Low-Slope Roofs Under Hail and Wind: Standards, Field Evidence and Next Steps

Where the hail falls

NOAA's Storm Events Database records 11,063 hail events of 1 inch or larger in Texas from 2016 through 2025, more than any other state (NCEI). Kansas, Nebraska, Oklahoma and Colorado follow:

A state event count is a record of reports, not of damage, but it shows where large stones are reported most often.

The three standards behind storm ratings

FM 4470: the whole roof assembly

FM Approvals' Class 4470 examination standard (April 2022 edition) evaluates single-ply, modified bitumen, built-up and liquid-applied roof assemblies as complete systems, from the deck up, for fire, simulated wind uplift, hail, water leakage, foot traffic and more. Hail ratings run Moderate, Severe and Very Severe (MH, SH and VSH), and Class 1-MH is the minimum for certification. Wind ratings start at Class 1-60 and go as high as Class 1-990 in steps of 15. The standard also prohibits substituting components in a certified assembly without the certification agency's authorization.

ANSI/SPRI/FM 4435/ES-1: the roof edge

The edge standard covers fascia, coping and gutter systems. According to NRCA, Section 1504.6 of the 2024 International Building Code requires metal edge systems on low-slope built-up, modified bitumen and single-ply roofs to be designed for wind loads under Chapter 16 and tested to ES-1's RE-1, RE-2 and RE-3 methods. SPRI's position is that the testing must be done for each fabricator that produces the product.

ASCE 7: the design wind loads

ASCE 7 is the load standard that the International Building Code adopts by reference, per ASCE's introduction to the 2022 edition. NRCA's free Roof Wind Designer calculates roof system design wind loads under the 2005, 2010, 2016 and 2022 editions.

The detail on each is in our guides to hail on single-ply membranes and wind uplift at corners and edges.

What the field investigations show

The Roofing Industry Committee on Weather Issues, founded in 1990, sends volunteer teams to inspect roofs after major hailstorms and hurricanes and publishes the results as free reports. Its hail work began with roughly 100 roofs after the April 21, 2004 Oklahoma City storm and has since covered Dallas-Fort Worth in 2011, North Texas in 2016 and the Texas A&M campus in 2025.

Two findings stand out for low-slope owners. After the April 11, 2016 North Texas storm, RICOWI concluded that hail size was the primary factor in roof performance and that membranes over high compressive strength substrates held up better. After Hurricane Ian in 2022, its teams found that all but one of the seven multi-ply bituminous roofs they inspected had peeled back at corners and perimeter edges, the areas of highest uplift. RICOWI also has a directory page on the National Roofing Directory.

After the storm

SPRI's maintenance guidance is short: inspect low-slope roofs at least twice a year and after any major storm, check the perimeter for loose metal or fasteners, look closely at flashings around penetrations, and clear the drains. Our after-the-storm guide covers hidden damage, documentation and the repair traps that can void a rating.

For commercial roofing contractors and manufacturers

If you install, inspect or supply low-slope systems in storm country, make it easy for building owners to find you. Claim your listing on the National Roofing Directory, and compare its membership levels: Claimed, Verified, Gold and Platinum, where Verified means a person checks your phone, email, website, logo, services, rating, insurance and license.

The Roofing Wire, NRD's free weekly newsletter, adds a Storm Watch email of National Weather Service alerts for the states you choose and a monthly tool giveaway. Industry bodies with NRD directory pages include SPRI, NRCA and IIBEC.

Frequently asked questions

What does an FM hail rating mean for a TPO roof?

It rates the tested assembly, not the membrane alone. FM 4470 evaluates the full roof assembly from the deck up, and substituting components without the certification agency's authorization is not allowed, so the rating applies to the listed combination of membrane, insulation, cover board and attachment method.

Is ES-1 testing required by the building code?

Yes, where the International Building Code is adopted. Section 1504.6 of the 2024 edition requires metal edge systems on low-slope membrane roofs to be tested to ES-1's RE-1, RE-2 and RE-3 methods, and gutters that secure the membrane edge to be tested under SPRI GT-1; SPRI's 2018 statement quotes the same requirement as Section 1504.5 of the edition then current.

Where can I read RICOWI's storm investigation reports?

RICOWI posts its hail and hurricane reports on its own website at no charge. The list runs from the 2004 Oklahoma City hailstorm to Hurricane Ian and the 2025 Texas A&M hailstorm.