Many a seasoned incident commander will be sure to tell you that, out of the myriad of tasks that must be performed at a structure fire, there are at least a half dozen or so that must be accomplished within the first five minutes of the fire. One of these tasks is ventilation of some sort, depending on the situation found upon arrival. The benefits of proper, coordinated ventilation are plentiful, including removing heat and smoke, making the engine company's push into the building a bit easier, and even creating an avenue for the rescue of trapped occupants. While most of these ventilation tasks are in support of an offensive attack, the trench cut is certainly not one of them.
The trench cut (otherwise known as "strip ventilation") is a long narrow ventilation hole that acts as a fire break on the roof of a structure that is being attacked by fire. This tactic is used when confronted with a stubborn concealed fire that is difficult to reach, or a fire that has a stronghold on the attic or cockloft space of a long, narrow building. It can also be used for a top floor fire within a building with a choke point, or "throat" in the roofline where a trench can be cut (see Photo 1). The main focus of this defensive tactic is to cut off fire extension to an area of the building that has not been involved in fire; this means that a portion of the building will be "written off," so to speak.
Keep in mind that many buildings will not possess the necessary attributes to employ a trench cut. For example, H-shaped buildings would benefit from the use of the trench cut during operations, as the building will have a choke-off point in the center of the structure, which can act as a pinch point for smoke and heat (see Photo 2).
Older, narrow Type III (ordinary) constructed taxpayers would benefit from this tactic, as the construction method used in the roof area may provide crews with an increased operational time frame to complete the trench. Adversely, newer lightweight strip malls would not be a positive candidate for this tactic. The main reason is the safety concerns when dealing with lightweight construction. Many of these newer strip malls are Type II construction, containing steel bar joists with light decking for roof sheathing. On top of the sheathing would be membrane-style roofing materials, hot tar layers and insulation materials as well. Cutting through this material would tend to decrease the efficiency of the saws, requiring a considerable amount of time to complete roof operations. As a rule, this type of construction does not lend itself to extended roof operations (see Photo 3).
Furthermore, buildings with a considerable amount of square footage on the roof would not be candidates for this technique. Consider a strip mall that measured 50 feet deep and 200 feet long, with four stores within the building. Cutting a trench in this roof would require two long cuts of 50 feet each to make the trench. Relief cuts in the trench itself would have to be cut at a maximum of every 5 feet, to keep the roof trench sections manageable for crews. This would add over 30 more feet of cuts to be made, which puts the total area to be cut over 130 feet! That is a significant amount of roof material that would need to be cut, requiring additional companies that may only be available through additional alarms to perform the cutting, and be available to open the trench when coordinated with fire attack. Perhaps these resources can be utilized while employing a different technique to halt the fire spread, such as pulling large areas of ceilings from within the fire building after the roof has been vented, and placing larger handlines into operation to stop the spreading flames.