Trenching Roof Is Success In Test on Burning Barn
features
Instructor
Fire Service Institute, U. of Illinois
Does trenching a roof work? Instructors of the Fire Service Institute at the University of Illinois recently had an opportunity to test trenching when a horse barn at the Champaign County Fairgrounds was burned as part of re cruit school training.
Trenching is opening up a roof by cutting a trench all the way across a roof to limit the spread of fire. If possible, a conventional hole over the hottest part of the fire should first be made. This tactic is not yet common in the Midwest.
The T-shaped horse barn was about 225 feet long with 30-foot-wide wings at one end of it. The wings were one story high and the center section was two stories.
Prior to starting any fires, a trench— two rafters or about 3 feet wide—was cut at the midpoint of the south one-story section. This trench extended clear across the roof from side wall to side wall. We did not cut the ridge board, although some departments suggest cutting the ridge to prevent fire from crossing the trench by following the underside of the ridge board. This did not prove to be a problem in our fires. We did open clear down to the lower edges of the roof.
One slope of the roof was cut with a power saw, and the other side with two axes. Although the saw was faster, it was also found practical to use axes to open a hole of this size. However, at an actual fire, the site of the trench should be farther ahead of the fire if axes are being used instead of a saw.
Hand lines only used
The Urbana Fire Department supplied 400 feet of 5-inch hose to feed the Fire Service Institute’s 750-gpm pumper. Two 11/2-inch hand lines, a 21/2-inch hand line and a deluge set with a variable 375 to 1000-gpm tip were used. All fire fighting was done with hand lines only. The deluge set was placed inside the building just behind and beneath the trench so the 375-gpm stream could be used to help push the heat and smoke up and out of the trench if necessary.
As heated gases rise and mushroom across the underside of the roof, they reach the trench, and the natural convection currents move the heat upward through the trench and out of the building, thus stopping the horizontal spread of fire. Once sufficient heat has built up, horizontal spread can occur rapidly.
During our test fires, flashover of the rafters was occurring at the rate of one every four seconds. Even with this momentum, the trench did a textbook perfect job of limiting horizontal spread. Each of the six fires in the trenched wing of the building was extinguished with hand lines. In most of the fires, we at-
tacked through end doors for test purposes and actually were driving the fire into the building toward the trench. However, even under this adverse condition, the trench still cut off the fire spread.
Master stream effect
On two of the fires, we charged the deluge set at 375 gpm. In one case, this was done to see if the stream, discharging up through the trench, not at the fire, would help push the fire up and out of the building. It did. The second use of the deluge set was during the most severe fire when the entire wing up to the trench was burning, and flames moved past the trench for a short time. The fire was quickly pushed back, and though flames extended down to head height briefly, the 375-gpm gun was not abandoned, and the situation was rapidly controlled by the deluge set and the 21/2-inch hand line with no structural involvement past the trench.
We were quite pleased with the results of the trenching operation. It proved trenching will work to limit horizontal fire spread. Without the defensive trench, any of these fires undoubtedly could have destroyed the barn.
The only change we will make next time is to cut a wider trench. On a building of this size, we think a 4 to 6foot-wide trench might perform even better and provide a margin of safety.
Trenching does not replace normal offensive ventilation to permit interior fire fighting, but it can povide a strong defensive position to stop the spread of fire in large buildings.
