Understanding the ignition of solids is a critical parameter for assessing the fire performance of materials. Mass timber, favoured for its sustainability benefits, is being used in large open-plan buildings with exposed ceilings. Ignition of an exposed timber ceiling can lead to a rapid transition to flashover, yet research on ignition in the ceiling orientation is limited. This study compares the ignition behaviour for timber samples in both the floor and ceiling orientation when exposed to external radiative heat flux. A novel setup using the Fire Propagation Apparatus (FPA) was developed to test both orientations under exposure to external radiative heat fluxes of 20–60 kW/m 2 . Compared with the normal orientation, the inverted orientation exhibited higher effective ignition temperatures and thermal inertia, and gas analysis revealed elevated CO and CO 2 concentrations prior to ignition, indicating reduced mixing efficiency. The findings demonstrate that changing the orientation alters fluid dynamics and boundary-layer behaviour, requiring greater energy absorption and higher pyrolysate concentrations to achieve flaming ignition. The results provide critical insights for performance-based design for exposed timber ceilings. • Inverted orientation increased ignition delay by up to 74 %. • Higher ignition temperature for inverted orientation (331 °C vs 302 °C). • Effective thermal inertia increased from 0.47 to 0.54 kW 2 s/m 4 K 2 when inverted. • CO data reflects a change in mixing time for the inverted orientation.
Madden et al. (2026) studied this question.