Stage 1 (~2:15pm to 2:35pm). The fire starts at the foot of the northeast stairwell in the basement. It soon ignites the stairs and burns at low intensity for some time, undetected. A small plume of heated air and combustion gases (red-purple curled outline) is carried by convection through a masonry ventilation hole (large purple arrow) into the cockloft, where it heats the underside surface of the roof (red dot).
(Note: In all of the sketches, the roof height has been greatly exaggerated for clarity.)