All fires require three elements: a source of ignition, combustible materials, and oxygen.
A fire will continue to burn until actively extinguished or until the combustibles or oxygen is exhausted. If not otherwise extinguished or exhausted, the fire will continue burning and at a certain point will suddenly and very rapidly expand from a small, localised fire to one filling the entire room. This transformation, which occurs in seconds, is called “flashover”.
Whether a small fire spreads and creates a flashover is governed by how materials in its vicinity react to fire. Today, our furniture and electronics contain significant amounts of plastics and other synthetics – and thus considerable amounts of fuel – which means that a fire develops 5-10 times faster in a living room with modern furnishings compared to a living room furnished as it would have been in the 1950s. This video shows how fast a fire develops in a modern living room relative to a legacy room.
After flashover occurs, the fire begins consuming all combustibles in the room and will soon be starved for oxygen. The heat is intense and combustibles even within the building structure can be feeding the fire. Like a monster trying to breathe, the flames will extend from the room where it started through any open doors and/or by breaking through windows to the outside sucking up fresh oxygen and feeding on fresh combustibles on its way through the building and up the façade. If the façade is made of combustible materials, this adds more fuel to the fire. The fire will only stop if actively extinguished or it runs out of combustibles to consume.