Benefits of Evaluating Interply Bitumen in Bituminous Roofing Membrane Samples in Assessments of Hail Impact Distress

Hail impact evaluations of multi-ply bituminous roofing membranes such as built-up roofing (BUR) membranes, often include laboratory testing of field-procured membrane samples. Such testing is performed to further evaluate the samples for distress present within the membrane cross section. Popular tests performed by forensic laboratories in the United States include delamination, wherein a sample’s individual plies are separated for observation with interply bitumen still present, and desaturation, wherein all bitumen is dissolved from a sample so the bare felt reinforcement mat can be directly evaluated. This paper addresses the potential shortcomings of using desaturation testing alone when evaluating roof membrane samples for hail impact distress, as distress or other conditions can be found in the bitumen that is not identifiable after the bitumen has dissolved. As the bitumen is the waterproofing element of the membrane, identification of bitumen damages and conditions is an important part of any evaluation that is missed from desaturation alone. This paper presents the findings from multiple roof membrane samples that were subjected to discrete impact forces, similar to hailstone impacts, and then examined by both delamination and desaturation. It is shown that distress to the bitumen may exist at impacted locations, without distress being identifiable on the reinforcing mats after desaturation. This testing is also supplemented with discussion of actual conditions found in delaminated membrane samples from actual hail-damage investigations. The potential for the mistaken identification of “false positives” for impact distress when evaluating desaturated felts is also discussed.