Ask An Expert is a question-and-answer column designed to address common questions related to forensic investigation and property damage. Each month we’ll feature one or two questions submitted by you – our readers and customers – and provide detailed, easy-to-understand answers. Email your questions to [email protected] or submit your questions here.
Does hail typically damage EPDM roofing? I have an older EPDM roof with tears along the north edge. The contractor claims it is from hail, but I believe it is from shrinking and subsequent tearing or splitting of the membrane at the edge of the roof. How do you differentiate hail damage and shrinkage?
Chris Scallion, P.E.: Ethylene propylene diene monomer (EPDM) roof membranes are typically installed in two basic configurations, both of which are resilient to hail impact. Ballasted membranes are loose-laid in place and covered with rounded river rock to prevent wind uplift. The rock is commonly about 1 ½ inches in diameter and provides a means of protecting the underlying membrane from debris impact, inadvertent man-made damage, and falling hailstones. Controlled laboratory testing, and field experience, has found that hail smaller than 2 ½ inches in diameter lacks sufficient mass to damage a sound, ballasted roof covering.
EPDM membranes are also installed as mechanically attached or fully adhered systems, in which the membrane forms the top surface of the roof system. EPDM is a resilient material with good flexibility properties. Adhered and mechanically attached EPDM membranes can sustain impact from hailstones up to 2 inches in diameter.
As EPDM ages, oils within the membrane degrade and volatize due to heat and ultraviolet (UV) light exposure. This is part of the normal weathering process. A loss of mass and volume occurs as these oils leave the membrane, resulting in shrinkage. Shrinkage is typically evident along the edges of the membrane, at the perimeter or along the backside of parapet walls. If the membrane shrinks substantially, it will pull away and can tear or “tent” along parapet walls.
When hail damages an EPDM membrane, it results in localized, often semi-circular tears or circular marks of “spider web” cracking. Excessive shrinkage results in extended linear cracking, and is easily differentiated from hail damage. The service life of an EPDM membrane can vary depending on the membrane thickness and manufacturer. One of the leading manufacturers of EPDM, Firestone, sells EPDM membranes with 15-, 20-, 25-, and 30-year warranties. Older membranes are more susceptible to shrinkage failure. Excessive membrane shrinkage can also leave EPDM more susceptible to hail impact. Tenting membranes that are unsupported and no longer bear on a firm, underlying surface can be susceptible to damage from hailstones smaller than a sound membrane in contact with a firm substrate.
 Sidney H. Greenfeld, Hail Resistance of Roofing Products, U.S. Department of Commerce, National Bureau of Standards, August 1969, page 6.
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About the Expert
Chris Scallion joined Donan in 2012 as a forensic engineer based out of the firm’s Raleigh, North Carolina office. Mr. Scallion has worked in structural design, building envelope science and roofing. His areas of expertise include but are not limited to flood damage, construction defects and steep and low slope roof systems. Mr. Scallion’s other project capabilities include commercial and residential roofing, flood and hurricane damage, wind and tornado damage, water intrusion, crawlspace moisture, construction defects, structural and foundation failures. View Chris’s full professional profile here.