Designing for the Impacts of Very Severe Hail

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Sponsored by GAF
By Jennifer Keegan, AAIA

FM’s LPDS 1-34 identified the VSH region as encompassing Oklahoma, Kansas, Nebraska, South Dakota, most of Texas, and parts of Montana, North Dakota, Minnesota, Iowa, Missouri, Arkansas, Wyoming, Colorado, and New Mexico. Note that the top 5 states listed above with the highest number of hail events are the first 5 states listed in the VSH region.

Figure 5: FM’s LPDS 1-34 map outlining the different hail categories—moderate, severe and very severe. The Very Severe region is most commonly referred to as “Hail alley.”

FM Approvals is a third-party testing and certification laboratory with a focus on testing products for property loss prevention using rigorous standards. FM Global, through the loss prevention data sheets, requires the use of FM Approved roof systems.

FM Approvals designed the hail tests for all regions with a 15 year mean recurrence interval. The Moderate Hail zone is based on an equivalent hail size of less than 1.75 inches; the Severe Hail zone is based on 1.75- to 2-inch size hail; and the Very Severe Hail zone is based on greater than or equal to 2-inch diameter sized hail.

Figure 6: Description of FM Approval hail regions.

Roof systems seeking the VSH designation are tested on accelerated aged roof membranes that are subject to a more severe hail impact (higher impact energy). The test protocol evaluates damage to the roof membrane as well as the coverboard located beneath the membrane. VSH testing of roof systems is considerably more difficult to pass than the previous Severe Hail rating.

The VSH test follows ANSI FM 4470, which has requirements for single-ply, polymer-modified bitumen sheet, built-up roof (BUR) and liquid applied roof assemblies for use in Class 1 and noncombustible roof deck Construction, and includes launching a 2-inch diameter ice ball from a hail gun, propelled at 152 to 160 ft/s, resulting in an impact energy of 53 to 58 ft-lb tested on the roof system with three different conditioned roof membranes:

  1. An unaged sample
  2. A sample after 1,000 hours of UV weathering; and
  3. A sample after 1,000 hours of UV weathering and 1,000 hours of heat aging

The roof membrane and field seam cannot show any signs of cracking or splitting under 10X magnification and the coverboard substrate cannot crack in order to pass this test.

In contrast, the Severe Hail rating has an impact energy of 14 ft-lbs and is only tested on unaged and UV weathered roof membrane samples, not heat aged membrane. The VSH test is the most stringent of tests designed to identify systems that can better protect buildings and ultimately reduce claims.

Research on the Impacts of Hail

FM 4470 calls for a pass/fail rating based on whether or not the roof assembly has been visually punctured or dented by the ice ball impact.5 One study that evaluated the impact resistance of roof membranes at various thicknesses and attachment methods found that as long as impact was not above a fastener, the membrane was not punctured, even with a 45 mil membrane. Note that this study was completed on unaged material, with all testing completed at room temperature, which does not replicate conditions for an aged roof exposed to a hail event, nor does it replicate the testing requirements for VSH assemblies. However, the findings summarize a few important concepts in regards to protection against hail events.

The impact resistance of the roof is really a system discussion, above and beyond just the membrane. All of the elements of the roof need to be considered in tandem to truly understand how they can work together to protect the building from the damaging impacts of hail.

The study highlighted this concept by evaluating the impact effects of hail on varied membrane thicknesses, as well as coverboards, and attachment methods. Each of these elements will influence the system's performance. This study showed the benefits of using a thicker roof membrane, especially a fleeceback single-ply membrane. It also revealed that coverboards and insulation can also be damaged by hail. So focusing solely on the damage observed to the roof membrane doesn’t tell the full story, especially with larger hailstones. This is why the VSH acceptance criteria includes the entire assembly, not just the roof membrane.

Another study6 aged the membrane to the 30-year heat-aged equivalence point and tested a standard Thermoplastic Polyolefin (TPO) membrane and one specifically designed for high-temperature resistance and long-life (long-life TPO) in demanding applications. Fleeceback TPO membrane is specifically designed to enhance impact resistance from hail, even as it ages and demonstrates the best performance. The extra cushion provided by the fleece renders the membrane more forgiving to impact.

When VSH assemblies first came to market, plywood and OSB were used as “coverboards” to achieve that rating, which are very labor intensive to install. Manufacturers of coverboards and roof membranes have teamed up to provide enhanced solutions to the VSH conundrum. New glass mat roof boards (coverboards) on the market specifically designated for VSH have increased the number of systems available, enhanced performance, and expanded designers capabilities by passing the VSH testing with simultaneously fastened or “gang fastened’ systems, meaning the roof insulation and coverboards are secured to the roof deck with fasteners concurrently extending through all layers (as shown in Figure 7). There are now solutions for projects that may benefit from mechanical attachment, including weather limitations, existing deck constraints, high uplift pressures or designers preference. Additionally, mechanically attached systems are often quicker, easier and more cost effective to install than adhered systems, which could offset the cost of the VSH assembly.

Figure 7: VSH systems. Left is simultaneously fastened 60 mil Fleeceback TPO over glass mat VSH roof board and Polyiso Insulation. Right is 60 mil Fleeceback TPO over glass mat VSH roof board adhered in low rise foam ribbons to mechanically attached Polyiso Insulation.

The early VSH assemblies were not successful with mechanically attached coverboards or single ply membranes. The impact of the ice ball would result in a failure. However, with the new glass mat roof boards, mechanical attachment of the coverboards and insulation (simultaneously fastened systems) is possible as shown in Figure 7.


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Originally published in August 2022