Final answer:
An incident commander would order a trench cut to create a firebreak during a structure fire, preventing the spread of flames across the roof. It is a last-resort, defensive strategy that requires careful planning due to its risks.
Step-by-step explanation:
An incident commander would order a trench cut in the situation of a structure fire, particularly when the fire is spreading quickly through a roof. The purpose of a trench cut is to create a firebreak or stop line, which aims to prevent the flames from spreading across the roof to other parts of the building. Trench cuts are a defensive firefighting strategy that can be very effective but also requires careful planning and execution.
Executing a trench cut involves cutting a section out of the roof, usually several feet wide, down to the roof deck, which is intended to eliminate fuel and create a gap that fire cannot easily jump across. This is often done as a last resort when other methods to control the fire's spread have been ineffective. However, it is a risky maneuver, not only because of the inherent dangers of working on a compromised roof but also due to the potential impact on the structural stability of the building. It is crucial that an incident commander assesses all risks involved before ordering a trench cut.