The fire broke out as students were preparing for their Degree Show. Eyewitnesses said that the fire appeared to have started when a projector exploded in the basement of the Charles Rennie Mackintosh building just before 12:40 pm. Investigators later determined that the cause was not a faulty projector, but "a canister of expanding foam" used in close proximity to a hot projector, causing flammable gases to ignite. According to "The Scotsman" newspaper, the use of aerosol cans is against school policy. The report from the Scottish Fire and Rescue Service found that the design of the building contributed greatly to the spread of the fire: "the number of timber lined walls and voids, and original ventilation ducts running both vertically and horizontally throughout the building" as well as "a vertical service void", which "ran the entire height of the building … allowed flames, hot gases, and smoke to travel". Fire and smoke dampers, which are intended to prevent the spread of fire and smoke through ducts, had not been retrofitted. In addition, an intended fire suppression system for the building had not been completed. A school staff member was on hand when the blaze first ignited, but was unable to contain the fast-spreading flames.