Hazard External firespread
Knowledge and understanding
Understand all associated hazard knowledge
External firespread may develop if the fire breaches the external envelope of the building. This may be a result of external window glazing or wall panels failing, or via open windows. External firespread may compromise compartments above or below the fire floor, irrespective of the materials used in construction. External firespread may also occur between adjacent compartments on the fire floor.
External firespread may also occur if there is a fire in an outside source such as refuse or vehicles or spread from an adjacent building.
External firespread under a building canopy may build up and retain sufficient heat to impact on nearby structures.
External firespread may interact with façade assemblies, including cladding systems. When assessing the extent of external firespread the complete cladding system, including fire barriers, should be considered. Once affected, from either an internal or external source, the cladding system may contribute to external firespread.
Fire may spread along the external surface of cladding; the materials used will affect the rate of firespread.
Cavities may exist as part of a cladding system or facade assembly. Cavities may also be created by damage, delamination or movement. Fire may spread within cavities, with flames becoming elongated due to the restricted space. The flame length may increase regardless of the materials used in construction, resulting in rapid, hidden firespread if appropriate fire barriers are not present.
The type of external cladding system used above 18m on buildings is regulated. This includes a restriction on the surface spread of flame, typically Class 0. There are also regulations on the materials used within the façade assembly, including insulation and any other filler material.
However, only the cladding used above 18m in height must comply with the regulations for buildings of this height, which may lead to the use of different cladding materials within the same building.
Dependant on the materials used, there is the potential for undetected firespread within the cladding system; this can lead to a second seat of fire if the fire spreads up a cavity and breaks out higher up the building. Although there are regulations in place relating to the materials used on buildings above 18m, fire spread within the cladding system may still occur. If the cladding system is contributing to fire propagation, then the fire may affect multiple storeys simultaneously making firefighting more difficult. Typically, buildings are not designed to support severe fires on multiple floors. Fire spread of this nature may have detrimental impact on the existing evacuation strategy.
Thin, flat or curved, building components, for example, glass that is used for glazing or structural purposes, may travel or float (also known as planing) a significant distance from the building. Poorly fitted window frames may lose integrity due to their exposure to fire or heat.
Green roofs and walls can be a source of fuel for a fire, allowing external firespread. Green wall frames or modules may be hazardous if they detach and fall.Weather conditions, such as wind strength and direction, may impact on external firespread. For further information about fire phenomena, including the Coandă effect, and its impact on external firespread, see hazard Uncontrolled ventilation