A recent laboratory study conducted by Jim D. Koontz and Associates Inc. (JDK), Hobbs, NM, shows that both new and aged non-reinforced EPDM roof assemblies offer a high degree of hail resistance over a variety of roofing substrates.

The impact of a 3" hailstone is clearly shown in this photo of an EPDM membrane tested over OSB. (Photo courtesy of Jim D. Koontz and Associates Inc.)

A recent laboratory study conducted by Jim D. Koontz and Associates Inc. (JDK), Hobbs, NM, shows that both new and aged non-reinforced EPDM roof assemblies offer a high degree of hail resistance over a variety of roofing substrates.

The EPDM roof systems tested in the study “provide excellent resistance to large hail,” says Koontz, whose company is actively involved in roofing materials research, test standard development, and roof auditing and failure investigation.

Of the 81 60 mil, non-reinforced EPDM “targets” installed over polyiso, wood fiber, plywood and OSB board, 76 EPDM roof assemblies retained their waterproofing integrity when impacted by hail up to 3" in diameter. The field aged samples were collected from six states across the country and ranged in age from five to 20 years.

“Owners of critical facilities, such as hospitals, schools, computer centers, airports and sensitive government buildings have come to realize the importance of installing a hail resistant roof assembly over critical facilities,” according to Koontz’s report. “The use of non-reinforced EPDM can provide an additional level of long-term protection.”

Field experience from the examination of thousands of roofs has shown that hail damage to a roofing system can be the result of several factors. JDK included the following criteria in its study:
  • Diameter of the hail
  • Type of roofing system
  • Age of the roof
  • Substrate beneath the primary roof system
  • Surface temperature at the point of impact
Historically, the hail resistance of roofing products have been tested by dropping steel balls or darts on to the roofing product. However, within the last few years, greater consideration has been given to impacting targets with ice spheres.

“The use of ice spheres, obviously, comes closer to replicating what occurs during a real hailstorm event,” says Koontz, who has spent the past 30 years studying ice sphere impact testing of roofing products with a variety of hail guns.

Koontz used 4' x 4' EPDM targets, which were all fully adhered to their substrates. Each target with substrate was mounted vertically. Hailstones measuring 1.5", 2.0", 2.5" and 3.0" impacted the targets at a 90-degree angle.

According to Koontz, prior research and experience has shown that roof assemblies exhibit different levels of impact resistance depending upon surface temperature.  In order to replicate the cold rain that accompanies a  hailstorm, the test targets were sprayed with water at 40 degrees F during testing.

“Some geographical areas of the United States are clearly more prone to severe hail events,” says Koontz. “Roof assemblies should be capable of resisting impact from reasonably expected hail storms for a given geographical area.

“Just as roofs are required to perform in various meteorological events,” continues Koontz, “such as wind, snow, and rain, a roof should be able to withstand some degree of hail impact over its expected service life.”

For mechanically attached systems, the researchers ensured that the EPDM assemblies were impacted both in the field of the roof and directly over the mechanical fasteners and plates. Koontz found that damage did occur over the fasteners and plates with a combination of either 1.5" or 2" ice spheres for both new, heat aged and field aged EPDM.

Koontz defined failure in this test as a “visible split or cut in the surface of the EPDM” membrane. Even though the mechanically attached membranes did not “fail” the test due to impacts on the fasteners and plates, damage did occur to key elements of the roofing system. For this reason, contractors and specifiers looking to maximize hail resistance may want to consider a fully adhered EPDM roof system that eliminates fasteners and plates entirely. Attaching the insulation or cover board with a urethane adhesive instead of mechanical fasteners would also take fastener damage out of the equation.

To obtain a copy of the full report on Koontz's research, please contact ERA at www.epdmroofs.org.

Hailstorms likely to increase

According to a recent Dun & Bradstreet study, building owners paid more for property insurance last year in the wake of a series of catastrophic hailstorms that produced a nation-leading $1.5 billion in claims from one state alone.

In 2006, Indiana-not Texas or Oklahoma-led the nation in hail-related claims.

“It was a bad year,” says Gregg Huey CEO of Indiana Farmers Insurance. “And our state isn’t even in the ‘hail belt’.”

In fact, a Dec. 2007 Purdue University study found that the number of days favorable  for severe hailstorms could more than double in some parts of the country by the end of the century.

“This study makes a strong statement that these severe weather events will become much more common than they are today,” said Noah Diffenbaugh of Purdue’s Climate Change Research Center.

All the more reason why architects and contractors should install the most hail resistant roof systems available. In addition, reducing or preventing roof damage from hail impact can lower insurance costs and save property owners hundreds of thousand of dollars in a single storm.

Strengthening EPDM specs

One of the strategies that contractors and architects can pursue to enhance impact resistance is to add “durability” to the roofing assembly. Durability can be defined as a combination of roof system “strength” and “flexibility,” as well as exemplary hail and roof traffic resistance. By specifying a cover board of any type-be it wood fiber, fiberglass or gypsum-the designer adds durability to the roof system.

A cover board is defined as a relatively thin (5/8") semi-rigid board installed between the insulation and the roofing membrane.

There are several commonly used cover boards for contractors and specifiers to choose from:
  • Asphaltic board
  • Plywood (OSB)
  • Mineral fiber board
  • Wood fiber board
  • Perlite
  • Paper-faced gypsum
  • Glass-mat gypsum
For years, the National Roofing Contractors Association (NRCA) has recommended the use of cover boards with polyiso insulation to minimize problems with facer-sheet delamination; cavitation at the edge of the board; cupping or bowing of the board; shrinkage; and crushing or powdering.

In areas where hailstorms are common, one major retail chain now specifies gypsum cover board for all of its facilities to keep them in operation and maintain profit flow after a severe hailstorm.

“At one of our Nebraska facilities, golf ball sized hail destroyed the single-ply roof,” says the manager of roofing programs for the retailer. “When we rebuilt that roof, we put 1/4" of glass-mat faced gypsum board between the foam insulation and the membrane. In a subsequent storm, 4" diameter hailstones destroyed trees, automobiles and roofs of all types. But our store came out of it with no fractures in the membrane.”

Considering all the benefits that a cover board provides, the price of entry is relatively low. According to one Registered Roof Consultant who specifies gypsum on many of his projects, the cover board adds 5% to the cost of the roof, but “gives the roof 25% more life over the long run. It’s a return on investment that we’re more than willing to make upfront.”