First of all, let me be clear that this is not an indictment of the method itself. You will not read any dispute or argument here against its effectiveness and rapidity. Nope, it’s good alright. A solid ventilation operation was born out of the necessity and a belief in the importance of peaked roof ventilation. These firefighters developed, evolved, and refined a vertical ventilation method that accomplishes the tactical requirements and at the same time keeps the brothers on solid working platforms in case things go bad. On the contrary, maybe it’s too good, too fast, and too easy! So much so that firefighters that don’t study their first due areas and the changing construction within them might be setting themselves up for a failure of the worst kind. The question I pose is whether or not this roof operation has evolved enough? Anyone that knows me is well aware of my proclivity towards and general belief in the value of topside ventilation and roof operations as a whole in relation to overall incident success. However, today’s disconcerting matrimony of internet videos and a chronic malaise towards truly educating ourselves on the ever changing arena in which we do battle has led to an increased reliance on Band-Aid cure all’s to real first due challenges. Without replicating, matching, and ultimately testing these methods in and on the buildings that you respond to you could be placing yourself and your company into a high risk scenario without even realizing it. A review of the method would be prudent prior to continuing this conversation. You can find the video of the operation under the Fire Engineering Training Minutes Truck Company section.
So what am I yammering on about? Well hopefully you all just reacquainted yourselves with the method so that we can get into the guts of the operation. The portion of the video that peaks my interest plays from the 2:15 mark to the 2:30 mark. You see I respond to and perform my topside vent operations in a city with a predominant amount of pre-engineered, lightweight trussed roofs. And, this concern of mine stems from a very distinct structural design or lack thereof. Where a conventional rafter or even older legacy roof system derives its strength and general load carrying capability from the rafters resting across the outer load bearing walls and joining in the middle at the ridge beam; the lightweight, pre-engineered truss system does not. What’s a ridge beam? A ridge beam or ridge pole as it is also called, is simply a piece of dimensional lumber that is hung and supported from gable to gable or bearing wall to bearing wall. The roof rafters were then miter cut, nailed to the ridge board and laid across the outer load bearing wall. A collar beam would then connect the rafters from the C-side of the roof to the rafters from the A-side of the roof thus adding to the roofs overall stability and rigidity. Load carrying capacity is derived from the dimensions and on center spacing of these roof rafters. This “Cut and Stack” style of roof construction was utilized up until the mid to the late 1960s give or take, depending on your part of the country. The lightweight roof system followed next with several generations of truss construction. Lightweight construction is made up of pre-engineered trusses that are built off site and then trucked to the site. Once there, they are hung in place at 24” intervals and a “filler” or “stiffener” stud (2”x 4” or 2”x 6”) is toenailed in between each truss at the peak or highest point of the ridge. This along with the plywood or O.S.B. attached using “diaphragm nailing” techniques provides resistance to “shear” force (namely wind) and any other lateral movements that might cause the trusses to fall over like dominos. The absence of a true ridge beam is kind of a big deal when discussing roof operations on a burning building, especially when the Milwaukee operation is 100% reliant upon the strength of said ridge! It seems everyone has lightweight trussed roofs in varying amounts. And, in case you were wondering, yes they can engineer steep pitched roofs using pre-engineered trusses. One just needs to Google Image Cathedral Truss or examine the attached pics. Yet in their operation there is no sounding or diagnostics performed prior to ascending and straddling the ridge to set the ladder. I never read or hear anybody throw out an F.S.A. (firefighter service announcement) regarding this method, "warning, not intended for use on pre-engineered trussed roofs". Again, the problem does not lie in the operation, the problem lies with our firefighters. Much like the need to add the “I“ to the acronym V.E.S. arose out of the always present human factor. The same issue is present here. We all know how much firefighters love systematic, step by step operations that spell out how to accomplish an otherwise precarious tactic on the fire ground. A system that takes the thinking out of it for us! The reason I chose the Milwaukee method to examine is because of its popularity. This operation is now all over the web and has been replicated on several other popular websites. We need to ensure that we understand the differences in building construction and their performances under fire conditions. While this is an outstandingly efficient operation for steep pitched roof ventilation, we must comprehend and appreciate the risk with performing this or any other type of roof ladder operation on a pre-engineered, lightweight trussed roof. If a steep pitched roof needs to be cut and your truck was not able to get the address, then roof ladders it is. Just make darn sure that you perform your diagnostics on the way up and do your best at sounding this roof prior to committing yourself to it and basing your safety solely on the expectation of a ridge beam which might not be there in the first place. On a lightweight system, when possible, using the security of an aerial ladder or platform may be a more reliable option depending on the time it takes for deployment. It is imperative that we become thinking firefighters who are not afraid to ask questions and who understand building constructions generational differences and the impact of the fire and the work that must be performed has on them.
Below is a pic of a Cathedral Truss prior to ridge stiffener installation.
Left and below are pics of a conventional rafter system with an
obvious "true" ridge beam.
Below is a pic of lightweight, pre-engineered trusses being set and hung prior to the 2"x 4" stiffeners being installed.
Below is a pic of an attic fire in a Lightweight, pre-engineered truss assembly. You can see the 2" x 4" stiffeners in place and how their placement at the ridge positions them in the prime location to sustain the bulk of the fire damage.
Below is a pic of a diagnostic cut. This cut can undress the entire roof system and only takes seconds to accomplish with a chainsaw. Smoke & fire conditions, construction type & direction, decking, covering type & thickness will all be identified in 8 to 10 seconds. Spend the time to increase the chances of a successful operation.