Fire breaks out when heat, fuel, and oxygen meet. Without intervention, a fire will only come to an end under two conditions: when the fuel has run out, or when the oxygen supply is exhausted. In an enclosed space, when a fire can deplete most oxygen, the flames will die down while the fuel continues to burn in a smouldering state under pyrolysis. Pyrolysis does not require oxygen, so it can take place without fire at or above 500C and turn organic compounds into charcoal, tar, or non-condensable gases. These gases have extremely high thermal values ready to burst into fireballs as soon as fresh oxygen re-emerges from any new opening. This is called "backdraft", and is what happened in the Mountain B Pub fire.
Mountain B was an enclosed building with a lot of flammable material, including the ceiling and wall insulation. The building was officially permitted for use as a restaurant. However, it was operated as a pub with live music. Post-fire investigation by the Building Inspectors Association found that aside from the flammable ceiling and sound-absorbing ovary sponge walls, flammable materials were added to fill the gaps in the walls to prevent the sound escaping.
