What is the primary cause of rollover in a fire situation?

Rollover is a phenomenon that occurs in fire situations when unburned combustible gases accumulate in the upper layer of a room or area and then ignite, often resulting in a rapid flame spread across the ceiling. The primary cause of rollover is indeed the ignition of these combustible gases that have spread throughout the fire area, as indicated in the correct answer.

This occurs typically in the growth stage of a fire when temperatures rise, leading to the production of gases from burning materials. If the conditions are right, including sufficient heat and oxygen, these gases can ignite, causing flames to roll over the surface. Understanding this dynamic is crucial for firefighters, as it can pose a significant risk if not recognized and managed properly. Recognizing the signs of rollover can help in fire suppression efforts and enhance the safety of firefighting operations.

In contrast, the other options reference scenarios that contribute to fire behavior but do not specifically trigger rollover. For example, materials igniting all at once or an explosion would cause fire development but would not define the rollover event as it pertains to gas ignition. Rapid burning due to increased oxygen influences fire dynamics, but the specific phenomenon of rollover is directly tied to the ignition of the accumulated gases.