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Very severe hail (VSH) is FM Global’s mapped building-hazard designation for areas where hail larger than 2 inches (51 mm) is expected at a 15-year mean recurrence interval (MRI). It is planning guidance for buildings and roof-mounted equipment—not a live storm warning, a promise that a particular storm will produce 2-inch hail, or proof that a roof or wall product has passed a VSH test.
That distinction matters. The VSH label comes from FM Global Data Sheet 1-34, Hail Damage. The sheet’s map category, a project’s construction requirements, an insurer’s engineering review and a manufacturer’s product rating are related but separate questions.
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What does “very severe hail” mean?
In FM Data Sheet 1-34, a very severe hail hazard area is one in which the hail size exceeds 2 inches (51 mm) for the 15-year MRI. FM uses that threshold to divide geographic areas on a hail-hazard map and to organize loss-prevention recommendations for buildings, roofs and roof-mounted equipment.
The phrase therefore describes a location-specific design hazard. It does not describe the weather happening outside at this moment. A site can be inside a VSH map area on a clear day, and a storm that produces hail meeting a weather-service warning threshold can occur outside an FM VSH area.
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FM’s definition is quoted in the sheet’s glossary as: “Very Severe hail hazard area: Areas in which the hail size exceeds 2 in. (51 mm) for the 15-year mean recurrence interval (MRI).” The current sheet, including its map figures and recommendations, is the controlling reference for applying the designation to a project.
How are moderate, severe and very severe hail areas separated?
FM keeps the recurrence basis constant at a 15-year MRI and changes the hail-size band. The three categories are:
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| FM category | Hail-size threshold | Common interpretation |
|---|---|---|
| Moderate hail (MH) | At or below 1.75 inches (44 mm) | Mapped moderate hazard for the 15-year MRI |
| Severe hail (SH) | More than 1.75 inches (44 mm) through 2 inches (51 mm) | Mapped severe hazard for the 15-year MRI |
| Very severe hail (VSH) | More than 2 inches (51 mm) | Mapped very severe hazard for the 15-year MRI |
These are map bands, not a ranking of an individual storm’s intensity and not product-performance grades. The boundary between severe and very severe is the 2-inch value; the boundary between moderate and severe is 1.75 inches. All three use the same 15-year MRI concept.
What does the 15-year mean recurrence interval actually tell you?
MRI is a planning statistic attached to the mapped hazard. It is not a timetable saying that one damaging hail event will occur every 15 years, nor does it identify the year or month of the next storm.
For design work, the MRI lets FM compare locations using a consistent probability basis. “More than 2 inches for the 15-year MRI” means the mapped hail-size estimate exceeds 2 inches when evaluated using that recurrence framework. It does not mean every address will experience a qualifying event on a predictable 15-year cycle, and it does not supply a forecast for an individual thunderstorm.
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When communicating the designation to owners or occupants, describe it as a long-term hazard classification. Avoid translating it into statements such as “2-inch hail will hit here every 15 years” or “the next event is due.” Those statements go beyond what the map definition supports.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchIs very severe hail the same as a severe-thunderstorm warning?
No. FM’s VSH category is a mapped building-design designation; a National Weather Service (NWS) severe-thunderstorm warning is an operational alert about a current or imminent storm.
The NWS glossary defines a severe thunderstorm as one that produces a tornado, winds of at least 58 mph (50 knots), and/or hail at least 1 inch in diameter. A warning is issued when radar indicates those conditions or a reliable report supplies qualifying wind or hail information, and the public is advised to seek safe shelter.
| Question | FM VSH designation | NWS severe-thunderstorm process |
|---|---|---|
| Primary purpose | Long-term property-loss prevention and design guidance | Immediate public safety alert |
| Hail criterion | More than 2 inches (51 mm) for a 15-year MRI | At least 1 inch in the severe-thunderstorm definition |
| Geographic basis | Mapped hazard area in Data Sheet 1-34 | Storm-specific warning polygon and forecast area |
| What it predicts | Relative planning hazard for a location | A qualifying storm threat now or soon |
A storm can satisfy the NWS severe criterion with 1-inch hail and still be outside an FM VSH area. Conversely, a site can be mapped VSH even when no warning is active. Do not use an NWS warning as a substitute for checking the FM map, and do not treat the FM map as a public alert.
How does a VSH map affect building design?
Data Sheet 1-34 connects the mapped hazard to loss-prevention decisions. On a project, the map can trigger a closer review of the roof assembly, rooftop equipment, skylights, heat and smoke vents and other components exposed to impact.
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The practical sequence is:
- Identify the exact site. Use the current Data Sheet 1-34 map for the project’s country and region. Do not rely on an old screenshot or a map copied into a prior specification.
- Confirm the applicable edition and scope. FM describes its data sheets as property-loss-prevention guidance. A 2021 technical bulletin from GAF says FM intends the sheets for FM-insured buildings, while some designers use them for other buildings; it advises verifying whether the sheet applies to the project.
- Read the recommendations for the mapped category. Determine which roof-system, attachment, edge, penetration, skylight and vent provisions the current sheet calls for in a VSH location.
- Coordinate with the design team. The architect, structural engineer, roofing designer, manufacturer and insurer may each have requirements. The final assembly must satisfy the project’s governing documents rather than a generic interpretation of the map color.
- Document the decision. Keep the map reference, sheet revision, selected assemblies, attachment details and any insurer comments in the project record.
The designation is therefore a trigger for disciplined specification, not a standalone construction detail. Data Sheet 1-34 may recommend FM Approved roof systems and special provisions for skylights or heat and smoke vents in VSH locations, but the applicable design depends on the current sheet, the building and the project requirements. A recommendation in the sheet should not be presented as a universal building-code mandate.
Does a VSH map area mean a roof or wall product is VSH-rated?
No. Geographic hazard classification and product testing are different evidence layers. A map tells you which hazard category applies to a location; a rating tells you what a particular assembly or product demonstrated under a named test procedure.
A technical advisory bulletin explaining Data Sheet 1-34 states that the hail sizes used to define moderate, severe and VSH map areas “do not correlate to the testing procedures to determine hail ratings.” In other words, seeing “more than 2 inches” on a map does not prove that a roof membrane, insulation board, wall panel or skylight was tested with a 2-inch hailstone, and it does not establish a pass/fail result.
One example of a separate test is FM Approvals Standard 4881 for Class 1 exterior wall systems. Its VSH rating example uses projected ice balls with a nominal 2-inch (50.8 mm) diameter and a minimum kinetic energy of 53 ft-lb (71.8 J). Those conditions belong to that standard and the exterior wall-panel systems it covers. They should not be transferred to a roofing assembly or treated as the definition of the FM VSH map zone.
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What changed in the current FM hail maps?
Currency matters because the mapped hazard can be revised as the underlying hail data and regional analysis change. FM’s revision history records an April 2025 interim revision updating the United States and Australia maps with the latest hail data. An April 2026 interim revision added worldwide hail maps.
The appendix identifies separate figures for the contiguous United States, Australia and other world regions. A project outside the United States should not be assigned a U.S. map category by analogy. A project in the United States or Australia should likewise use the current regional figure rather than an older map embedded in a consultant’s template.
For work being designed, renovated or reinsured in November 2026, retrieve the current Data Sheet 1-34 document, check its revision history and retain the version used for the design decision.
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What are the most common mistakes when using the VSH designation?
- Treating VSH as a live alert. Use local weather authorities for immediate warnings; use Data Sheet 1-34 for long-term property planning.
- Reading MRI as a schedule. A 15-year MRI is not a promise of one event every 15 years and does not forecast the next storm date.
- Equating map size with test size. The 2-inch map threshold does not automatically describe a product test. Check the exact approval standard and assembly listing.
- Assuming a product label covers the whole assembly. Ratings can depend on membranes, cover boards, insulation, adhesives, deck type, fasteners and installation details. A component’s marketing description is not a substitute for the complete approval or listing.
- Using an outdated or wrong-region map. FM’s 2025 and 2026 revisions changed map coverage. Confirm the region and revision date.
- Turning FM guidance into a code claim. Determine which requirements come from the adopted building code, the owner’s criteria, the insurer, the contract documents or FM guidance.
- Skipping rooftop equipment and openings. Hail losses can involve equipment, skylights and vents as well as the field of the roof. Review every exposed element identified by the current sheet.
What should a project team record before approving a VSH design?
A concise record makes the designation auditable and prevents later confusion between a map category and a product rating. Include:
- Project address, country and regional map figure.
- Data Sheet 1-34 revision date, including confirmation that the April 2026 worldwide-map update was considered where relevant.
- Mapped category and the threshold wording: VSH means more than 2 inches (51 mm) for the 15-year MRI.
- Applicable owner, code, insurer and contract requirements.
- Roof-system and attachment specifications, including deck, insulation, cover board, membrane, fasteners and edge conditions.
- Details for skylights, heat and smoke vents, rooftop equipment and other penetrations.
- Exact product approvals or test standards cited by the manufacturer, without substituting the map definition for the test evidence.
- Open questions sent to FM, the insurer, the manufacturer or the design professional, plus the resulting decisions.
This documentation also helps during replacement work. If a roof is repaired after hail, the team can distinguish a weather event, the site’s mapped hazard and the performance evidence for the replacement assembly rather than assuming that one label answers all three questions.
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Why the distinction matters to owners and occupants
Owners often encounter the term in insurance engineering reports, roof specifications or manufacturer literature. The safest plain-language explanation is: “This location is mapped for a higher long-term hail hazard, so the project should follow the current FM loss-prevention recommendations and use assemblies with documented performance where required.”
That wording communicates the consequence without overpromising. It does not claim that every storm will produce 2-inch hail, that a building is guaranteed to avoid damage, or that one manufacturer’s system is automatically acceptable. Those outcomes depend on the complete design, installation quality, maintenance, exposure and the requirements governing the project.
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How should you explain VSH in one sentence?
Very severe hail is FM Global’s map category for locations where hail larger than 2 inches (51 mm) is associated with a 15-year mean recurrence interval; it is building-design guidance, not a storm warning or a product certification.
Frequently Asked Questions
Does a VSH designation mean my building must be reroofed immediately?
No. The designation identifies a planning hazard; it does not by itself create a universal replacement deadline. Whether work is needed depends on the roof’s condition, the current project or insurer requirements, applicable codes and the assemblies specified for the building.
Can a weather app tell me whether my address is in a VSH area?
Weather apps provide forecasts and alerts, not FM’s property-hazard classification. Use the current regional map in Data Sheet 1-34 and verify the exact site with the project’s design or insurance professionals.
Is a product marked “2-inch hail” automatically FM VSH approved?
No. Ask for the complete approval or test listing, including the standard, assembly build-up, deck, attachments and installation limits. The 2-inch value in the FM map definition does not correlate automatically to product-rating test procedures.
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No. The designation is used for building loss-prevention planning and can affect roof systems, rooftop equipment, skylights, heat and smoke vents and other exposed components. Separate standards may address exterior wall systems or other product types.
Where can I verify whether FM guidance applies to my project?
Start with the current FM Data Sheet 1-34 and then confirm scope with the owner, insurer and design team. FM’s guidance is intended for FM-insured buildings, while other designers may use it voluntarily; applicability should be verified rather than assumed.
