What should firefighters do after a proper size-up in backdraft conditions?

In backdraft conditions, which are characterized by a rapid buildup of flammable gases in a confined space, conducting a proper size-up is crucial. After assessing the situation, the next appropriate action is to consider vertical ventilation of the roof. This approach helps to release the accumulated heat and gases, which can reduce the likelihood of a backdraft occurring when fresh air is introduced.

Vertical ventilation involves making openings in the roof to allow smoke and hot gases to escape from the structure. By doing this, firefighters create a safer environment to conduct interior operations. This tactic helps to lower the temperature within the structure and ultimately prevents an explosive backdraft from happening when the oxygen is reintroduced to the fire.

The other options, while relevant in different contexts, do not address the immediate danger posed by backdraft conditions as effectively. Evacuating the structure is vital in life-threatening situations but does not mitigate the risk of a backdraft if firefighting efforts are to continue. Waiting for more firefighters could lead to a delay that may increase risks, and spraying water on the fire without properly ventilating first can actually trigger a backdraft by introducing oxygen abruptly into a hot, smoke-filled environment. Thus, considering vertical ventilation is the most strategic and effective action following a size-up in