Investigating the thermal degradation of multi-board decking materials ignited by firebrand piles

Background Firebrand piles are a major contributor to structural losses during wildland–urban interface fires. Among potential recipient fuels, decking materials exhibit ignition susceptibility that is strongly dependent on material type. However, experiments on representative multi-board decking specimens exposed to deposited firebrand piles remain limited. Aims This study quantifies the thermal degradation of multi-board decking specimens ignited by firebrand piles under various conditions. Methods Experiments were conducted on 60 × 60 cm2 multi-board decking, testing two materials: pressure-treated wood (PTW) and wood–polymer composite (WPC). The applied conditions included different wind speeds, material moisture contents (MC) and firebrand pile orientations. Experiments extended beyond firebrand pile consumption. Key results WPC showed low sensitivity to firebrand pile exposure across all conditions. Conversely, PTW exhibited high susceptibility to firebrand pile ignition, with increasing wind speed significantly elevating thermal degradation and burn-through, while being necessary for back-surface flaming. Reducing MC played a secondary role, whereas pile orientation showed the least influence. Conclusions PTW was substantially more susceptible to firebrand pile ignition than WPC, with thermal degradation strongly governed by the applied conditions. Implications Experiments on representative multi-board decking specimens are essential to evaluate scale effects and assess the applicability of small-scale observations to larger decking assemblies.