autoextension

Autoextension is the vertical spread of fire from one storey of a building to the next, typically via the windows. A fire on a lower floor produces flames and hot gases that project outward through broken windows; if those flames reach the window opening of the floor above, they can ignite material inside, especially curtains, blinds, or interior finish. The phenomenon occurs because flames follow the natural path of least resistance and heat rises, making it a serious hazard in multi-storey buildings where window spacing and separation are inadequate.

The mechanism depends on several factors: window size and spacing between floors, the intensity of the fire below, prevailing wind direction, and what combustible material sits near the upper window opening. In a typical scenario, flames shoot out of a lower-floor window to a height of 1 to 1.5 metres or more, depending on the heat release rate of the fire. If the floor-to-ceiling height is standard (around 3 metres), and the upper window sill is positioned where projecting flames can reach it, autoextension becomes probable.

Prevention and building design

Building codes address autoextension through several methods: increasing the vertical distance between windows on successive floors, using spandrel panels or other non-combustible barriers, or installing external fire escapes that protect window openings. Some jurisdictions mandate minimum separation distances, often 1.2 to 1.5 metres between the sill of one window and the head of the window below. Materials used to fill spandrels must have high fire resistance ratings, typically non-combustible as defined in local standards.

Window design itself matters. Larger windows project flames farther outward; smaller windows may reduce the risk. The material of the window frame (aluminium versus steel, for example) affects how quickly the assembly fails and how flames escape. Broken glass is assumed in autoextension scenarios; intact glazing provides temporary protection but fails under radiant heat and thermal shock.

Autoextension is distinct from other vertical fire spread mechanisms such as external wall flame spread, which occurs across the outer surface of the facade, or internal spread through shafts, cavities, and structural voids. It is also separate from the spread that occurs when flames enter ventilation ducts or when hot smoke moves between floors via normal openings. Understanding autoextension is crucial for fire safety engineers designing tall residential and commercial buildings, particularly in jurisdictions with high population density where storey separation failures can lead to mass casualties.