Trenching Strategy: Drawing a Line in the Sand


Any fire operation presents two challenges. First, there is the extreme physical effort required to extinguish the fire. Second, and no less important, is the subtle balancing act that firefighters must perform in determining realistic goals given the fire conditions and resources available. How long is an aggressive attack viable, and how much of a building can be saved?

A fire on the top floor of a multistory building may be the most physically taxing of all the incidents to which we respond. The challenge of long hose stretches combined with the exhaustion of walking up stairways in bunker gear will rapidly deplete a firefighter’s energy. An effective strategy for a top-floor fire must consider this fatigue element. In buildings of ordinary brick and wood-joist construction, that strategy must also factor in the condition of the cockloft area directly above the fire.

A cockloft is the open area that lies between the roof and the ceiling of the top-floor occupancy. It will vary in size from a few feet to a space that is almost large enough to walk through. The cockloft’s normal, positive function is to provide an insulating air space that helps reduce room temperatures for top-floor residents in hot weather. However, at a fire operation, it provides a direct path for fire to spread rapidly to all sections of a building. An unchecked fire in a cockloft space has the potential for extensive physical damage and will challenge a fire department’s resources.


Fire Department of New York units confronted these physical and strategic challenges at a difficult top-floor fire in a Bronx apartment building. On April 6, 2013, at 0955 hours, an alarm was received for a fire in a building on the corner of Cedar Avenue and Fordham Road. The first units arrived at the scene within four minutes and transmitted a “10-75” radio signal for a fire on the top floor of an apartment house.

The fire building was an occupied, six-story, 125- × 100-foot building of ordinary brick and wood-joist construction divided into two separate (A and B) wings (Figure 1).

Figure 1. The Fire Building


The initial response of four engines, three trucks, a squad, and a rescue faced a heavy volume of fire venting from windows of the top-floor apartment near the front of the A wing. Our dispatchers received numerous phone calls for heavy smoke and fire throughout the area since the building was clearly visible from both the Bronx and Manhattan.

I arrived at the scene and assumed command of the fire five minutes after the first-arriving unit. The initial chief (Battalion 19) met me in the street for a face-to-face appraisal on the status of the operation and the location of our units. A hoseline was already in position on the top floor of the A wing, primary searches were underway, and roof venting was being initiated. The second-arriving battalion chief (Battalion 27) was positioned on the top floor to supervise the interior fire attack.

All of the elements of an exceptionally challenging incident were in place. The fire (which turned out to be intentionally set) was in a location that would require extensive hose stretches and had the potential to endanger a large portion of the structure. The building was fully occupied, and we were receiving several reports of occupants in distress. In addition, the volume of fire was far beyond what you would normally face in this type of occupancy at 10 a.m. on a Saturday morning.

I immediately transmitted a second alarm in anticipation of the additional help we would require and assigned the chief of Battalion 19 to supervise operations on the roof. With a battalion chief already in position on the top floor, the status of the roof and cockloft became the next priority, given the amount of fire and the construction of the building.

The initial report from Battalion 19 was not encouraging. Heavy fire was burning through a vent hole that had been cut over the location of the fire in the A wing. In addition, heavy smoke was showing throughout the roof area of the B wing as well as in the narrow “throat” area between the A and B wings. We now faced the possibility of fire burning throughout the entire cockloft and extending into the apartments in the B wing. I transmitted a third alarm just eight minutes after the second alarm to obtain additional help for what was becoming a rapidly expanding operation.

At this point, a number of tactics had to be quickly accomplished and coordinated to establish a strategy that would hold the fire in check. A truck company was ordered to the top floor of the B wing. By opening small inspection holes in the ceilings and surveying with a thermal imaging camera, truck members determined that fire had not yet extended into those apartments. Engine companies stretched a precautionary hoseline to the truck’s location to extinguish any fire that might appear.

Roof Tactics

A trench was cut and opened up in the roof of the A wing approximately 20 feet from the location of the initial vent hole (Figure 2). This position took advantage of the narrow distance between the stair bulkhead and the exterior parapet to separate the fire area from the rest of the A wing. A trench is essentially a narrow slice cut out of a roof area. It spans the entire width of a roof section and can confine a cockloft fire by venting; removing combustible material; and, if necessary, providing a means of operating a hoseline into the cockloft.

Figure 2. Trench Cuts


The exact situation in the remainder of the cockloft remained uncertain at this time. In anticipation of fire possibly spreading through the entire roof area, I ordered a trench to be cut, but not opened, in the throat area (photo 1). In a worst-case scenario, this offered a chance to protect the B wing apartments if the fire bypassed our efforts to hold it to the A wing.

(1-2) Photos by author.
(1-2) Photos by author.

Small examination holes (photo 2) were made on both sides of this trench to monitor fire spread in the cockloft. At the same time, a hoseline was stretched to the roof to protect operating personnel and to provide a means of extinguishing fire if conditions required that the trench be opened up.

photo 2

Battalion 17 responded to the scene as the safety officer and was ordered to monitor the physical condition of the roof and cockloft areas. Fire had been burning in the cockloft for several minutes at this point, and there was a concern that localized sections of the roof would weaken and collapse. When the third alarm was transmitted, a battalion chief was assigned to respond to the operation in a New York City Police Department helicopter. His aerial view of the building provided an additional perspective of roof conditions.

Even with two hoselines extinguishing fire in the A wing and precautionary lines in position in other sections of the building, it remained uncertain at this point whether or not the aggressive interior attack would be successful. In anticipation of the need for a Plan B, I positioned Tower Ladder 46 at the throat area of the building (photo 3). This location would allow it to both extinguish fire in the A wing and protect the vital throat area, which presented a direct path for fire extension to the B wing.

(3) Photo by Lou Minutoli.
(3) Photo by Lou Minutoli.

Battalion 27 reported that he had fire in two separate apartments in the top floor of the A wing. From my perspective in the street, I could see that some visible fire was being extinguished only to light up again minutes later. There were also pockets of fire in the cockloft that the interior hoselines could not reach.

We had now fought an aggressive interior attack for some time and were not making satisfactory headway. In any operation, if you do not get the sense that you are defeating the fire, then the fire and gravity are in the process of defeating you. I decided to withdraw all firefighters from the top floor, conduct a roll call to ensure they were in safe locations, and used the tower ladder to knock down the fire from the exterior. Battalion 19 remained on the roof to monitor conditions in the throat and B wing and to ensure that all personnel were a safe distance from the tower ladder.

Operating an exterior heavy-caliber stream into an occupied building is an unusual tactic that must be performed only with rigid precautions in place. First, all firefighting and civilian personnel must be removed from danger areas. Then a clear announcement should be made on the fireground radio prior to starting the stream so that units in other parts of the building can monitor conditions and report any adverse effects that may occur. The exterior stream should be operated for a minimal time to avoid a heavy accumulation of water in the building. The intent is to knock down heavy fire rather than completely extinguish it.

The brief use of the tower ladder successfully checked the fire in the cockloft that the units had not been able to extinguish with the interior handlines. Following a careful evaluation of structural conditions on the roof and top floor, engine companies were recommitted to the interior to eliminate the remaining pockets of fire. It was discovered that fire had burned through the floor of one of the top-floor apartments to the floor below. This type of construction will often have voids that run the full height of the building. Truck personnel were assigned to examine all floors down to and including the basement. No further extension was found. At 1127 hours, the incident was finally placed under control.

This operation called for an initial strategy capable of aggressively attacking the fire while ensuring that likely paths of extension were examined and protected. It also required a willingness to change the strategy when the volume of fire proved overwhelming for the interior attack. And, as always, the strategies would not have worked without the hands-on tactics that firefighters accomplished despite the extraordinary challenges they faced. Although the top front section of the A wing was destroyed, the rest of the building was protected (photos 4 and 5). Trenching tactics in particular were a key to the successful outcome of this incident.

(4-5) Photos by Lou Minutoli.
(4-5) Photos by Lou Minutoli.
(4-5) Photos by Lou Minutoli.


From a strategic point of view, there are two kinds of trenches. One type is cut and opened up with the intention of isolating a fire to one area of a building. The other is preemptively cut but not opened. The preemptive trench is designed to provide a second line of defense if the first trench is unsuccessful in containing the fire. Both types were used at this operation.

Do not open a trench before the initial vertical ventilation hole is cut directly over the fire’s location. Opening up a trench prior to the creation of the vertical vent hole will draw heat and flame through the cockloft to the trench’s location.

A trench should ideally be at least three feet wide and about 20 feet from the initial vertical vent opening. It should be cut in the narrowest possible section of a roof, since it can require considerable time and personnel to complete and implement. Outside building walls, substantial fire divisions, and roof bulkhead structures can be used to define the perimeter of the trench and minimize the amount of saw cutting required.

Photo 6 shows the location of the initial vertical vent hole. As illustrated in photo 7, the trench was opened in a narrow section of the A wing to allow for a quicker cut and to provide sufficient distance from the vent hole. Regardless of where the trench is located, small examination holes should be cut on both sides of the trench to warn firefighters of any fire movement in the cockloft.

(6-7) Photos by author.
(6-7) Photos by author.
(6-7) Photos by author.

Safety Considerations

Trenching is a complex tactic with numerous dangers. A dedicated sector commander should be assigned to exclusively supervise that aspect of the operation. Given the noise created by power saws, the sector commander or his delegate should remain in a quiet section of the roof where important fireground radio communications can be monitored.

A cockloft fire has the potential to quickly trap firefighters. Before starting any trenching activities, establish a means of rapid egress from the roof. At this fire, numerous aerial and tower ladders were positioned for this purpose (photo 8). If fire escapes are on the building, be aware that not all of them will extend up to the roof.

(8) Photo by Lou Minutoli.
(8) Photo by Lou Minutoli.

Once a trench is opened, all personnel must stay off the roof on the fire side of the trench. A charged hoseline should be in position near the nonfire side. This line can be used to protect firefighters and to extinguish fire in the trench. However, prior to operating this line, you must communicate and coordinate with any firefighters working below.

Operating a saw is a hazard in itself. Only trained personnel should be given this assignment. The saw operator should have a guide firefighter in place behind him, and a supervisor should oversee both members. The operator and guide person can use hand signals to communicate over the noise of the saw. All other firefighters should maintain a safe distance from the point of saw operation.

Finally, there is the danger of getting too narrowly focused on performing the hands-on work. Immersion in the tactic should not keep you from continually evaluating the strategy. Consider whether the trench is working. Is it advisable to start a second, more distant, trench, or is it possibly time to get off the roof altogether?

When Not to Trench

Do not consider trenching unless sufficient resources are available. It takes considerable time, effort, and personnel to successfully establish a trench. Smaller departments in particular must consider whether this would be the best use of their personnel given other fireground demands.

Do not use a trench if there is any doubt about the stability of a roof. Evaluate fire damage and the roof support construction. The size of a building is also a factor. A large strip mall, for example, may be 100 feet deep. Trenching such a building would call for two straight 100-foot cuts followed by numerous cross cuts and then the laborious process of removing the roofing material. Even large departments would find this difficult to accomplish in a reasonable time.

At many operations, the basic firefighting premise of strategically placing a hoseline allows us to perform aggressive searches while the fire is being confined and extinguished. Advanced fires sometimes call for other tools in a firefighter’s tactical “toolbox.”

When an interior attack is not capable of extinguishing a top-floor fire, an incident commander is forced to take a broader perspective of the operation. He must judge just how much can be accomplished given the resources he has to work with, the extent of the fire, and the construction of the building-in short, determine where he can draw the line between what can be saved and what will be lost to the fire. The trench cut is a preemptive tactic that can often define exactly where that line stands.

THOMAS DUNNE is a deputy chief and 32-year veteran of the Fire Department of New York with extensive experience in Mid-Manhattan and the Bronx. He has been the incident commander at hundreds of fires in residential, commercial, and high-rise buildings. He is a Fordham University graduate and is an adjunct instructor for the National Fire Academy. He has written numerous articles for Fire Engineering and has presented at conventions, colleges, and fire academies across the country. He writes and lectures on a variety of fire service topics through his “Third Alarm Fire Training” seminars.

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