The Trench Cut - When, Where and How

Many of the ventilation tactics used on today's fireground are in support of an offensive fire attack, while the trench cut is used in many capacities.

  • Where is the fire? The first step of performing the trench cut requires the crews to place a large vent hole (10 feet by 10 feet) over the fire, to allow heat and smoke to exit the structure, and to buy the roof team time to complete the trench. Unfortunately, this step is quite frequently overlooked, and the fire tends to roar right past the prospective trench area.
  • What is exiting the roof? Is there already fire or heavy smoke coming through the roof in an area that has failed? If this is the case, the prudent fire officer may re-think the decision to trench the roof (see Photo 4).
  • What materials make up the Roof? Knowing the materials will allow your crews to bring the right blades for the saws and other necessary tools to open the trench.
  • Is the roof spongy/sagging/blistering in areas? Be sure to identify any areas that are weakened and failing; operations may have to be adjusted due to roof conditions.

Starting The Operation

Gaining access to the roof for this operation should begin from a well supported area of the roof, preferably from the opposite side of a fire wall or other substantial separation. At least two means of egresses shall be in place and secured for rapid egress from the roof area should the operation turn for the worse. A minimum of two fully staffed companies are a good start for this type of operation and the incident commander (IC) will need to consider relief for them during the operation. Approach the roof from a safe area, and start off by making a large vent opening directly over the fire to allow smoke and heat to exit the structure. This will also buy some time for the companies to complete the trench cut.

Once that opening has been completed, look for a suitable spot to cut the trench, at least 20 feet from the original ventilation opening. Look for some existing roof openings that can assist in making the trench cut (vent chases, scuttle hatches, bulkhead doors, etc…) and cut from these openings; the existing openings will lessen the amount of area the crews will have to open up. Look for a pinch point or "throat" area of the building that will facilitate cutting the trench. Optimally, limiting the length of the cut from side to side will decrease the time needed to make the cut and it will get the crews off the roof faster.

Prior to cutting the trench, the crews should make a few inspection holes between the existing vent opening and the area for the trench to be cut. An inspection hole is a series of three Kerf Cuts (a cut in the roof material the same width of the rotary blade in use) in the shape of a triangle. These inspection holes will serve as early warning as the fire progresses in the direction of the trench. A few inspection holes should also be cut beyond the trench area as well; if the fire has jumped the trench area, it may be necessary to move the trench cut further down the roof (see Photo 5).

Making The Cut

Once a spot is chosen for the trench, check the direction of the roof rafters. Cutting the trench in the same direction as the roof rafters are running can save the roof crew valuable time. For example, if the trench is cut with only one rafter within the trench sides, the trench panels can be pivoted open on only one nailed rafter. If the roof rafters are wide enough apart, the trench may be able to be cut completely within the rafters, and the sections can be lifted out entirely. If the trench is made perpendicular to the direction of the rafters, many more relief cuts will be required to open up the trench panels.

Starting from the upwind side, with the crews located between the trench and the means of egress, make two straight cuts approximately three feet wide, from one wall completely to the opposite wall of the building. Remember: do not cut the trench between you and your egress! It will be necessary to put perpendicular relief cuts throughout the length of the trench cut. Make the relief cuts at approximately every five feet down the length of the trench cut; this will keep the sections of the trench cut manageable for crews to manipulate open.

Opening The Trench

The trench cut is just one part of the overall strategy on the fireground; a major portion of the operation involves a second part, the actual opening of the trench. Therefore, it is imperative that strict coordination between the suppression teams and the roof teams be adhered to. Command should have large hose streams placed underneath the area of the trench cut, to suppress any fire that may come towards the opening. When ready, the roof team shall open up the trench, and punch out the ceiling below the cut area. Once opened, do not stand by and admire your work; get off the roof immediately. The introduction of products of combustion with the fresh air above the roof may result in vent-point ignition, so it is critical that a protective hoseline be in place to assure the safety of the roof teams as they exit the roof.