Flashover

A flashover is the simultaneous ignition of all flammable material in an enclosed area.

Flashover occurs when the majority of surfaces in a space are heated to the point at which they give off flammable gases that are hot enough to ignite themselves (i.e. they do not need to be touched by flame to start burning). Prior to flashover, flammable gases may be given off but are not hot enough to ignite without a spark.

The classic example of flashover is where a piece of furniture is set alight in a domestic room. The fire on the furniture produces a layer of hot smoke in the room. The radiated heat from this layer warms up the other surfaces in the room, causing them to exude combustible gases. When the surface temperatures are high enough, these gases auto-ignite and (in the space of a few seconds) practically every surface in the room is on fire.