Flashover occurs when the majority of the exposed surfaces in a space are heated to their autoignition temperature and emit flammable gases (see also flash point).
Flashover normally occurs at 500 °C (932 °F) or 590 °C (1,100 °F) for ordinary combustibles and an incident heat flux at floor level of 20 kilowatts per square metre (2.5 hp/sq ft).
The fire involving the initial piece of furniture can produce a layer of hot smoke, which spreads across the ceiling in the room.
Firefighters memorize a chant to help remember these during training: "Thick dark smoke, high heat, rollover, free burning.
Traditionally, black, dense smoke was considered particularly dangerous, but history shows this to be an unreliable indicator.