Fire testing and failure mechanism of roof decking

Abstract

This study investigated the fire performance of roofing materials under controlled laboratory conditions, with a focus on the ignition mechanisms of roof decking and their implications for fire testing and material flammability. Mockup roof assemblies, comprising asphalt shingles, underlayment, and wooden decking, were tested at both bench and large scales. Large-scale testing employed a modified version of the ASTM E108 Burning Brand test, the standard used to classify the fire performance of roof coverings. Bench-scale evaluation was conducted using a cone calorimeter equipped with a specialized sample holder adapted from ASTM E3367 and designed to accommodate multilayer assemblies. This configuration (commonly referred to as the Cube test) was intended to replicate fire exposure conditions similar to those in ASTM E108. In both test configurations, flaming beneath the roof decking occurred without any apparent perforation, that is, without the formation of breaks or fissures in the roof assembly. Video analysis of the tests revealed that ignition on the underside of the specimens was initiated by intense smoldering, which eventually transitioned into flaming. This ignition mechanism, known as the smoldering-to-flaming transition, represents a fundamentally different failure mode from burn-through, in which flames penetrate through openings in the structure. This study experimentally demonstrated and mechanistically explained this transition as a possible cause of the long-standing reproducibility issues in ASTM E108. Underside flaming occurred only when airflow increased oxygen supply, with air velocity controlling the transition through competing effects of enhanced char oxidation/pyrolysis and convective cooling. These findings emphasize the need for detailed airflow characterization beneath roofing test assemblies for a reproducible and reliable assessment of roof assemblies.