Hail and Single-Ply Membranes: What FM 4470 Tests and What Real Storms Show

What counts as single-ply

FM Approvals' Class 4470 standard defines single-ply roof covers as factory-produced sheets, typically EPDM, PVC, TPO, CSPE or variations of them. The same standard rates hail resistance for the whole assembly, so a membrane's rating depends on the substrate and attachment it was tested with.

How FM 4470 tests for hail

The standard has three hail classes, and Class 1-MH (Moderate Hail) is the minimum for certification.

• Moderate and Severe Hail. Two identical samples are built on the chosen substrate: one as manufactured and one weathered for 1,000 hours in an ultraviolet cabinet. Each takes at least ten impacts from a 1.19-pound (1 kg) steel ball. For MH the ball drops 81 inches, about 8 foot-pounds of energy; for SH it drops 141.5 inches, about 14 foot-pounds.
• Very Severe Hail. Three samples, including one UV-conditioned or heat-aged, are tested at a roof surface temperature of 40 to 45°F and struck at least three times with 2-inch ice balls made to ANSI FM 4473. Each impact targets 53 to 58 foot-pounds of energy, and the target points include a field seam, the open field and, on mechanically fastened systems, a fastener and stress plate.
• Passing. Neither the membrane nor a field seam may crack or split when examined under 10x magnification. For a VSH rating, the substrate beneath the membrane must not crack either.

That last condition matters in the field: RICOWI's inspections, below, tie membrane damage to what lies beneath the sheet.

What the storms show

The Roofing Industry Committee on Weather Issues publishes its hail investigations free on its reports page. Two recent Texas storms are the most useful for single-ply owners.

North Texas, April 11, 2016

For its North Texas report, RICOWI's teams inspected 27 single-ply roofs and rated seven severely damaged, four of them after hail of 2.5 inches or more. Two unballasted thermoplastic membranes were badly damaged by hail of only 1.25 to 1.5 inches, with fractures at the edges of fastener plates; both were older, one 12 to 15 years and the other more than 15. Three EPDM roofs hit by 1- to 3-inch hail showed no documented punctures or fractures.

Across all roof types, the investigators found the greatest change in damage level between 1.25 and 2 inches of hail, and more than 43% of membranes over low compressive strength substrates had significant damage, against 25% over high compressive strength substrates. Their summary for single-ply was favorable: only a small number of roofs were damaged by hail under 1.5 inches, and several performed adequately with 2-inch or larger hail. For low-slope roofs generally, age stopped mattering once hail exceeded 3 inches.

Texas A&M, March 15, 2025

RICOWI's campus report, released January 7, 2026, documented stones of about 1 to 2.25 inches. It describes TPO as fairly resilient because of its flexibility and elongation, but liable to puncture or tear depending on age, condition and impact energy. Even a TPO sheet showing only divots can sit on partially crushed insulation, leaving voids that weaken support for the membrane and cut its thermal value. One roughly 14-year-old TPO roof showed widespread hail spatter with no observable surface damage. The team did not do destructive testing or look at membrane undersides, so hidden fractures were outside its scope.

Specifying for hail country

NOAA's Storm Events Database records 1,986 hail events of 2 inches or larger in Texas from 2016 through 2025, followed by 838 in Kansas and 679 in Nebraska (NCEI). Roofs in those markets see ice at and above the VSH test size. Four points follow from the evidence above:

1. Match the FM hail class to the local record, and remember that the SH steel ball carries far less energy than the VSH ice ball.
2. Treat the substrate as part of the hail design; RICOWI's damage rates were much lower over high compressive strength boards.
3. Watch fastener plates on mechanically attached systems, both a VSH test location and the failure point in North Texas.
4. Keep the assembly as tested. FM 4470 prohibits substituting components in a certified assembly without the certification agency's authorization.

For contractors and manufacturers

Membrane makers with pages in the National Roofing Directory's company directory include Carlisle SynTec Systems, Johns Manville and Elevate. Contractors can claim a listing, compare membership levels and sign up for The Roofing Wire, whose Storm Watch email sends National Weather Service alerts for the states you pick.

Frequently asked questions

Can 1-inch hail damage a TPO roof?

It can, but RICOWI's North Texas data suggests it is uncommon. Only a small number of single-ply roofs there were damaged by hail under 1.5 inches, and the two thermoplastic failures from smaller hail were older roofs that fractured at fastener plates.

What separates FM Severe Hail from Very Severe Hail?

The projectile and the energy. SH uses a steel ball dropped to deliver about 14 foot-pounds, while VSH uses 2-inch ice balls at 53 to 58 foot-pounds on cold samples, and a VSH assembly's substrate must not crack.

Why does the insulation under the membrane matter for hail?

Because hail can crush what it lands on. RICOWI found far less damage on membranes over high compressive strength substrates, and its 2025 report notes that crushed insulation can leave voids under a TPO sheet that looks intact.