What is it called when only the gases are burning at the top of a structure?

The scenario described, where only the gases are burning at the top of a structure, is known as rollover. This phenomenon occurs when the combustible gases that have accumulated in the upper areas of a room ignite and burn without any flames present at the lower levels. Rollover can be an indicator of an impending flashover, as it signals high heat conditions and the presence of flammable gases that are ready to ignite more violently if provided with an ignition source.

During rollover, fire is typically seen moving across the ceiling, which can manifest as a blue or yellow flame. It's a critical phenomenon for firefighters to recognize, as it indicates the state of the fire and potential risks involved, helping to inform their strategy and tactics for fire suppression and safe entry.