Evaluating the thermal response of large-scale decking assemblies exposed to firebrand piles

Firebrand accumulation in the form of piles significantly contribute to structural losses in the Wildland-Urban Interface. Decking materials may be particularly vulnerable to ignition from firebrand piles. However, large-scale studies replicating real-world conditions are still lacking. This study investigates the observed thermal response of Pressure Treated Wood (PTW) and Trex Enhance (Trex) composite decking under exposure to 8 grams firebrand piles (10 cm × 5 cm x 2 cm) at large-scale decking assembly (60 cm × 60 cm). Experimental conditions involved different constant wind speed (1.4 m s-1 and 2.7 m s-1); moisture content (MC) of tested materials (PTW: 26% and 7%; Trex: <1%); and firebrand pile orientation (0°: 10 cm side parallel to airflow; and 90°: 10 cm side perpendicular to airflow). For PTW experiments, three key events were observed: 1) Burn-through: when the samples experienced vertical propagation until a glowing-edged opening >5 mm appeared at the back surface; 2) Back surface flaming ignition: when transition from smouldering to intense flaming ignition took place at the back surface; and 3) Back surface re-ignition: when flaming ignition appeared second time at the back surface but at lower intensity. For PTW, the higher wind speed (2.7 m s-1) was the dominant factor associated with significantly increased likelihood of burn-through and was a necessary factor in inducing back surface flaming. The low MC (7%) also increased the propensity for burn-through and back surface flaming, though to a lesser extent. Although firebrand pile orientation showed insignificant impact on the overall thermal response, it still played a critical role. The 90° orientation increased the burn-through propensity, while 0° orientation displayed a greater tendency for back surface flaming and its associated characteristics. Compared to PTW, Trex exhibited considerably lower combustion intensity and duration, with no observations of burn-through or back surface flaming in any experiment.