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Commercial Roofing

Protecting Your Building from Hail Roof Damage

By Dawn Killough

May 13, 2022

Close-up of hail impacting dark shingles.

Up to 25% of the United States experiences large hail episodes—and when they do, hail can cause severe damage to roofs and the HVAC equipment stored there. Hailstorms account for 70% of the average annual property losses from severe convective storms in the US. Building owners can protect their buildings and equipment by designing their buildings and roofs to withstand the damage caused by large hailstones.

FM Global, an insurance company that specializes in loss prevention and testing, provides a standard for testing and approving assemblies that provide greater hail resistance. Building owners and designers often rely on FM Global's resources to specify desired roofing assemblies and construction methods. These resources include RoofNav assemblies from FM Approvals as well as loss prevention data sheets and Approvals Standards. Well-maintained roofs that meet FM's hail testing and approval requirements are better equipped to protect buildings from hail and water damage that occurs during storms.

Assessing Risks from Hail Roof Damage

Hail can cause physical damage to a building's roof and the equipment stored there, including dents, cracking, mineral loss on modified bitumen sheets, and compound delamination. Older roofs or those in poor condition may be more susceptible to damage from hail. Roof equipment, including cooling fins and condenser coils, can also be damaged from falling hail. Skylights are subject to hail punctures, which are the main cause of water damage to buildings and their contents.

In the US, the most severe hail generally occurs in the Midwest and central contiguous states. According to FM, very severe hail can be found in the region from North Dakota to Texas and Colorado to Missouri. Roofs, roof assemblies, and equipment in this region should be designed to meet FM's specific hail requirements to better withstand hail roof damage.

Defining Very Severe Hail

FM Approvals has recognized three classes of hail: moderate, severe, and very severe.

Moderate hail (MH) is less than 1.75 inches in diameter, which is about the size of a cherry. Severe hail (SH) is 1.75 inches to 2 inches in diameter, similar in size to a golf ball. Very severe hail (VSH) is 2 inches or greater in diameter, similar in size to a billiard ball (or larger).

FM's Very Severe Hail (VSH) certification was added in 2016 based on data from the National Oceanic and Atmospheric Administration's National Weather Service and National Center for Environmental Protection's Storm Prediction Center. This data showed a concentration of reports of hail larger than 2 inches in diameter in the very severe hail region.

Passing a Very Severe Hail Test

FM tests assemblies in accordance with FM 4470:

  • To test an assembly for moderate hail, a 2-inch-diameter steel ball weighing 1.19 pounds is dropped from 81 inches above the assembly. Two samples, unweathered and weathered, are tested 10 times each. This test provides an impact energy of 8 foot-pounds.
  • Testing for severe hail, the same 2-inch-diameter steel ball is dropped from a height of 141.5 inches above the assembly. This test provides an impact energy of 14 foot-pounds.
  • When testing whether a roof assembly can withstand very severe hail, a 2-inch-diameter preformed ice ball is launched at the assembly using compressed air. Three samples are used in this test: one unconditioned, one UV weathered, and the third UV weathered and heat aged for 42 days prior to testing. This test provides an impact energy of between 53 and 58 foot-pounds.

After the test is performed, the roof assembly is inspected for dents, cracking, and delamination. Assemblies that pass the test are certified as meeting the FM criteria. You can find an online database of VSH-rated roofing assemblies here; GAF has over 1600 single ply VSH-listed assemblies.

The FM standard recommends that building owners inspect their roofs on a regular basis and repair deficiencies; the standard also advises replacing roofs where deficiencies cover a large area or when the roof is nearing the end of its useful life. An owner should also inspect heat and smoke vents and skylights for cracking, yellowing or discoloration, cloudiness, or other indications of damage due to weathering and aging. Skylights showing damage should be replaced as needed, and hail guards on HVAC equipment require inspections to ensure they are in place and free of damage.

For more information about GAF and very severe hail, contact your local commercial roofing contractor or complete our very severe hail contact form.

About the Author

Dawn Killough is a freelance writer in the construction, finance, and accounting fields. She is the author of an ebook about green building and writes for construction tech and green building websites. She lives in Salem, Oregon with her husband and four cats.

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The initial and supplemental roof insulation ultimately saves a net of 705 metric tons of carbon over the life of the building.If you want to see more examples like the one above, check out PIMA's study, conducted by the consulting firm ICF. The research group looked at several DOE building prototypes across a range of climate zones, calculating how much carbon, energy, and money can be saved when roof insulation is upgraded from an existing baseline to current code compliance. Their results can be found here. Justin Koscher of PIMA also highlighted these savings, conveniently sorted by climate zone and building type, here.Support for Carbon Investment DecisionsSo how can you make sure you address both operational and embodied carbon when making "carbon investment" decisions? We've prepared a handy chart to help.First, when looking at lower-embodied-carbon substitutions for higher-embodied-carbon building materials or systems (moving from the upper-left red quadrant to the lower-left yellow quadrant in the chart), ensure that the alternatives you are considering have equivalent performance attributes in terms of resilience and longevity. If an alternative material or system has lower initial embodied carbon, but doesn't perform as well or last as long as the specified product, then it may not be a good carbon investment. Another consideration here is whether or not the embodied carbon of the alternative is released as emissions (i.e. as part of its raw material supply or manufacturing, or "cradle to gate" stages), or if it remains in the product throughout its useful life. In other words, can the alternative item be considered a carbon sink? If so, using it may be a good strategy.Next, determine if the alternative product or system can provide operational carbon savings, even if it has high embodied energy (upper-right yellow quadrant). If the alternative has positive operational carbon impacts over a long period, don't sacrifice operational carbon savings for the sake of avoiding an initial embodied product carbon investment when justified for strategic reasons.Last, if a product has high operational carbon savings and relatively low embodied carbon (lower-right green quadrant), include more of this product in your designs. The polyiso roof insulation in our example above fits into this category. You can utilize these carbon savings to offset the carbon use in other areas of the design, like aesthetic finishes, where the decision to use the product may be discretionary but desired.When designing buildings, we need to consider the whole picture, looking at building products' embodied carbon as a potential investment yielding improved operational and performance outcomes. Our design choices and product selection can have a significant impact on total carbon targets for the buildings we envision, build, and operate.Click these links to learn more about GAF's and Siplast's insulation solutions. Please also visit our design professional and architect resources page for guide specifications, details, innovative green building materials, continuing education, and expert guidance.We presented the findings in this blog in a presentation called "Carbon and Energy Impacts of Roof Insulation: The Whole[-Life] Story" given at the BEST6 Conference on March 19, 2024 in Austin, Texas.References:Architecture 2030. (2019). New Buildings: Embodied Carbon. https://web.archive.org/web/20190801031738/https://architecture2030.org/new-buildings-embodied/ Carbon Leadership Forum. (2023, April 2). 1 - Embodied Carbon 101. https://carbonleadershipforum.org/embodied-carbon-101/

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