Why are windows broken or holes cut in the roof during a fire?

As a fire burns, it expands upward and outward. The fire creates superheated gasses and by products of combustion, which need to be removed from the building to make the building more tenable for the occupants and firefighters. Breaking the windows and /or cutting holes in the roof (called ventilation) stops the damaging outward movement and enables firefighters to fight the fire more efficiently, resulting in less damage to the structure and its contents. When at all possible, firefighters are instructed to open windows instead of breaking them, if they can. In situations where it is unknown if the occupants of the building are still inside or dangerous combustion levels have accumulated in the building, then the firefighters will break windows. At times a ventilation hole cut in the roof of a building fire will also draw the fire away from unburned sections of the building, giving firefighters extra time to attack the fire and reduce the overall damage of the fire.