Trench Cuts: Where Do They Work?
By Nate Demarse
Nationwide, there seems to be a lot of confusion regarding the use of the trench cut. In this “Trench Cut” series, we will discuss the background of the trench cut and the specific types of structures for which it was designed. As with several concepts in the fire service, the trench cut has been adapted and is now being utilized on other building types. In some cases this adaptation may work. However, we will discuss specific building types where trench cuts may not be the answer to a successful outcome. In some cases, it may even hinder the operation by “distracting” essential resources which may be better utilized by attacking the fire where it is NOW, instead of backing off, writing off an entire portion of the building and stopping it LATER. This is especially true in departments where manpower issues are common or constant.
The trench cut is one component of an overall strategy. Simply cutting a trench in a roof will not stop a rapidly extending cockloft fire. Several other things must be coordinated for a successful operation. Other components include: Removing and accounting for members from the area that we are giving up, completely pulling the ceilings directly below the trench and stretching additional lines to key locations above and below the trench. The overall strategy will be briefly touched in this drill. In an upcoming drill, step-by-step considerations for trench cuts will be discussed from a roof firefighters perspective
As stated above, the trench cut is one component in an overall strategy. Trench cuts can be used to cut off a rapidly extending cockloft fire at a pinch point. The pinch point or throat could be defined as an area where a building narrows sufficiently to perform a successful operation before the fire can extend past that point. This designated pinch point must be far enough ahead of the advancing fire to allow the trench to be completed, but not so far away that too much of the building is given up needlessly.
The trench cut is a defensive tactic and should be viewed as cutting a “fire break” in the roof. Although, the opening will allow fire, smoke and gases to vent from the cockloft area, the trench is NOT intended to act as a ventilation opening. A large ventilation opening must be cut over the fire BEFORE starting a trench cut. Failure to provide this large ventilation opening over the fire will ALMOST ALWAYS result in the fire jumping the trench and extending into uninvolved areas. In fact, if a normal vertical ventilation opening is not provided and a trench is cut and pulled remote from the fire area, it will certainly cause the fire to extend towards the trench (a new ventilation opening/path of least resistance) and into the uninvolved area that you are working to save. Additionally, the large vertical ventilation cut will buy extra time that is needed to make your additional cuts for the trench.
The trench cut was designed for buildings that have similar features as the examples below:
The first two photos are two different angles of the same building in the Bronx, NY. The arrows point to the throat where a trench could be cut relatively quickly to isolate the fire from the other wings. You can see how the narrowing of the building in the throat area would allow a trench cut to be placed with minimal effort and maximize the chances of cutting the fire off. The area where the arrows are pointing are 10-15 feet wide at their widest point. Keep these numbers in mind as you read further to other building types.
Here is another example using more recent building construction in a suburban setting:
This example in Westmont, IL has similar features as the building in the Bronx. While not a 6 story non-fireproof building, it does have it’s own version of a pronounced “wing” and a “throat” area. A properly placed trench could be successful in this building. This throat or pinch point is approximately 15 feet wide.
The trench cut was never intended for use in the following buildings discussed below:
This 1 story Class III strip mall/taxpayer measuring 150×50 is NOT a candidate for a trench cut. The likelihood of a trench cut being successful in this type of building is nearly nil. The features of this type of building do not provide a pinch point or throat area to successfully cut a trench. Lets discuss the tasks that MUST take place in order to cut a PROPER trench cut with a SUCCESSFUL outcome.
An initial ventilation hole measuring approximately 10′x10′ must be cut over the main body of fire. Most likely, with heavy fire conditions in the cockloft this cut should be extended to slow the lateral fire spread in the cockloft. Depending on the problems encountered, cutting and extending this initial ventilation opening could take several members operating two saws, five to fifteen minutes. To perform a trench cut correctly, this initial ventilation cut MUST be completed and therefore cannot be bypassed. At this point, we are approximately 15 - 20 minutes into the operation and we haven’t even started the trench.
To effectively cut a trench in this roof, you will have to cut the ENTIRE DEPTH of this building twice. This will equal 100 feet of linear cutting. Relief cuts every 3-5 feet along the trench will account for another 17 - 30 feet of cutting. Relief cuts are necessary so the roofing material can be pulled from the trench in pieces. To perform a trench cut operation CORRECTLY, the ceiling below must also be pulled so hose lines can be operated into the cockloft. The ceiling must be pulled from the front wall to the rear wall and ideally 2 to 3 bays wide (2 to 3 feet). That is approximately 150 square feet of ceiling that must be pulled. In old commercial buildings such as this, several ceilings including tin ceilings will severely hamper the operation. Manpower will also be needed to stretch and operate hose lines into the cockloft from above after the trench is pulled.
To summarize the numbers:
- Members on the roof will need to do approximately 130 feet of linear cutting (not including multiple inspection holes & the initial large ventilation hole).
- Members below will have to open up approximately 150 square feet of ceiling.
- Members are needed to stretch multiple lines above and below the trench (not including the lines that are needed to attack the main body of fire)
This operation will take far too long to perform and require more manpower than most departments are able to muster. Use your available resources to stretch and advance additional 2 1/2″ lines, pull ceilings and expand the initial ventilation cut to slow/stop the fire travel in the cockloft and extinguish the fire. A rapidly extending cockloft fire would almost certainly overrun any trench operation started on this type of building before it could be completed.
Another Trench Cut No-Go:
Garden apartment or townhouse type complexes such as this one in Westmont, IL are also NOT candidates for a trench operation. Although the middle area of the building is a more narrow than either end, it is still 60 feet deep (30 feet from the soffit to the peak). Once again, there is no “pinch point” or throat to successfully place a trench cut quickly to effectively cut off a rapidly extending fire. The same problems present themselves here as they did above. These problems include over 200 feet of linear cutting, pulling massive amounts of ceiling and the associated manpower issues. However, there are a couple of advantages to this building type. The roof deck will typically be constructed of plywood or OSB and will only have one or two layers of shingles present and the interior ceilings will most likely be constructed of sheetrock. However, these two advantages should NOT lead you to the conclusion that a trench cut operation should be conducted on this type of building.
If you are going to spend time cutting 200 feet of roofing material and pulling 150 square feet of ceiling, do it near the seat of the fire! If placed strategically, you could essentially cut the ENTIRE roof off of the fire apartment. How’s that for a ventilation opening to stop the lateral spread of fire?
Some officers and firefighters will argue tooth and nail that trench cuts should be utilized on the building types discussed above. Some departments will say that they have been successful in stopping fires in these building types by utilizing a trench cut. I ask those members and those departments this question: How rapid is the fire REALLY extending if you are able to spend 30 minutes to CORRECTLY perform ALL of the tasks described above to utilize a trench cut to stop the fire? I will lay money on the probability that in the majority of the cases nationwide, where a trench cut was credited with saving the building, it was most likely the aggressive operations of the interior companies opening the ceilings and exposing the fire in the cockloft from below. These companies in some cases probably worked without a primary ventilation hole which exponentially complicated their tasks. In most of those cases, the trench cut was probably not warranted but seemed to coincidentally “work-out”.
This drill summarizes when and where trench cuts may or may not be warranted. I wanted to touch on the background and concepts of trench cuts before diving straight in on the “how-to” steps of cutting one.