BY JOHN MITTENDORF
Over the years, numerous aspects within the American fire service have dramatically changed so that they have collectively both enhanced fireground operations and steadily degraded the fireground to the detriment of suppression personnel.
From the perspective of enhanced fireground operations, improved fire apparatus, superior protective equipment, thermal imaging cameras, communication capabilities that were only a dream a few years ago, and other similar items have dramatically improved the effectiveness of suppression operations.
However, from the perspective of a fireground that can be a detriment to firefighter safety and suppression operations, we now have a modern thermal layer (smoke, fire gases, and heat) that has become more dangerous because of the increased use and resultant combustion of synthetic materials, flashovers that have replaced backdrafts as a result of a more volatile thermal layer, and building construction that has lost a significant amount of integrity and resistance to fire through the use of alternative materials and a reduction in the size of structural members (lightweight construction).
Ventilation operations can eliminate or minimize the hazards from the modern thermal layer and flashover. However, ventilation operations are often conducted within or on top of buildings, which immediately should raise a concern when these operations must be applied to modern building construction that is continuing to offer less time before collapse when it is exposed to heat and/or fire.
Notice that heat was used with fire in the last sentence. The reason is that some materials used in lightweight construction can cause rapid failure in the presence of moderate heat without direct impingement of flame. As an example, every firefighter should read and thoughtfully consider the article “The Pros and Cons of I-Joists” by Chief Kenneth Morgan of the Clark County (NV) Fire Department (Fire Engineering, August 2010). The article chronicles how a small fire in a pile of textiles on the floor in a commercial building that caused only smoke and heat to rise to the ceiling compromised numerous roofing system I-joists. This article gives a good example of how some types of lightweight construction can be compromised in a short time without direct exposure to flame.
The ability of moderate heat to compromise conventional structural members (2- × 6-inch or larger solid lumber—photo 1) was largely unknown before because of the size/mass of older construction. Unfortunately, the common modern structures—residential and some commercial buildings—involve nothing more than the use of 2- × 4-inch or 2- × 6-inch lumber for vertical walls, 2- × 4-inch trusses held together with gang-nail plates (photo 2) and/or glue (photo 3), and the increasing use of I-joists that have oriented strand board (OSB) for the stems for roof (and floor) assemblies. In most cases, the walls and roof are then covered with OSB material for shear strength for the walls and a roof decking. Interestingly, the ½-inch 4- × 8-foot OSB panels routinely used for exterior shear panels and roof decking (photo 4) are in reality 7⁄16 inch thick! Interestingly, this “common” home is comprised of 2- × 4-inch trusses (photo 2) and covered with 7⁄16-inch OSB!
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| (1) Photos by author except photo 10, which is courtesy of Kurt Zingheim. |
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Note: The primary reason solid conventional lumber has been replaced by micro-lam, BCI, OSB, I-joists, and other forms of lightweight lumber is highlighted by an interesting statistic that mirrors the harvesting of lumber in this country. In the past 25 years, 17 mills have closed in Southern Oregon, and Oregon’s timber harvest in 2009 was close to the Great Depression-era lows. Have you noticed that a noteworthy amount of lumber now comes from Canada? Additionally, the wood currently used for lumber is dramatically different from the wood used for lumber 30 years ago (see “Modern Wood an Additional Fireground Hazard?” above).
If the American fire service is now routinely encountering structures with structural members that burn faster and hotter and are held together by gang-nail plates and/or members that use “glue” for strength and can fail when exposed to compromising levels of heat and/or fire, then it is mandatory to apply this dilemma to one of the most popular forms of ventilation—vertical ventilation and, more specifically, ventilation operations on lightweight roofs. For this article, the term “lightweight roofs” includes metal roofs such as corrugated metal, metal deck built-up roofs, and wood roofs with structural members less than 2 × 6 inches such as BCI, I-joists, 2- × 4-inch trusses, and other similar configurations that would not be considered “conventional roofs.”
Before we look at the question of the viability of ventilation on lightweight roofs, let’s quickly review a few considerations as applied to vertical ventilation on conventional and/or older roofs that was often enhanced by three primary considerations:
- Since lightweight truss construction was virtually unknown and/or not used in this configuration, the size of ordinary conventional construction (i.e., 2 × 6 inches or larger) had the potential to provide time for various roof ventilation operations such as opening skylights, penthouse doors, and scuttle covers, and then also cutting ventilation openings, which can be a time-intensive operation.
- Because older construction resisted the effects of fire for longer time frames than modern lightweight construction, it was often possible to cut an offensive ventilation opening directly near or over a fire as in photo 5 (heat hole) and then cut a defensive opening (strip or trench), if necessary, as in photo 6.
- A basic rule that was often applied to older roof ventilation operations went something like this: “As you walk across a roof, use your feet (or a sounding tool) to feel the roof, because if it feels strong, it probably is.” More often than not, this was a workable roof ventilation rule, and it still can be on older roofs.
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However, when we apply these same three factors to lightweight roof construction, a completely different set of considerations should be immediately apparent and should instantaneously raise the question of the viability of roof-ventilation operations on lightweight roofs, particularly when comparing the difference between photos 1 and 2. Although roof ventilation operations are not for every fire department, they are a very effective method for minimizing the heat, smoke, and fire gases within a contaminated environment, particularly from the upper portion of the environment. However, vertical ventilation is most effective when accomplished before or no later than during attack operations, and it can be a time-intensive operation. When the word “time” is applied to ventilation operations on lightweight roofs, a significant concern immediately surfaces—the national average for the collapse of lightweight construction (when exposed to fire) is between five to seven minutes. If fire is exposing lightweight roof members during the arrival of suppression personnel or if fire begins to expose lightweight roof members while suppression personnel are preparing to go to the roof, is there sufficient time for a safe roof operation? The answer is, probably not!
VIABILITY OF VENTILATION OPERATIONS
Therefore, from the perspective of this article, the answer to the question of the viability of ventilation operations on lightweight roofs is no and a conditional yes! Let’s take a look at why both of these answers can simultaneously be true:
- Roof ventilation operations on lightweight roofs are unsafe when fire or sufficient heat has or is compromising roof structural members in an area where ventilation operations are necessary. Therefore, roof ventilation operations on lightweight construction should be safely conducted on only those portions that are not exposed to fire. This may mean that, in some cases, defensive operations (strip/trench ventilation) may be the only safe option.
- Roof ventilation operations on lightweight roofs can be safe when fire or sufficient heat has not or is not compromising roof structural members where ventilation operations are to be conducted. This is the conditional portion of the yes answer.
To illustrate the two preceding viewpoints, let’s look at photo 7. For this discussion, let’s assume this commercial building is 100 feet long, the roof has been constructed from 2- × 4-inch trusses held together with gang-nail plates, the roof has been sheeted with ½-inch OSB, and there is a fire in the building that has not extended into the attic space. In this scenario, what is the chance of a collapse of the roof’s structural members? Answer: minimal to zero.
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Next, let’s assume the fire has extended into the attic area (photo 8) and is exposing the roof’s structural members. In this scenario, what is the chance of collapse? Answer: The section of roof exposed to fire will collapse unless the fire is quickly extinguished before collapse.
Last, in photo 9, the portion of the trusses that were exposed to the fire have collapsed. Additionally, let’s assume the intact portion of the roof still standing has not been exposed to fire or a lengthy significant amount of heat. Now, let’s ask a question that applies to exterior roof personnel (and also interior attack personnel). As depicted in photo 9, will the portion of the roof that has collapsed result in a collapse of the unburned roof, or will the unburned section of the roof remain as depicted? The general answer is, the burned section will not cause the unburned section to collapse, as the unburned section is attached to the perimeter of the building and the connecting medium between the last truss that fails and the first truss that does not fail is ½-inch OSB, which is not a sufficient material to pull the remaining unburned trusses into the building. This is best illustrated in photo 10. This fire was in one unit of a strip commercial with an I-joist roof and OSB decking. Notice the burned section collapsed, but the unburned portion did not.
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So, how does this discussion apply to lightweight roof operations? If you are standing on a section of a roof without fire and/or high heat levels, you likely are not on a section of roof that will suddenly collapse. However, if you are standing on a section of lightweight roof with fire and/or high heat levels, you likely are on a section of roof that can suddenly collapse without warning. Remember, falling debris always has the right-of-way!
BASIC SAFETY RULES
Now, let’s apply the previous viewpoints to a roof ventilation team that has been assigned to ventilate the roof in photo 11 and look at some basic safety rules of engagement. Remember that ventilation operations should be preceded by a set of standard operating procedures and the ability to constantly follow appropriate safety rules that are applicable to each specific ventilation operation.
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Size-up. In no particular order, you notice the following:
- The structure is a two-story dwelling attached to a one-story dwelling.
- Both structures appear to have been built in the past 20 years.
- The second floor and the attic of the two-story dwelling appear to be well involved, and there appears to be some smoke (light in color but with some pressure and growing volume) from the one-story structure where it is attached to the two-story building.
- You quickly determine there is likely extension of smoke and/or fire into the one-story structure and/or attic. You also notice there is some smoke from an attic vent, which indicates extension of smoke into the attic of the one-story structure.
- As you approach the structure, you notice the rafter tails are exposed on both structures and they are 2 × 4 inches in size, which indicates 2- × 4-inch truss construction (remember that this primarily applies to residential structures). Commercial structures can be modified to make their roof construction appear larger (photo 12). Also, the painted wood decking above the exposed rafter tails appears to be OSB.
- You determine from the perspective of a team assigned to roof ventilation that strip/trench ventilation on the roof of the single-story building and close to the two-story structure would limit any horizontal extension of fire across the attic of the one-story structure.
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Ladders. Consider two ladders per roof to be the minimum for roof operations. In this scenario, ladder away from the fire (right side of structure), which allows you to start from the strong area and work back to your means of egress. Always know where and how to exit the roof and whether or not you have an alternate means of escape.
Basic equipment. The following tools and equipment are recommended as a minimum for performing vertical ventilation: full protective clothing; breathing apparatus; pickhead ax; pike pole; rubbish hook or other suitable tools such as a halligan or a hook; a power saw (chain or rotary); and a portable radio. When taking power equipment aloft, work in teams of two as a minimum, and emphasize safety.
First member to the roof. The first ventilation member to a roof should have a sounding tool. Is the roof strong enough to support the weight of personnel, or has the decking been burned away under the roofing material? Always evaluate the integrity of a roof before stepping onto it. Additionally, when the first ventilation member sounds a roof to determine its safety, that member also determines the path of travel for other firefighters.
Read the roof. Before venturing off the ladder, personnel must take the appropriate time to read the roof. In this case, the following factors are visible:
- The roof is not of sufficient pitch to require a roof ladder.
- The roof is covered with composition shingles.
- Smoke of moderate color, density, pressure, and volume appears to be issuing from a roof vent close to the two-story structure.
- There is a good chance of extension of fire from the two-story structure into the one-story structure.
- You remember to consider that the building and roof may be under demolition, in addition to the fact that gravity wants the building.
Determine ventilation feasibility. Now, we get to the primary roof ventilation rule of whether you should be on the roof or not; this can be termed “Determining Ventilation Feasibility.” Prior to committing personnel on a roof for ventilation operations, personnel must (not should, as the word denotes optional) determine whether or not ventilation operations are feasible. This entails knowing the type of roof construction and the location of fire in relation to the roof structural members and roof personnel. When these two factors are known, then roof personnel should have a basic idea of the structural stability and amount of time available for a roof ventilation operation. Since the introduction of lightweight construction, the days of randomly walking out on a roof and cutting a ventilation opening are over!
Determine the type of roof. Before personnel venture away from their ladder, it is essential to know what the roof supporting the ventilation team is comprised of (if you don’t already know from your size-up). Guessing can be deadly. Some items to consider are type of roof—conventional or lightweight construction; roof covering—slate or tile, which can make it difficult to traverse; and other similar considerations. To underscore an important point, consider the following: A flat roof is a very common roof. It may be found in many areas, usually covered by composition-type materials. However, unless personnel are familiar with a particular building and roof, the structural members that support it are an unknown. The roof may be of lightweight concrete, 2- × 12-inch joists and 1- × 6-inch sheathing, 2- × 3-inch I-joist and ½-inch OSB, or open-web construction covered by corrugated metal. All of these roofs may look similar from the top, yet each will react differently when exposed to fire. Therefore, before committing personnel on an unknown roof, determine the type of roof and whether it will provide adequate time to perform the intended ventilation operations.
This can be easily accomplished by taking a plug out of a roof to help you determine the type of roof construction. A plug is a small triangular piece of composition (only) cut with a hand tool or power saw and removed to reveal the type of roof decking below. As an example:
- If the plug reveals corrugated metal decking (photo 13), the roof is probably a metal deck, built-up roof, which in reality is a metal lightweight roof.
- If the plug reveals 1- × 6-inch sheathing (photo 14), the roof is probably of conventional construction, as 1- × 6-inch sheathing has not been used for more than 50 years. Additionally, multiple layers of composition material should also be visible.
- If the plug reveals ½-inch plywood (photo 15) or OSB, the roof is likely a lightweight roof. Additionally, expect to see minimal layers of composition material. Although in this scenario you noticed the exposed 2- × 4-inch rafter tails from the ground, you made a quick check on the roof and noticed OSB decking with one layer of composition. You quickly determined you are likely standing on 2- × 4-inch trusses covered with ½-inch OSB.
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Determine the location and extension of fire. Prior to initiating ventilation operations, you must determine the location and extension of fire (if you don’t know). What did you see in your initial size-up? What areas of the roof are issuing fire or smoke? Ventilators, vent pipes, skylights, heat blisters, and melting snow are excellent indicators. Consider the color, temperature, and pressure of any visible smoke. If necessary, make a small opening by your means of egress with a power saw or an appropriate hand tool. Make a small (small is defined as not large enough for a firefighter to inadvertently step into and break an ankle, foot, and so on) hole in the roof decking to see whether smoke or fire is below. Such an opening is often referred to as a kerf cut, an indicator hole, or an inspection hole. If any smoke is present, consider its characteristics. For example,
- A lack of smoke would be an indicator that fire is not in the immediate area (photo 16).
- Cold, white smoke would indicate that fire isn’t in the immediate vicinity of the opening (photo 17).
- Hot, black smoke under pressure is a positive indicator that the fire is near (photo 18).
- Fire is an indicator that you may be in the wrong place at the wrong time (photo 19).
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For this scenario, let’s assume you make a quick indicator opening and notice white smoke with moderate pressure and little heat similar to photo 17. You now initially know the status of the attic (in your area) has not been compromised and you can proceed with your ventilation operation. However, it can be advantageous to make occasional indicator openings as you approach the intended area of your operation to verify the status of the attic.
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From the opposite point of view, let’s assume you initially make an indicator opening and see what is shown in photo 19 or possibly photo 18. In this case, roof ventilation is not a safe operation, and your next priority would be to exit the roof in a timely manner. If you think making indicator openings requires too much time, it is preferable to falling into a burning structure with a collapsing roof.
Walk the strong areas of a roof. Ventilation personnel should always concentrate on walking the strong areas of a roof, whether during fireground operations or training evolutions. In this scenario, it would be the perimeter or the ridge of the roof.
Sound your path of travel. It is possible for plywood/OSB decking to weaken or burn away without burning through the composition covering. Therefore, although a roof may look normal from the top, it may not support any weight. Keep the sounding tool in front of your body to provide maximum stability in case you encounter a weak portion of the roof. Sound the roof along your entire path of travel.
Work from the weak area to the strong area/means of egress. Start ventilation cuts in the weakest portion of the roof (near the fire), and work away toward the strongest portion (unburned section). Spend as little time as possible on the weak portion. In this scenario, you would start as close to the two-story structure as possible and work back to your ladder.
Keep the wind at your back. When possible, plan ventilation cuts so the wind is at your back, with smoke and heat moving away. Today, personnel are plagued with the by-products of plastics, which have been proved to be carcinogenic. Consider using breathing apparatus when ventilating while in smoky environments.
Cut only as deep as necessary. Personnel using power saws for ventilation purposes must control the depth of the cut and know what is needed to accomplish specific ventilation operations. Unless otherwise necessary, make ventilation cuts through the roof decking only. Cuts deeper than the roof decking increase the possibility of severing structural members. For this roof in our scenario, a cut deeper than two inches can begin to compromise the structural members of the lightweight roof.
Placement of ventilation openings. Offensive ventilation openings are normally placed over or as close to the seat of a fire as possible. Defensive ventilation openings are placed ahead of a horizontally extending fire to change the horizontal direction to a vertical direction. In this scenario, let’s assume the roof ventilation team discovers that fire is just starting to extend into the common attic of the single-story structure. Cut a strip/trench cut in the roof of the single-story structure as close to the two-story structure as roof integrity will allow. If there is one advantage of lightweight construction, plywood/OSB materials can be rapidly cut with power saws.
Note: The success of strip/trench ventilation openings depends on completing the cut from one side of the building to the opposing side and coordinating with engine company operations to ensure personnel can pull the ceiling area under the strip/trench opening so a hoseline can be used to extinguish any visible fire and/or begin to extinguish the fire back to its source. A strip/trench opening without a coordinated attack from below can be ineffective!
Don’t be a roof shepherd. When roof ventilation operations are finished, personnel should not spend unnecessary time admiring their handiwork (photo 20); there are stronger and safer areas to rest and other fireground priorities that likely need to be completed. When roof operations are completed, exit the roof in a safe and timely manner.
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Path of egress. Always know how to exit a roof safely. Generally, leave the roof the same way that you climbed onto it. Unless conditions have changed, consider the original path to be the best way off. If your path to the roof was safe, it should still be safe for your exit from the roof.
Thermal imaging camera. More departments are successfully using this tool for roof operations. It can locate an area under a roof that is exposed to fire and also some structural members (larger members retain more heat than their surroundings). However, the value of this tool is based on the expertise of the operator.
JOHN MITTENDORF was a 30-year veteran of the Los Angeles City (CA) Fire Department when he retired as a battalion chief. He has an associate degree in fire science and has nationally and internationally instructed in fireground operations for the past 25 years. He was the recipient of the Fire Engineering 2008 Lifetime Achievement Award and is a member of the editorial advisory board of Fire Engineering. He is the author of Truck Company Operations, Second Edition (Fire Engineering, 2010) and presenter of the forthcoming DVD Ten Commandments of Truck Company Operations—An All-Day Seminar.
Modern Wood an Additional Fireground Hazard?
Most firefighters are well aware of the inherent hazards of lightweight construction, as it has been commonly used by the building industry in various configurations since about 1960. These hazards are a result of the reduced size of structural members in tension and compression configurations and the use of gang-nail plates or glue instead of 8⁄16 penny nails or steel plates and bolts in older truss construction.
However, it is easy to overlook the fact that the wood currently being used is significantly different from the wood that was used in older construction. From a logging perspective, old-growth trees are just a memory, with “new-growth” trees (or second-growth trees) and “plantation trees” normally replacing the older trees. This has resulted in wood that is not only different but that also burns significantly hotter and faster. Today’s dimension lumber is smaller than in the past: 2-inch × 4-inch actually measures 1½-inch × 3½-inch.
Interestingly, the lumber industry, in about 1986, changed its rating system from Utility (utl), Standard (std), Construction Grade (cons), and Select (sel) to #3, #2, and #1 (best).
Old-growth trees produced a wood that was denser and had a reduced level of pitch (which burns like a petroleum-type product). Additionally, it was not uncommon for wood—particularly wood that was to be used for structural members—to be cut from the “heart” of the tree (which was commonly hundreds of years old—hence, its maximum density and strength). Interestingly, Douglas fir was the standard for exterior bearing walls (the good stuff), with white fir or hemlock used for interior walls (the cheaper stuff). Conversely, new-growth trees are less dense and have a higher concentration of pitch compared to old-growth trees. This has resulted in wood that is lighter in weight, is capable of burning more rapidly and producing a larger amount of heat energy, and in “cheaper” types of wood being used for structural members. When these factors are combined, it is no surprise that modern buildings built with this type of lumber used in various lightweight configurations are collapsing faster and burning hotter than the buildings of yesterday.
