Got Hail? Proving the performance of EPDM roof assemblies

EPDM targets and impact procedures
Manufacturers provided a total of 81 test targets constructed with 1.52-mm (60-mil) non-reinforced EPDM for impact testing. The new, heat-aged, and field-aged targets are listed in Figure 5. The field-aged and exposed EPDM samples were collected from six states and ranged in age from five to 20 years. It is important to note some of these field-aged samples—listed in Figure 6—represent more heat exposure than would typically be found in the Canadian climates, translating to an added safety factor for buildings in this country. The U.S. samples taken from the Great Plains—a region experiencing climatic weather conditions similar to those of the plains of Alberta, Saskatchewan, and Manitoba—provide a good barometer for the performance of roofs in these provinces.

The artificially heat aged samples were prepared at Cascade Technical Services of Hillsboro, Oregon. The samples were heat-aged for 1440 hours at a temperature of 116 C (240 F). The 1.2 x 1.2-m (4 x 4-ft) EPDM ‘targets’ were installed over a variety of substrates that included polyiso and wood fibre insulation, plywood, and oriented strandboard (OSB). Fully adhered EPDM was utilized in the target construction. Figure 7 indicates the material age, substrate, and number of samples of each prepared.

Each target with substrate was vertically mounted. Hailstones measuring 38, 51, 63, and 76 mm (1 ½, 2, 2 ½, and 3 in.) impacted the targets at a 90-degree angle at velocities listed by NBS. To replicate severe weather conditions, such as cold rain during a hailstorm, the test targets were sprayed with water at 4 C (39 F). Prior research and experience has shown roof assemblies exhibit different levels of impact resistance depending upon surface temperature.

The various targets were impacted both in the ‘field area’ and also directly over fasteners and plates utilized to secure the substrate below the EPDM. Failure was defined as a visible split or cut in the surface of
the EPDM.

Impact results
Of the 25 ‘new’ EPDM targets tested, 24 were not damaged by the 76-mm (3-in.) hail balls. None of the 20 ‘heat-aged’ targets failed when impacted with 76-mm hail balls.

The ‘field-aged’ EPDM target samples included 18 over a 51-mm (2-in.) thick polyiso insulation substrate, and 18 over a 13-mm (½-in.) thick OSB substrate, supported by 38-mm (1 ½-in.) thick polyiso roof insulation. Fourteen of the EPDM targets adhered directly over the polyiso did not fail when impacted with 76-mm hail balls. One sample failed with a 76-mm hail ball, a second sample failed with a 63-mm (2 ½-in.) hail ball, and the two other samples failed with a 51-mm diameter hail ball. None of the 18 EPDM ‘field-aged’ targets over OSB were damaged by 76-mm diameter hail balls (Figure 8).

Hail and roofs
Hail forms in the core of a thunderstorm. Water vapour in warm, rapidly-rising air masses (convection currents) condenses into water at higher, cooler altitudes to produce heavy rain showers. If it is cold enough, ice crystals can form around minute particles, such as dust whipped up from the ground, and increase in size as more water freezes onto their surfaces. When the ice pellets are too heavy for the ascending air currents to lift, they fall as hail. They may become larger, heavier and more damaging if they collect more water on the way down.

Hailstones have a minimum diameter of 5 mm (0.2 in.)—smaller and they are defined as snow or ice pellets. Hail can grow larger than 100 mm (4 in.), reaching the size of grapefruit. It can hit the ground at 130 km/h (80 mph), causing severe damage to crops, houses, and vehicles, along with injuring people and animals.

As shown in Figure 9, hail occurs right across Canada, but more frequently in Alberta, the southern Prairies and in southern Ontario. Roof assemblies should be capable of resisting impact from reasonably expected hail storms for a given area. Just as roofs are required to perform in various meteorological events (e.g. wind, snow, and rain), a roof should be able to withstand some degree of hail impact over its expected service life.

This article’s co-author has examined hundreds of EPDM roofs impacted by hail. Two noteworthy investigations include a telephone building in Fort Worth, Texas, that was impacted by softball-sized hail in 1995. The non-reinforced EPDM over polyiso did not fail. A second investigation was at the University of Nebraska in Kearney. All campus buildings covered with non-reinforced EPDM survived softball-sized (i.e. 96-mm [3.8-in.]) hail. The manufacturer of the roof was notified of the performance of the aged EPDM assembly. The gravel built-up roofing (BUR), metal, slate, clay tile, and modified bitumen (mod-bit) roof covers on the 65 other university buildings all failed.

During the examination of hundreds of roofs, direct impacts over fasteners and plates used to secure underlayment have been extremely rare. Damage observed of that kind has not constituted a failure of the entire roof, and has been repairable. The increasing use of adhesives to fasten insulation and coverboards is eliminating the already unlikely chance of damage caused by hail-ball impact with mechanical fastener plates.

The new, heat-aged, and aged non-reinforced EPDM tested within this study provided excellent resistance to large hail. Of the 81 targets installed over polyiso, wood fibre, plywood, and OSB, 76 did not fail when impacted with hail ice balls up to 76 mm (3 in.) in diameter.

The overall results of this research clearly indicate non-reinforced EPDM roof assemblies offer a high degree of hail resistance over a variety of substrates, validating empirical experiences. The impact resistance of both the field-aged and heat-aged membrane also clearly demonstrates EPDM retains the bulk of its impact resistance as it ages.

Owners of critical facilities (e.g. hospitals, schools, data centres, airports, and sensitive government buildings), who demand durability and long-term service lives of their roofs, have come to realize the importance of installing a hail-resistant assembly. The use of non-reinforced EPDM can provide an additional level of long-term protection.(Koontz’s “A Comparative Study of Dynamic Impact and Static Loading of One-Ply Roofing Assemblies” is a reprint from ASTM Special Technical Publication (STP) 959 1988 (1988): 24).

In heating-dominated northern climates like Canada, roof system designers should consider using black EPDM in fully adhered assemblies where the top layer of cover board is set in urethane adhesive—beads or full-coverage sprayed polyurethane foam (SPF). From a design standpoint, this assembly has many built-in safeguards, such as the durability of non-reinforced EPDM without any hard insulation fastening plates or fastener heads directly under the membrane. A ballasted EPDM roofing assembly utilizing black EPDM can be a prudent choice for hail resistance.

Further, roof cover durability in hail events is linked to membrane thickness. Designers should consider specifying greater roof membrane thickness.

From empirical observation by the authors and by testing, several key characteristics of roof systems designed for hail resistance can be summarized:

  1. Utilize non-reinforced roof covers with greater thickness—2.28 mm (90 mil) over 1.52 mm (60 mil).
  2. Provide high-density support for the roof cover (i.e. do not allow the membrane to be depressed under hail ball impact).
  3. When used, place screw fasteners and stress plates below the cover board, and do not place below roof covers.
  4. Ballasted roof systems provide the greatest protection.
  5. Specify roof covers with a history (i.e. at least 20 years) of general in-situ performance to ensure performance as the membrane ages and is exposed to ultraviolet (UV) radiation and climatic conditions.

It is important to remember this hail testing was completed in the United States, but the data can be useful to design/construction professionals north of the border as hail is not aware of territorial lines. As damage from these storms can be quite dramatic, it will be important for Canadian testing agencies to consider this in the future.

Jim D. Koontz, PE, RRC, has been involved in the roofing industry since 1960 and began testing roofing materials in 1976. President of Jim D. Koontz & Associates, he has experience as a roofer, estimator, consultant, lecturer, researcher, and expert witness. Koontz has authored numerous articles relating to roofing material/product research, including research on single-ply products and hail/wind impacts. He can be reached at

Tom Hutchinson, AIA, FRCI, RRC, is a principal in Hutchinson Design Group. A licensed architect and registered roof consultant, he specializes in roof design, contract document preparation, specifications, inspections, and the determination of moisture penetration and failure of existing roof system. Hutchinson is a former president of RCI Inc. He is also a Certified Energy Professional in Chicago and secretary of the Conseil International du Bâtiment/Réunion Internationale des Laboratoires d’Essais et de recherches sur les Matériaux et les constructions (CIB/RILEM) International Joint Committee on Roof Materials and Systems. Hutchinson can be contacted via e-mail at

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