By Christopher M. Scallion, P.E.
Forensic Engineer, DONAN
Commonly called “flat roofs,” most commercial buildings are actually clad with roof systems that provide about 1/8 to 1/4 inch per foot of positive drainage. These roof systems are constructed differently than most steep-slope roof coverings and are typically watertight rather than water-shedding assemblies.
The most common commercial roof coverings are single-ply membranes, multi-ply asphalt systems, and metal panels. Common single-ply membranes include TPO (thermoplastic polyolefin), PVC (polyvinyl chloride), and EPDM (ethylene propylene diene monomer). TPO and PVC are both thermoplastic membranes, typically white in color, while EPDM is a more traditional black rubber material. The most common asphalt-based roof systems include modified bitumen and built-up asphalt roofing.
Severe winds affect all of the above low-slope roof systems in a similar manner. Wind uplift pressures are a function of wind speed and the roof height above ground. The resultant uplift pressures are highest at the corners and along the perimeter of a building. As a result, wind damage is typically seen first at the edges of the roof. Once the roof system is compromised, a cascading failure can occur, and wind damage is often characterized by roof coverings that have been lifted and peeled back from the edges.
Single-ply roof systems damaged by wind may appear loose and wrinkled while metal panels are commonly kinked and folded sharply. When severe winds damage a roof system, collateral indicators of high winds are commonly observed. Items such as satellite dishes, antennas, and other roof-top equipment can be blown from the roof or displaced before wind speeds reach a velocity that will damage a sound roof covering.
While wind damage usually appears similar regardless of the roof covering, hail damage to commercial roof coverings is more variable depending on the membrane in place. In fact, the size of hailstones necessary to cause damage to various roof coverings can vary from as little as ½ inch in diameter to over 2 ½ inches in diameter.
Metal roof panels subjected to hail impact commonly sustain dents. While unsightly, indentations from hail impact do not affect the watertightness of the roof panel. If the metal panel finish is cracked or chipped, the panel can rust over time, shortening its lifespan. Unless the panel becomes split or fractured, the indentations are considered cosmetic.
Thermoplastic membranes like PVC and TPO can withstand impact from hailstones up to 1 ¾ inches in diameter depending on the condition of the roof system. As these membranes age, plasticizers in the material can leach out of the membrane, leaving it more susceptible to damage. In severe cases, hail as small as ½ inch in diameter can cause fractures in a deteriorated thermoplastic roof membrane. When hail damages a thermoplastic membrane, cracks in crow’s feet or concentric semicircle patterns form. Rigid cover boards can be installed below the membrane to improve resistance to hail impact.
EPDM membranes are inherently more flexible than most thermoplastic materials. A sound EPDM roof covering can withstand impact by hailstones up to 2 ½ inches in diameter.  When EPDM roof coverings do sustain damage, surface cracks can occur similar to thermoplastic membranes. In extreme instances, hail may puncture the EPDM if the underlying substrate is soft or malleable. Most single-ply membranes are firmly supported in the field of the roof, but may be stretched above the surface along the perimeter or at rooftop equipment. These areas are easily fractured by incoming hailstones and typically sustain damage before the central roof areas.
Modified bitumen and built-up asphalt roof (BUR) systems are resilient to hail impact based on their installed configurations. Built-up asphalt roof systems are commonly gravel surfaced. The gravel provides protection to the underlying roof membrane from both foot traffic, ultraviolet (UV) exposure, as well as hailstones. BURs can also be installed in a smooth-surfaced configuration, but these coverings are more vulnerable and can sustain damage from 1 ½-inch-diameter hailstones.
Modified bitumen roof assemblies are typically constructed in two plies, with a base sheet and cap sheet. The material itself is manufactured from asphalt blended with rubber or plastics to provide a more flexible and long-lasting roof covering. The most durable modified bitumen membranes are granule surfaced, similar to a common asphalt shingle. Modified bitumen roof coverings in good condition can easily withstand impact from hailstones 1 ½ inches in diameter or larger. Hail can displace the granules or fracture the underlying reinforcement of the modified bitumen. When blisters or ridges occur in BUR or modified bitumen membranes, the unsupported area will be more susceptible to damage from incoming hailstones.
As with most building materials, the age and condition of commercial roof coverings has a great impact on their resiliency to wind and hail. Properly secured roof membranes can withstand significant wind forces, but an improperly attached membrane can be blown from the roof during moderate gusts. Similarly, most commercial roof coverings are resilient to hail impact. As these materials age and deteriorate they quickly become more susceptible to damage.
Minimum Design Loads for Buildings and Other Structures (Reston, VA: American Society of Civil Engineers).
 Simulated Hail Damage and Impact Resistance Test Procedures for Roof Coverings and Membranes,” Crenshaw and Koontz, RICOWI Meeting, Dallas, Texas, October 27, 2000.
 Sidney H. Greenfeld, Hail Resistance of Roofing Products, U.S. Department of Commerce, National Bureau of Standards, August 1969, page 6.
 “Hail Damage to Built-Up Roofing,” Timothy Marshall and Scott Morrison.
 Roofing Industry Committee on Weather Issues, Hailstorm Investigation, Oklahoma City, Oklahoma, April 2004.
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