Ventilating Lightweight Roof Assemblies: The Inverse Trench Cut

BY JOHN “SKIP” COLEMAN

When I came on the job in 1975, I may have been in one of the last recruit classes that actually trained on how to use a Browder life net. For you younger firefighters, this was a large steel-framed folding canvas net with 32 coiled springs (that question was on my first lieutenant’s test) and was about 12 feet in diameter. Carried on trucks, it was used to catch civilians who jumped from windows at fires. At least 12 firefighters and one jumper were needed to safely use the device. Our small class had seven recruits; we had to bring out another company and needed all the instructors to conduct the evolution. Because of the personnel required, the nets were soon removed from the truck companies and put into museums.

What does that have to do with trench cuts? Staffing! The same holds true for trench cuts, which necessitate at least two truck companies to complete. Although one company can certainly complete a trench cut, time is a critical element when considering a trench cut. Staffing, time, and safety are three reasons to rethink the way we look at trench cuts and where we make them at a fire in an occupancy that has a lightweight roof assembly.

 

TRENCH CUT THEORY

 

A trench cut is an opening made the full distance between two exterior walls or other firestops. The width of the trench should be at least three feet. A trench cut may be made in any direction regardless of the type of roof.1

Generally, a trench cut is considered in commercial or multifamily apartment building fires involving the attic, cockloft, trussloft, or roof assembly. The cut is intended to create an opening between two exterior walls that removes the fuel—i.e., the roof assembly (remember the fire triangle—heat, fuel, and oxygen) from the advancing fire. After making the cut, you may place a hoseline on the trench cut’s nonburning side to suppress and darken any fire that may attempt to jump the cut.

The cut also creates a long narrow vent hole, which will assist in venting the approaching heat and fire.

The first step in creating a trench cut is to create a vertical ventilation hole over the fire or wait until the fire vents itself. This initial act buys the vent crew time to perform the trench cut. Using crews to vent a roof in a lightweight roof assembly can be dangerous. Additionally, effective trench cuts are extremely labor- and time-intensive.

Francis L. Brannigan, author of Building Construction for the Fire Service, states, “It’s not safe to put a firefighter above or below a truss assembly involved in fire.” On the whole, this advice is extremely wise. However, it should not be taken at face value. Let’s look at two fire scenarios in relation to the above statement.

In 1988, five firefighters were killed in the Hackensack, New Jersey, Ford dealership fire. The fire started in an attic area above the dealership, and the roof assembly was supported by five 75-foot bowstring truss assemblies spaced 16 feet apart. The space between the ceiling and the roof assembly was used for storage. The fire area below the roof had few, if any, interior walls. When the truss assembly failed, the roof, the materials stored below it, and the ceiling assembly collapsed onto the service area floor. There was nothing substantial to stop or slow down the collapse.

The second scenario concerns a three-story garden apartment with a truss loft roof assembly spanning multiple sections of apartments. Garden apartment sections generally have four apartments per floor. Modern apartment buildings of this type (built in the past 30 years or so) should have draftstopping (nonfire rated materials such as plywood, gypsum board, or sheet metal) in attic trusses and in the floor trusses, located in line with the perimeter of the apartments. In essence, the draftstopping provides compartmentation in these void spaces, the size of the apartment above and below. Lightweight truss assemblies, generally Fink trusses, support the roof assembly. These trusses are generally constructed of 2 × 4 or smaller lumber and are placed on 16-inch centers. Generally, truss lofts are not used for storage.

The difference between the two buildings is what is below the trusses. There was nothing below the truss assembly at Hackensack Ford. But what is below the truss assembly in apartment buildings? Walls! Lots and lots of walls.

Figure 1 shows a typical four-apartment section of garden apartment; note the number of walls present. Regardless of whether it’s a load-bearing or nonload-bearing wall, the wall’s strength will mitigate the effects of a truss and roof assembly collapse.

Figure 1. Typical Garden Apartment Floor Layout
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Source: Managing Major Fires by John F. “Skip” Coleman, Fire Engineering, 2001.

Remember also that in lightweight construction, “lightweight” has two meanings. It means that the materials used are lightweight. It also means that the entire assembly is lighter overall compared with a conventional rafter roof assembly made from 2 × 6 or larger lumber.

In my 32 years on the Toledo (OH) Fire Department, I had responded to many dozens of fires in multifamily apartment buildings with lightweight roofs. I do not remember any general and precipitous collapse of such assemblies at any fire in which conditions allowed firefighters to enter the building.

Let me rephrase that. I had been to a few fires in these occupancies where, on arrival, the amount of fire involving that apartment building section prohibited any fire crew to enter. Picture an apartment building totally involved in fire. Buildings exhibiting this much fire have a propensity for total collapse. However, apartment buildings with the amount of fire that allows firefighters to enter the building involved, in my opinion, have a slim chance of a general and precipitous collapse.

Going back to Brannigan’s statement concerning truss assemblies, I agree that it is not safe to put firefighters above a truss assembly that is involved in fire. I do not believe in the blanket statement that firefighters should not go under a truss assembly involved in fire. Whether or not to go under a truss assembly involved in fire depends on what’s under the truss assembly. In apartment buildings laid out as in Figure 1, the wall assemblies will prevent a general and precipitous collapse that would injure a firefighter. Although a truss member may go through the drywall and into the living space below, a total collapse is unlikely.

If you do not believe it is safe to operate directly under a truss assembly with fire above it, under those circumstances (as in Figure 1), it is relatively safe to operate in an adjoining room or area. This is especially true if necessary searches are in progress.

 

INVERSE TRENCH CUT

 

Probably somewhere around 2000 or 2001, I was dispatched to a midmorning fire in an apartment building on Hill Avenue in Toledo. The winds that morning were extremely heavy, as can be seen photo 1. Fire entered the attic area and began to travel to unaffected portions of the apartment complex.

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(1) Side A of the fire building. The wind was coming from the A/D corner, going toward the B/C corner. (Photos by author.)
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(2) As the trench was being pulled two apartments to the left, we brought this civilian out of her second-floor window, on the building’s D side.

I decided to send a battalion chief and a crew inside an adjoining section of the apartment building toward which that fire was traveling and directed them to take a 2½-inch line to the top floor and pull the ceiling for the entire width of the building. I then told them that, after the ceiling was pulled, not to allow the fire past the trench. They did as directed—pulled a ceiling and stopped the fire’s advance with what I now call an “inverse trench cut.”

Instead of going above the fire onto a roof assembly and cutting a trench with a carbide-tipped chain saw or rotary saw, which is extremely tool-, labor-, and time-intensive, we went below the fire and pulled drywall ceiling with six- and eight-foot pike poles. As the fire approached, instead of removing the fuel by cutting away the upper portion of the roof, we took away one leg of the fire triangle by removing the heat from the fire by cooling the roof assembly’s underside with a 2½-inch handline.

Conducting an inverse trench cut is a two-step process. It is assumed that conditions are such that the evolution can be done safely and from the most advantageous location. The best location to perform an inverse trench cut is in a common area of the apartment building, such as a hallway. Pull the ceiling area along the wall closest to the advancing fire. This will provide the best protection for firefighters if there is a partial collapse of the roof assembly.

To make an inverse trench cut, first pull a three-foot section of ceiling along the wall in the hallway closest to the advancing fire. This opening should be from wall to wall as much as possible (Figure 2).

Figure 2. Making the Inverse Trench Cut
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Source: Adapted from Managing Major Fires by John F. “Skip” Coleman, Fire Engineering, 2001.

Second, position a 2½-inch hoseline in the hall to darken any advancing fire in the truss loft area. Hopefully, the draftstopping or fire-rated corridor walls (sometimes present when interior public hallways are used) will slow or stop the fire. This line should only be opened if fire is evident or imminent.

 

ADVANTAGES

 

Inverse trench cuts have the following advantages:

  • Cuts are made ahead of the fire just as a traditional trench cut is but are protected by the interior wall assemblies if there is a partial or total roof collapse, making them a much safer operation.
  • It takes less staffing to remove a three-foot section of ½- or 3⁄8-inch drywall using pike poles than to use power tools to cut away roofing materials.
  • The pike poles are much simpler and safer to operate than power saws.

 

In the garden apartments in photo 3, it is safest to conduct an inverse trench cut in the section of apartments next to the section involved. If this is a precautionary cut, then it is best to make it in the hallway if at all possible (Figure 2). That way, if the fire is stopped at the fire-rated separation wall, occupants may be able to sleep in their own beds that night. In a smaller fire with little involvement in the truss loft area, you can make the cut in the same section of the apartment complex as the fire but in the hallway if possible.

Click to Enlarge

(3) Arrow points to the location of the fire-rated wall that divides sections of the apartment building.

You can use inverse trench cuts effectively in strip malls as well. Move into an uninvolved occupancy, and pull the ceiling at the wall shared with the fire occupancy. Sometimes this can be accomplished with a hoseline stream. Then, position a 2½-inch line and await the advancing fire as described above. When necessary, cool the underside of the roof assembly.

This is another tool to put in your toolbox. It is not appropriate for every fire incident. Prefire surveys of commercial buildings in your area are essential to knowing when and when not to attempt any type of fire operation. It worked for us at the Hill Avenue fire and has worked at similar fires since then.

 

Endnote

 

1. Fire Department of New York, Firefighting Procedures, Volume 1, Book 4, March 15, 1997.

John “Skip” Coleman retired as assistant chief from the Toledo (OH) Department of Fire and Rescue. He is a technical editor of Fire Engineering; a member of the FDIC Educational Advisory Board; and author ofIncident Management for the Street-Smart Fire Officer (Fire Engineering, 1997), Managing Major Fires (Fire Engineering, 2000), and Incident Management for the Street-Smart Fire Officer, Second Edition (Fire Engineering, 2008).

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