What Hail Actually Does to a Spray Foam Roof, and Why FM's Very Severe Hail Zone Now Reaches All Five of Our States

What does hail actually do to a spray foam roof?

A coated spray foam roof handles hail better than the single-ply membranes we restore, because the foam is closed-cell and a cracked coating does not leak on the day of the storm. That advantage has a clock on it. Cracks left through a winter let ultraviolet light and water into the foam, and the repair grows from caulk-and-coat to scarify-and-refoam. Inspect after every storm over an inch.

FM Global rewrote its hail map in March 2018, and on the state list Carlisle published from it in August 2025 all five states A-1 works in sit inside the Very Severe Hail zone. The zone runs county by county, not along state lines. A foam roof inside it survives the storm. Whether it survives the winter after depends on who walked it in September.

Why does FM's Very Severe Hail zone now reach all five of our states?

FM Global Property Loss Prevention Data Sheet 1-34, Hail Damage, was rewritten in March 2018 and carries a July 2018 interim revision. It defines three hazard areas by the largest hail expected over a 15-year mean recurrence interval: moderate, where stones do not exceed 1.75 inches; severe, between 1.75 and 2 inches; and very severe, where hail exceeds 2 inches.

The map behind those definitions was built from nearly 350,000 hail reports collected by the U.S. government since 1955. FM Approvals described the result on October 18, 2018 as a greatly expanded Very Severe Hail zone extending from Texas to North Dakota and from Wyoming to Missouri. The earlier map had covered Oklahoma, Kansas and some northern Texas counties, according to Professional Roofing's December 2017 explainer, and was driven by FM's own loss history rather than the physical hazard.

Carlisle's guide to the FM VSH designation, updated August 2025, lists the states now inside the zone: Montana, Wyoming, Colorado, New Mexico, Texas, Oklahoma, Nebraska, Kansas, South Dakota, North Dakota, Minnesota, Iowa, Missouri, Arkansas, Louisiana, Mississippi, Tennessee, Illinois and Wisconsin. Every state A-1 works in is on it. The current map sits inside Data Sheet 1-34, which FM Global publishes behind a registration, and the zone boundaries run county by county rather than along state lines, so a specific building should be checked against the map rather than the list.

The weather record says the map is right. Vivian, South Dakota holds the national record hailstone, 8.00 inches across and 1.938 pounds, from July 23, 2010, per the NOAA state climate summary. On August 11, 2026 the National Weather Service in Rapid City logged 4.0-inch hail northwest of Oglala, 3.5-inch hail at Hermosa and a 111 mph gust northeast of Newell, with roofs partially or completely blown off multiple residences and businesses. On April 17, 2025 the Omaha office recorded 4-inch hail at Arlington, Nebraska. NWS Cheyenne's 2000 to 2023 climatology puts Cheyenne and Sidney above six severe-hail days a year, with 2-inch hail near Scottsbluff and Sidney one to two days a year, peaking in June.

What do the hail ratings on a roof actually measure?

FM Approvals Standard 4470 covers single-ply, modified bitumen, built-up and liquid-applied roof assemblies, which is the category a spray foam and silicone roof falls into. Every FM Approved assembly must reach at least Moderate Hail, Class 1-MH. FM 4470 was revised in late 2016 to add a Very Severe Hail class above the existing Severe Hail class, and the three now sit together as MH, SH and VSH.

The tests are physical. For MH, a 2-inch steel ball weighing 1.19 pounds is dropped from 81 inches onto an unweathered sample and a sample aged 1,000 hours under ultraviolet light, for about 8 foot-pounds of impact energy. SH raises the drop to 141.5 inches and 14 foot-pounds. VSH switches to 2-inch ice balls fired from an air cannon at 152 to 160 feet per second, 53 to 58 foot-pounds, which FM equates to a hailstone up to 2.5 inches, and adds a third sample that is weathered and then heat-aged for 42 days.

For comparison, the July 24, 2025 IIBEC review by Stephen Patterson and Jordan Beckner puts UL 2218 Class 4 at a 2-inch steel ball dropped 20 feet for 23.71 foot-pounds and FM 4473 Class 4 at a 2-inch ice ball at 23.75 foot-pounds. A VSH-rated low-slope assembly is tested at more than twice the energy of a Class 4 shingle.

Data Sheet 1-34 then tells an FM-insured owner what to put on a building inside the VSH zone. Table 2.2.1.1-1 recommends single-ply and modified bitumen assemblies carrying a VSH rating, or a concrete paver ballasted roof, and lists standing seam and lap seam metal at SH, asphalt shingle or tile at FM Class 4, and built-up roofing at SH as acceptable alternatives where VSH-rated roofs are unavailable. Liquid-applied and foam assemblies are not called out by name in that table, so a foam roof on an FM-insured building should be matched to its RoofNav listing before anyone promises a rating.

What happens when a hailstone hits a coated spray foam roof?

A spray foam roof is a layer of closed-cell polyurethane foam sprayed onto the deck or the old roof, under an elastomeric coating. On A-1's 20-year systems that coating is silicone at 38 mils dry film. The foam is the waterproofing. The coating protects the foam from ultraviolet light and foot traffic and adds the reflective surface.

When a stone lands, the foam's cell structure crushes and takes the energy. The result is a dent with a crescent or crow's-foot crack in the coating over it. The bigger the stone, the deeper the dent and the more likely the coating splits.

The Roofing Industry Committee on Weather Issues investigated the Oklahoma City hailstorms of 2004 and reported, as quoted in UPC Foam's hail guide, that "immediate leaks into the buildings would not be expected from the hail-caused fractures in the coating as the foam is closed-cell and would not allow liquid water to pass through." Of five foam roofs inspected, the three hit by hail from over 1 inch to 2 inches had coating fractures; the two hit by 0.5 to 1-inch stones did not.

The IBHS and RICOWI spray foam guide says the same thing in fewer words: hail damage typically does not cause the roof system to leak and can usually be repaired economically. On the day of the storm, the industry line is true.

So is hail damage to a foam roof cosmetic?

No. The Spray Polyurethane Foam Alliance's repair guideline SPFA-139, revised 2015, opens with the research finding that wind-driven missiles typically do not cause an SPF roof to leak and that the damage can usually be repaired without replacing the roof. Later in the same document it adds the sentence contractors quote less often: hail damage may not be discovered for months or years after a hail event, and in those cases the repairs may be more extensive because of ultraviolet degradation or moisture intrusion through breached coatings and damaged foam cell structure.

That is the whole argument in one document. A cracked coating is a repair. A cracked coating that spends a winter open can become a scarify-and-refoam. Ultraviolet light degrades exposed foam. Water gets into the crushed cells through the breach, and every freeze expands it. What starts as a caulk-and-coat item becomes removal of damaged foam, and on a heavily hit roof it becomes grinding off the top half inch or three quarters of an inch of the field and spraying new foam.

FM Global's own researcher, Andreas Muehlbauer, put the general roofing case bluntly in FM Approvals' October 2018 article: "Most roofing systems out there are not VSH rated and start to fail when hail reaches 1.5 inches (38 mm) in diameter." His colleague Bob Zemantic added that some ten-year-old roofs with no visible deficiencies are showing damage from one-inch hail. Age matters along with stone size, and a coating ages too.

What did a golf-ball hailstorm do to the Woonsocket school roof?

Woonsocket School District in Woonsocket, South Dakota is a 23,000 square foot roof A-1 restored in 2022. The original foam, installed by another contractor, had saturated under ponding water and was leaking into the school. A-1 cut out the wet foam, sprayed additional foam to build positive slope, and sealed the field under Gaco S20 silicone at 38 mils under specification SPF-S20-38-20, with a 20-year no-dollar-limit warranty on labor and material.

Then hail of golf-ball size and larger cut the silicone open across the roof. A claim was filed, the insurance adjuster approved it, and A-1 made the repair as soon as it was approved. Each impact cut was repaired individually with reinforced flashing-grade silicone. The roof was washed. A fresh S20 topcoat went over the whole field under Gaco's recoat specification RC-S20-38-20.

The roof did not leak after the storm. The cuts were closed before water could work into the foam, so the system never saturated, which is the whole point of the closed-cell argument above and the reason the repair had to be prompt.

The storm work took six working days: two of prep, two of cleaning, two of recoat. The school was not disrupted, and Gaco reinstated the 20-year warranty in full. The same storm on a conventional single-ply roof can mean wet insulation, a tear-off and a construction schedule wrapped around a school year. On a restorable roof repaired in time it was a recoat, and the warranty clock reset.

In A-1's October 16, 2023 proposal for the Barnett Fieldhouse in Rapid City, company president Karl Sybesma made the same point about restoration generally: because the work is not a tear-off, "the risk of exposure is nearly eliminated, while lowering labor costs and trips to the landfill." Woonsocket is that sentence applied to a hailstorm.

Was Woonsocket a one-off?

No. Storm repair, on A-1's own roofs and on other contractors', is a large part of A-1's work, and its inspection reports on coated foam roofs show the same pattern each time. Large hail splits the coating, which leaves the foam surface exposed to moisture and weather, and a split left unrepaired lets the foam become saturated.

The most recent is Freeman Shopping Center in Freeman, South Dakota, a 21,140 square foot metal roof A-1 covered with spray foam and an acrylic topcoat in 2021. A 2026 storm brought golf-ball-size and larger hail, jagged and plenty of it. The job went through insurance, and A-1 repaired and restored the system that summer.

Kent Sybesma, A-1's director of operations, describes the progression the way SPFA-139 does. A hail-cut roof lets in small amounts of moisture first. Those small amounts become saturated components, and saturated components leak over time. A repair made right away is less work, a better repair, and cheaper for the owner. Some owners are not concerned because the roof is not leaking yet, which is why A-1 writes up the damage in detail, so the owner can decide how to manage the roof with the facts in hand.

How is hail damage on foam graded and repaired?

SPFA-139 grades damage two ways, by severity and by extent, using a grid laid over the roof. Severity runs from light, dents about half an inch across or under an eighth of an inch deep, through moderate, half to three-quarter inch and under a quarter inch deep, to heavy, three-quarter to inch-and-a-half dents a quarter to half an inch deep, to severe, dents over an inch and a half across or half an inch deep. Extent is counted per 100 square feet, with fewer than 10 defects and more than 20 as the two bands and judgment in between.

The repair follows the grid. Light and moderate damage at low extent is caulked and coated. Light and moderate damage at more than 20 hits per 100 square feet is recoated to fill the cracks, with caulk in the deeper ones. Heavy damage means removing the damaged foam and caulking the holes, and at more than 20 hits per 100 square feet it means scarifying half an inch of the surface, refoaming and coating. Severe damage at high extent scarifies three quarters of an inch.

The counting is the part owners skip and adjusters do not. Fifteen 3-inch dents on 1,000 square feet may be easier to repair than hundreds of three-quarter-inch ones, as the guideline itself notes. A written count per grid square, with dated photographs and a moisture reading in the worst square, is the document that decides whether the roof gets a recoat or a refoam, and it is the kind of document an insurer asks for.

How does that compare with what hail did to The Monument's PVC roof?

The Monument in Rapid City is a 193,018 square foot civic complex whose mechanically fastened PVC membrane was more than 20 years old when A-1 restored it in 2023 and 2024. On August 23, 2023, A-1 and a Holcim representative inspected and tested the Barnett Fieldhouse roofs before the second phase was quoted. Hail damage was counted at roughly one hit per 250 square feet across the fields, previously patched with elastomeric products as a temporary measure. A moisture meter and core samples traced about 2,800 square feet of saturated insulation to ponding around unrepaired hail penetrations.

One inspection shows the difference between the two roof types. On the membrane, every unrepaired hail split was an open path to the insulation below, and the insulation held the water. On the foam roof at Woonsocket, the splits sat over closed-cell foam and the fix was a recoat. Both roofs were restorable. One needed 2,800 square feet of wet insulation cut out first.

The IIBEC 2025 review found the same pattern on a larger sample: most roofs damaged by 2-inch or larger hail had been installed over a soft substrate, and none of the roofs installed over dense cover boards were damaged, even by hail in the 3-inch range. A thin membrane over compressible insulation lets the stone punch through. Closed-cell foam is dense enough to take the dent and hold the water line.

Metal is the third case and the simplest. Jim Koontz and Troy White's March 2014 IIBEC study of 22 hail-struck metal roofs in Oklahoma City and Dallas found cosmetic dents, no functional damage to paint or plating, and no leaks. A dented roof that does not leak is often excluded under a cosmetic-damage endorsement; a metal roof whose fastener washers or seams were split by the same storm is a different claim.

What should an owner do in the first 30 days after a hailstorm?

The order matters because the repair class changes with time.

• Within a week: walk the roof or have it walked. Look for crescent cracks in the coating, dents you can feel underfoot, split pipe boots and flashings, and any place the foam shows white or yellow through the coating. Photograph with a tape measure in frame.
• Within two weeks: get the grid count done. A written SPFA-139 style count per 100 square feet, with a moisture reading in the worst square, fixes the repair class before weather changes it. Keep the report with the warranty file.
• Before the first hard freeze: close every open crack. Caulk-and-coat on light damage is a day's work. The same cracks after a winter of freeze-thaw are a refoam.
• If the roof is under a manufacturer's warranty: notify the manufacturer before repairs start. At Woonsocket the recoat ran under Gaco's own recoat specification, and the warranty came back in full.
• If a claim is likely: the count, the photographs and the dated report are the file. Do not let a temporary repair erase the evidence of the damage.

Where does this leave the hail-proof question?

No roof is hail-proof, and no honest contractor calls one that. What a coated foam roof offers is a wider margin between damage and leak, a repair that stays a repair if it is done in season, and, at Woonsocket, a warranty the manufacturer reinstated after the recoat.

Two limits are worth stating. First, Gaco's GacoRoofFoam F2733 with GacoFlex S2100 silicone earned an FM Severe Hail listing on June 14, 2016, at the time the only reflective silicone assembly with that classification by Gaco's account, but a listing belongs to a specific assembly, and we found no published Very Severe Hail listing for a foam-and-silicone system. The rating on your roof is whatever RoofNav says it is. Second, we found no UL 2218 Class 4 listing for any silicone coating, so do not accept that claim from anyone, including us.

What you can accept is the field record. In the Very Severe Hail zone, a foam roof that gets inspected every spring and fall, as the IBHS guide recommends, and repaired the season it is hit, is the roof that comes out of the next storm with a recoat instead of a tear-off.

What does this post not tell you?

This post describes FM Global and FM Approvals classifications, industry repair guidelines and A-1's own project records as of September 2026. It is not insurance or engineering advice. Hail ratings belong to specific FM Approved assemblies listed in RoofNav, and a manufacturer's warranty terms govern what any recoat reinstates. Confirm the listing and the warranty for your own roof before relying on either, and consult your insurance agent or a public adjuster on any claim.