Almost nowhere on the fireground is the understanding of paths of least resistance more critical than during ventilation operations. Just as proper and timely ventilation can effectively channel fire away from both victims and exposures, so too can misplaced, uncoordinated ventilation create havoc on a building and those still inside, including firefighters.
Contiguous structures with common cocklofts are often destroyed by inadequate ventilation, caused by a failure to take advantage of the most effective path of least resistance. This failure can be often identified by the one completely destroyed structure in the center of the row and the destroyed roof and top floors of all the adjacent buildings to the leeward (and if you really screw up, to the windward) side. The key for success is to vent early over the natural vertical arteries such as scuttles, bulkheads, and skylights. On a lower floor fire, this should be sufficient in regard to vertical ventilation. Opening and examining these natural arteries will take advantage of the most effective path of least resistance and channel the products of combustion upward. If the fire is on the top floor or in the cockloft, in addition to the aforementioned natural openings, cutting the roof as close as directly over the seat of the fire as is safe will also pull the fire up and out of the building. This will slow the horizontal fire spread under the roof. If the fire is not given an opportunity to travel upward (where it wants to go), it will follow the other path of least resistance, horizontally throughout the cockloft.
Lack of ventilation can also cause fire forces to be chased out the building at best and burned at worst. Suppose an attack team is positioned at the door to a burning apartment. To properly coordinate the attack, a vent team should be ready on the building’s exterior, somewhere opposite the nozzle, to remove windows to allow the smoke and heat to exhaust to the exterior, away from the attack team. This is how it is supposed to happen. This coordination of attack is critical. If this attack support is not in place, leaving the products of combustion (and steam) no way out of the structure, the products of combustion will be forced to take the path of least resistance out of the structure, which will now be via the entry point of the attack team. Many firefighters have suffered burn injuries and many buildings have been needlessly destroyed due to this lack of coordination and support.
The wind can also play a major part in influencing the path of least resistance. Wind can make the products of combustion take unusual paths of least resistance in a building that the attack team may not be ready for. Take for example, a fire in a building where the wind is gusting right into the fire apartment. Suppose also that the apartment door has been left open. Instead of allowing the fire to vent out the window, the wind has now changed the path of least resistance. When the attack team gets onto the fire floor and tries to advance down the hall or into the apartment, it may be overwhelmed by fire and heat being blown at them. In this case, the team may have to retreat to the safety of a stairwell while a master stream is used from the outside to knock down the fire, taking advantage of the wind-created path of least resistance. Uncoordinated exterior lines can have the same effect. This is why opposing streams and interior/exterior attacks are so dangerous. Exterior lines, which are usually larger, will always overpower smaller interior lines.