FM LPDS 1-34 includes commentary for designers. Table 2.3-1 provides two paths to comply with the VSH requirements—selection of a Very Severe Hail rated assembly, or a system with poured concrete or concrete pavers over the roof membrane. The commentary in section 3.1 states that concrete and concrete pavers can protect roof covers from hail larger than 2.5 inches.

Section 3.1 provides guidance of roof selection and states, “When using Table 2.3-1, selecting roofs with higher hail ratings, thicker membranes, and/or harder coverboards (such as gypsum or wood) can reduce roof and interior water damage from many hail storms.”

Section 2.2.1.1 directs users to RoofNav to find properly rated roof assemblies. As of June 2022, RoofNav offers over 16,500 VSH rated assemblies. Of the 11,000+ assemblies designated for new roof systems, nearly 40 percent are single-ply systems. Roof recover assemblies with VSH ratings are about 50 percent single-ply systems. Reroof assemblies are currently about 30 percent single-ply systems. The new coverboards on the market and ability to simultaneously fasten the systems will lead to an increase of available reroof assemblies with a VSH rating.

While LPDS 1-34 and RoofNav provide guidance and a user-friendly assembly selection tool, roof manufacturers are often a great resource to answer any questions about approved assemblies and the nuances found in the approval listings. Manufacturers are often able to assist designers in converting their specification for a VSH rated roof assembly.

While FM requirements only apply to FM insured buildings, the potential damage to the roof in these locations applies to all buildings. It is not uncommon for owners and designers that do not insure buildings through FM to elect to follow FMs guidelines to protect their investments and mitigate their design risk.

Roofs: The First Line of Defense Against Hail

Roofs are the first line of defense against severe weather events. With severe weather expanding beyond the traditional hail alley states—and larger hailstones developing more frequently—understanding material selection, aged condition, maintenance, and replacement options are key components to managing this risk. Choosing the right roof system could be the difference between weathering the storm or losing revenue from shutting down business.

Hail can severely damage a roof as low-slope roofs accept the full assault of the hail event. It’s important to quickly repair the damage to mitigate any resulting leaks and mitigate the risk of damage from subsequent events. Damage can include dents, cracking, crazing (circular microfractures), mineral loss on asphaltic sheets (primary UV protector), and exposure of roof membrane reinforcement.

Making the case for investing in a VSH roof system for owners can take several paths. For owners who have FM Insured buildings, they have a choice to make between increased premiums for a severe hail rated assembly or reduced premiums for a more costly VSH rated assembly. There should be a savings with installing VSH rated assemblies; the question is how much and is the owner willing and able to finance the upfront investment to upgrade.

There is more to consider than just the upfront costs of upgrading to a VSH assembly. Making the investment in a more robust roof system means less chances of roof damage that can lead to water intrusion that can cause disruption, damaged goods, mold growth, and/or loss of business. It can also lead to premature roof replacement that may not be covered by insurance.

There is more to managing risk than just the upfront investment in the roof assembly. Design that provides a rated assembly and quality materials are essential. Good workmanship during the installation is a critical component to a successful roof installation. Specifying third party inspections by a Registered Roof Observer (RRO) and quality assurance protocol during construction can help identify and resolve any issues before there is a problem. The owner also plays a part in the long term performance of the roof. Maintenance is essential to keep drains cleared and debris off the roof that could puncture or otherwise damage the membrane. Routine inspections should be performed to quickly identify any damage caused from maintenance, service contractors or storm damage, as well as signs of wear and tear and overall aging. A Registered Roof Consultant (RRC) can provide an independent assessment of the roof’s condition and develop repair recommendations as needed.

There are many buildings that are not FM Insured and still the owners have decided upon or agree to follow the requirements for VSH rated assemblies. Taking the precautionary measures available to protect their building can provide owners and designers alike with a sense of confidence in the longevity of their investment by mitigating their risk and protecting the occupants and contents.

Utilizing a fleeceback membrane over a standard smooth-back single ply membrane enhances the performance as it relates to impact. The fleeceback TPO membrane is specifically designed for durability in demanding situations in terms of impact and puncture resistance, even as it ages. Designers and owners alike often rely on the added cushion and related protection to justify the cost premium for many projects in VSH regions.

Fleeceback membranes can also be faster to install, depending on the time of installation. A fleeceback membrane adhered in low rise foam can be faster to install than adhered smooth-back membranes which can require adhesive application to the coverboard as well as the backside of the membrane, plus time for the adhesive to become tacky and flash off. A faster installation often results in a lower installation cost, which can increase the number of design options that meet the project budget.

While a fleeceback membrane is at a premium compared to the standard smooth-back counterpart, the enhanced performance with VSH as well as the extended warranty or guarantee coverage lends itself to be a worthwhile design consideration.

Designers previously relied on adhered coverboards and membranes for the VSH rating. Adhered systems are often more costly from a material and installation standpoint. Some designers prefer mechanically attaching roof systems, especially in higher wind uplift regions. Depending on the time of installation, project size and location, and crew size, there are potential labor savings with a mechanically attached system, which is one of the reasons the simultaneously fastened VSH assemblies have come to market in the past year, providing designers and contractors alike with more options which could result in cost savings to the owner. Utilizing a cushion of the fleeceback TPO membrane over a simultaneously fastened glass mat roof board (coverboard) provides the longest available warranty or guarantee coverage for VSH assemblies. The extended warranty along with the reduced labor cost can support the cost premium of the fleeceback material.

A roof recover is typically a more cost effective solution than a reroof, as there is a significant labor savings without removing the existing system and there is a savings on insulation materials. It is important to select an assembly that is approved for recover scenarios. The majority of the approved VSH assemblies are for new roofs. There is a much smaller percentage available for roof recovers. However, the approved assemblies for roof recovers is expected to grow over the next year as manufacturers continue to test assemblies with the new coverboards.

Pricing of the VSH coverboards can vary greatly depending on market conditions. Carefully consider any substitution requests for coverboard materials. While the intent may be to save money, FM still requires an approved VSH assembly and coverboards by different manufacturers are not interchangeable.

Hail to VSH Designations—What You Need to Know

In a nutshell, here’s what you need to know about VSH.

Roofs are the first line of defense against severe weather events, including hail. Insured losses from hail events have exceeded $10 billion in the United States annually since 2007. The geographic footprint of hailstorms has expanded and about two-thirds of the United States experiences 1-inch or larger hailstones. Nearly 10 percent more U.S. properties, more than 6.8 million, were affected by hail in 2021 than in 2020. Coinciding with the increase in properties affected by a damaging hail event in 2021, there was also an increase in claims, which rose to $16.5 billion from $14.2 billion in 2020.

With severe weather expanding beyond the traditional hail alley states—and larger hailstones developing more frequently—understanding material selection, aged condition, maintenance, and replacement options are key components to managing this risk. Choosing the right roof system could be the difference between weathering the storm or losing revenue from shutting down business.

FM Global requires VSH rated assemblies to be designed and installed for buildings they insure that are located within hail alley, which includes Oklahoma, Kansas, Nebraska, South Dakota, most of Texas, and parts of Montana, North Dakota, Minnesota, Iowa, Missouri, Arkansas, Wyoming, Colorado, and New Mexico. While FM requirements only apply to FM insured buildings, the potential damage to the roof in these locations applies to all buildings. It is not uncommon for owners and designers that do not insure buildings through FM to elect to follow FMs guidelines to protect their investments and mitigate their design risk.

Very Severe Hail rated assemblies have been required since 2018 for FM Insured buildings, there are exponentially more rated assemblies available. As of June 2022, there are over 16,500 VSH rated assemblies! How will you know which one to select? Membrane selection is critical for VSH prone regions. Thicker roof membranes, as well as higher performance grades that will remain pliable under heat and UV exposure over years in service will outperform standard grade materials. Fleeceback membranes also provide an added cushion layer that buffers the negative effects of hail events. High compressive strength coverboards were shown to be an effective means to enhance the performance of the roof system when exposed to hail events, and allow for simultaneous fastening of the insulation and coverboards in some rated assemblies.

While FM LPSD 1-34 walks users through the process of designing a roof for hail, and RoofNav is a web-based tool to assist with rated assembly selection, there is a lot more to know about roof system design and performance. Remember that hail is just one of the criteria in the design process. Building use, wind uplift, fire ratings, climate zone, and maintenance all need to be considered and incorporated into the design. Leveraging the expertise of a building enclosure consultant or an RRC will provide a roof design with longevity in mind and help mitigate risk.

End Notes

¹Georgia-Pacific Building Products. “Severe Weather Preparedness: A Q&A Conversation with FM Approvals.” Web. 15 June. 2022.
²National Oceanic and Atmospheric Administration. "NOAA Report The Frequency of Large Hail Over The Contiguous United States, 2003.” Web. 15 June 2022.
³Insurance Information Institute. "Facts + Statistics: Hail." Web 18 June. 2022
⁴Insurance Journal. "Nobody Seems to Know Why Hail Storm Losses Suddenly Began Rising in 2008”. Web. 22 June. 2022
⁵Bhawalkar, Sarang (PHD); Taylor, Thomas J. (PHD). "Puncture Resistance of Thermoplastic Single-Ply Roofing Membranes" Web. 23 June 2022.
⁶Bhawalkar, Sarang (PHD); Yang, Tammy (PHD); Taylor, Thomas J. (PHD). "Ice Ball Impact Resistance of Heat-Aged TPO Roofing Membranes.” Web. 23 June 2022.
⁷RICOWI, Inc. "Hailstorm Investigation Report North Texas - April 11, 2016." Web. 27 June 2022.
⁸Georgia-Pacific Building Products. "Severe Weather Preparedness: A Q&A Conversation with FM Approvals." Web. 29 June. 2022.

Jennifer Keegan, AAIA, has over 20 years of experience as a building enclosure consultant. She provides technical leadership within the industry and is an advocate for women within the industry.