Forensic evaluation of roofing membranes for hail damage is a large and growing industry throughout the United States, especially in the Midwest and across the South where hailstorms are prevalent throughout the year. For these evaluations, accurate assessment of the roof membrane and the causation of damage is an imperative. An accurate assessment requires knowledge of the potential causes of damage for the roof system, including determination of damage related to hail impact. Laboratory testing of membrane samples is often an important component of such an evaluation. Damage resulting from interply mopping voids, which are as-built conditions, is often misidentified as hail damage in multi-ply bituminous roof membranes. This misidentification frequently occurs because the characteristics of distress to the membrane surface caused by interply mopping voids can be similar to the characteristics of membrane surface distress caused by hailstone impacts. The purpose of this paper is to educate readers about interply mopping voids and the type of distress that they can cause. Examples involving roofing membrane samples cut from various structures and analyzed in a laboratory will be used to illustrate interply mopping voids and the damage that results from their presence. Finally, this paper will outline ways to better distinguish hail impact distress from interply mopping void distress when performing a forensic roof evaluation in the field.
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