material. Metal 26 gage and thicker will pass the UL 2218 impact test at a Class 4 level. In addition to thickness, metal becomes much stiffer when it is bent or seamed. The fact that some of the metal products are made to look like wood shakes, tile, or slate means the metal has been stiffened considerably just through the forming process. Alternative products, such as synthetic tiles, are generally made of either flexible material, such as rubber, or more rigid materials such as plastic, wood fiber, urethane, and recycled resins. These alternative materials can usually be manufactured to attain a Class 3 or 4 UL 2218 rating. Asphalt shingles manufactured with polymer-modified styrene-butadiene-styrene (SBS) or atactic polypropylene (APP) are more impact resistant than typical composition shingles manufactured using a glass fiber base mat or organic felt. The SBS or APP is blended into the asphalt to enhance flexibility, durability, crack resistance, impact resistance, and resistance to ultraviolet light. These and other modifications have enabled asphalt shingles to make the transition from being one of the most vulnerable roofing systems to one of the strongest choices available.
• Roof Covering: Use a roof covering that has rating of Class 3 or 4 when tested in accordance with UL 2218 or FM 4473. Be sure to look for the class rating on the product label because impact-resistant products often do not look any different than other comparable untested products.
• Roof slope: Use roof slopes of 6:12 (27 degrees) and greater. Particularly for new construction, the use of higher roof slopes (6:12 and greater) greatly enhances roof-system resistance to impacts from hail.
• Roof sheathing: Use 5/8 inch plywood roof sheathing supported by framing (trusses or rafters) spaced no more than 24 inches on center.
• Reroofing: When reroofing, remove the existing roof covering before installing a new roof covering.