As rooftop photovoltaic (PV) systems are increasingly deployed on taller buildings and across a wider range of building applications, localized overheating or initial fires caused by electrical faults, combustible roof-covering materials, or maintenance-related ignition sources may affect PV modules and contribute to subsequent fire spread over the rooftop system. In this study, a full-scale fire experiment was conducted on a flat-laid rooftop PV system using a nominal 100 kW n-heptane pan fire as a controlled localized external fire source to investigate the fire development and escalation mechanism of the system. The results show that the fire hazard was first and primarily concentrated in the confined under-panel space: the average cavity peak temperature of the ignited array reached 684.0 °C, with a local maximum of 853.4 °C, both significantly higher than the maximum upper-surface center temperature of 370.7 °C. The involvement of the waterproofing membrane in combustion was the key amplifying mechanism driving the transition from localized heating to a sustained high-temperature event; the average cavity temperature exceeded 500 °C after 234 s and remained above this threshold for approximately 201 s, with an average cavity heat accumulation index of 178.1 × 10³ °C·s. Compared with the lower upper-surface center measuring points, hazardous temperatures beneath the modules were reached earlier by 149, 193 and 247 s at the thresholds of 50, 100 and 200 °C, respectively. Under the tested configuration, these findings provide engineering insights for fire-risk identification, early monitoring, and fire-safe design of flat-laid rooftop PV systems in high-rise building applications.