How a fire spreads

Convection is the transmission of heat, not flames, within a liquid or a gas. As the liquid or gas is heated, it expands and becomes lighter thus rising and being replaced by the heavier liquid or gas. Therefore, in a high-rise building fire, these superheated gases rise up vertical shafts and spread to upper floors. Ignition is caused when the rising gases exceed their ignition temperature as they reach the upper areas.

Conduction is the transmission of heat through a material. For example, heat may be transmitted through steel roller doors and ignite combustible materials on the other side.

Radiation is the transmission of heat travelling through space until the heat waves are absorbed by other surfaces or materials. An example is when radiated heat from a burning building ignites combustible materials some distance away.

Direct burning
Direct burning occurs when flames make contact with a fuel and the fuel ignites.