Roof Impact

Aug. 1, 2011
A roof’s main goal is to keep water out, but some building owners put hail damage at the top of the list.

Readers of this column are aware of the many things we expect a roof system to do. The primary goal is to keep water out of the building, and the obvious secondary things include resistance to fire and wind. Property managers might then list flashings for walls, curbs, and HVAC next, with many experts believing that flashings are the primary source of problems on roofs.

Yet building owners in parts of Colorado and Wyoming may well move hail damage to the top of the list, as illustrated in the National Geographic hail map above.True, other states such as Texas and Oklahoma have more hail days, but not the large, damaging hailstones with diameters of 2 inches or more.

A pair of recent disasters illustrate Colorado’s vulnerability to severe hail damage. On July 13, three-quarter-inch hail pummeled Denver International Airport, the fifth busiest airport in the country, damaging three aircraft – two sustained damage severe enough to warrant pulling them out of the fleet – and stranding about 1,000 people in the airport. Just a few days later, an afternoon of hail in Denver’s Westwood neighborhood destroyed the roof of the Boys & Girls Club, a popular summer gathering place for over 1,700 kids. The storm punched nearly 9,000 holes in the roof and resulted in leaks that severely damaged the gym and game room. The damage was initially estimated at $200,000-250,000.

For other parts of the country, impact damage may be attributed to inadvertent abuse, such as other trades working on HVAC, for example.

Understanding Impact Resistance
Newspaper reports of hail damage may term the sizes of damaging hail as similar to a golf or tennis ball in size.

Below, ASTM D3746 impact energy is compared with that of FMGlobal and Underwriters Laboratories (UL) to determine roughly what impact energy corresponds with what size of hailstone.


Missile diameter (mm)

Mass (kg)

Distance (mm)

Energy (Joules)

ASTM D 3746





FM Class 1 SH





FM Class 1 MH





UL Class 1





UL Class 2





UL Class 3





UL Class 4





Simulated Hail Damage and Impact Resistance Test Procedures for Roof Coverings and Membranes, RICOWI, Crenshaw and Koontz 10/2000

FM also has a Specification Test Standard for Impact Resistance Testing of Rigid Roofing Materials under FMApprovals Class Number 4473, with the same energy requirements as UL. This test uses ice balls rather than the steel missiles of FM 4470.

For those of you who have never seen some of these impact test devices, above is a hail gun used by Jim Koontz to evaluate hail impact on photovoltaic cells.

Another important test for impact resistance is resistance to flying debris, a major concern in hurricane areas. If debris breaks windows or shutters during a severe storm, the interior of the building becomes pressurized, contributing to blow-off. This is tested with a cannon that fires 2-by-4 lumber at selected targets.

What to Look For in Performance and Resistance
When owners attempt to protect their building against an anticipated hurricane, they will very likely visit the nearest Home Depot or Loews to pick up plywood. Thanks to flying debris tests, FM proved that protecting windows with nominal half-inch plywood is inadequate to meet code, but one layer of three-quarter inch or two layers of half-inch plywood would do the job.

Observation of roof membrane systems in service reveals:

  • Use of higher density “cover boards” improves impact resistance.
  • Increased thickness of single-ply membranes increases puncture resistance.
  • Use of ballasted systems provides reduced impact energy on the underlying membrane.
  • Installing walk pads at access points can mitigate mechanical damage.

FM also reminds us that hail can damage roof-mounted equipment as well as the roof system. FM data sheet 1-34 states:

2.2.4 Roof-mounted equipment (sheet metal duct work, housings, fins, etc.) should be designed to resist large hailstones.

2.2.5 When outdoor equipment has not been designed to resist hail impact or does not have sufficient inherent strength to resist severe hailstorms, hail screens or guards may be placed over the equipment. This is particularly applicable to skylights. The protector may be deflected by impact; however, the energy of the stones will be considerably dissipated.

Certain equipment may permit attachment (without damage) of the screen or guard directly to it. This method is preferred, as the impact is transmitted directly into the equipment base, and no legs that bring the force onto the covering are necessary. Care is needed, however, in using this method, because a strong built-in flange or base must be present on the equipment for attachment and impact transfer. Bolting or welding to air conditioning fins, or to any thin metal that contains liquids or gases is not recommended. When it is not practical to attach directly to the equipment, a table-like support for the protecting device probably will be needed.

Two other impact tests have been in service since 1989. They are linked to levels of performance needed for a particular construction.

At the 1991 International Symposium on Roofing Technology, the concept of the FIT Classification for Roofing Systems was introduced. In this system, experts were asked to identify levels of performance needed for specific construction conditions. The letter F refers to fatigue resistance, while the I refers to puncture, and T the risk of slippage with temperature. For this column, we will look at the I designation, which considers the puncture of the membrane in service, both dynamic (e.g., a dropped tool) and static (a concentrated load such as dunnage supporting rooftop equipment). Both of these tests are now ASTM test methods, D 5602 for Static Puncture and D 5635 for Dynamic Puncture Resistance.

For more information on impact resistance, useful sites include:

FM Global Approval Standards 

FM Global Data Sheets

FM Global Downloads

FIT Classification for Roofing Systems (requires Adobe Acrobat Reader to view)

Simulated Hail Damage and Impact Resistance Test Procedures for Roof Coverings and Membranes by Crenshaw and Koontz at the October 2000 RICOWI meeting (requires Adobe Acrobat Reader to view)

D 5602: Static Puncture Resistance of Roofing Membrane Specimens and D 5635: Dynamic Puncture Resistance of Roofing Membrane Specimens

Roof-System Performance II

Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc. - The Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc. 

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