By: David Rickert
Prevalent throughout large cities in the Midwest and northeast, the two and a half story wood frame with a walk-up attic has been a fixture for over a century; and these structures present significant challenges for fire crews not commonly found in other types of structures.
There is a saying in the fire service, “as goes the first line so goes the fire;” in the case of the full attic fire, you can amend that to: “as goes the first truck so goes the fire.”
Achieving a successful outcome for these types of fires relies heavily on aggressive, coordinated truck work; so much so, that incidents in these structures are regularly referred to as, “truck fires.”In other words, an aggressive stance, which includes roof ventilation, should be undertaken by the first due truck company.
It is in fact paramount to successfully dealing with walk-up attic fires.
To gain an understanding of this type of fire ,one must first look at some of the properties and unique dynamics created by fire in a large peaked roof space.
Peaked Roofs
First and foremost, unlike fires on other floors, the peaked-roof creates a large area (the peak-space) for: gas-phase fuel, unburned pyrolizates, and flammable products of incomplete combustion to gather. This peak-space or accumulation zone (fig. 1) is an ideal place for an over-rich atmosphere to develop, and is notoriously hard to ventilate horizontally. The only safe and reliable way to get rid of these hot fire gases is to create a large opening above them. The size of this accumulation zone varies, and is a function of the distance from the peak to the top of the attic windows. This can be referred to as the “drop down.”(fig. 1) The drop down is the best way to quickly gauge the peak-space and determine the criticality of effecting rapid, topside ventilation. This distance can vary widely but is usually from four to ten feet. (On the model the drop down is seven feet ).It becomes this peak-space, or accumulation zone, that creates the greatest hazard.
Attic Windows
This graphic assumes fire involving the front of the attic and highlights some basic behavior: The fire has self-vented out the attic windows in front, grown quickly beyond control of the window, and forced into the peak-space. Fresh air can be drawn from a number of sources including: stairwells, between studs of balloon frame construction, attic windows, eave areas damaged by animals or neglect, and floor areas breached for chimneys, pipe chases, ducting, or conduit. This is important because it will determine not only how quickly the fire grows, but also how quickly an over-rich atmosphere will develop. The hottest, most turbulent atmosphere will be located directly above and adjacent to the fire. As one moves away from this point, the atmosphere, while still superheated, will become less turbulent and begin to stratify with the hottest least dense fire gases at the peak, and the coolest, densest smoke closest to ground level.
Even if the peak-space is ignored for a moment, window size and number are usually woefully inadequate to properly vent a fire in a full-sized attic. When most of these homes were built ,the attics were not intended for habitation. Furthermore the windows in these attics were incorporated for aesthetics, without any consideration given to purpose or practicality. It is not uncommon to have very large and elaborate windows toward the front or street-side of a structure, and have small or non-existent ones in the rear of the building. In addition, window placement is limited to the ends of the gable or house.
There is a common misconception that when heavy fire is venting from an attic window, no further ventilation is necessary, when in fact the opposite is true. This falls under the category of mistaking SOME ventilation for ADEQUATE ventilation; and is an all too common mistake when sizing up fire conditions in the attic space. Fire venting from a window should be used as an indication or indicator, rather than a contraindication that further venting is necessary. The reason for this is: venting of the window has now increased the volume and intensity of the fire to the point that the by-products of combustion(flame,superheated gases and smoke), will overwhelm the window opening and enter into the peak-space. This will rapidly degrade conditions and involve the rest of the attic. In addition, assuming the heat of the fire was hot enough to break the window and/or self-vent means the peak-space has already become fully- charged with smoke and heat. As such, this is not only an inaccurate or erroneous statement, but a dangerous assumption.
Large common Spaces
Attics are generally comprised of a single, large common area (floor to peak, front of the house to the rear.) When unfinished, these attics have a large amount of exposed, century old wood (joists, rafters, roof, and floor boards).This makes them capable of releasing huge amounts of heat and smoke during a fire; consider a barn fire on top of a two story house. This is significant not only in the amount of fire and heat that can be generated, but also critical fire behavior such as rapid fire events that will often involve or affect the entire attic space simultaneously. This is not a room fire, but rather an entire floor fire.
The presence of these peaked-roofs, attic windows, and large common spaces described above are the main reasons that truck crews need to consider roof ventilation a primary rather than a secondary concern during these types of fires. There are other considerations; however, barring a forcible entry problem that the engine cannot handle, or an immediate life hazard, NONE are as important as getting a truck to the roof and vertically ventilating.
Two questions: “What do we hope to accomplish with vertical ventilation?” And, “what are we really accomplishing by performing roof ventilation?” The simple answer of course is: to localize the fire and vent excessive heat, smoke and fire gases; and for real-world firefighting, maybe that is all we need to know. For the purposes of this article however, examining some of the lesser-known phenomena will serve us well in fully understanding why vertical ventilation is the keystone of a successful peaked-roof strategy:
Depressurize the space: A well-involved attic fire will pressurize the attic. This pressure will not be constant, but vary widely throughout the attic space causing the smoke and fire contained within to behave erratically; and forcing smoke and heat into places it would not normally exist. By creating a sufficiently-sized opening above the fire, you effectively depressurize the attic space and remove some of the unpredictability and aggressiveness of the fire.
Stabilize the atmosphere: The attic will contain smoke and fire gases in various states of flux, many of which are highly flammable. Moreover, these will be turbulently mixing within the attic space. This creates a highly unstable atmosphere. Highly unstable atmospheres are associated with all extreme fire events (flashover, backdraft, rapid fire development, smoke explosions, etc.). This unstable atmosphere if left unventilated or under-ventilated, will present many challenges to the advancing engine company. Introduction of hose-streams into such atmospheres can have unpredictable and sometimes disastrous results. Venting the roof accelerates the fire but at the same time removes many of the destabilizing elements from the peak-space. A well-vented fire is: more predictable, easier to locate, control, and ultimately extinguish.
Change fire momentum from horizontal to vertical: Even when attic windows are venting, the fire and associated hot gas layer is generally moving along a horizontal plane within the attic. It can move rapidly across this space and bank untenable heat right down to floor level. Venting the roof adds an important vertical element to the equation; and when it is of sufficient size, will change the momentum of the fire from horizontal to vertical, giving the engine crew critical headroom to operate safely and extinguish the fire.
The hole
Make it right: Locate the hole in the right place at the right time;
Make it big: Three or four foot ‘teepee,’ or ‘coffin-cuts’ will generally yield openings below ten square feet. This is better than nothing; however it will not be entirely adequate either. Cutting from the aerial or the ridge, or trying to reach out from a single roof ladder on steep roofs is generally: unproductive, dangerous and inefficient. Utilizing: proper techniques, holes as large as thirty, forty, or even fifty square feet can be accomplished quickly, even on steeply-pitched roofs;
Make it once: One big hole in the right place should do the trick, although there are exceptions to this rule;
Make it off: Once opened up, get off the roof; you can always go back up once the fire is knocked down; It may not always be possible to place the hole directly over the fire. This does not mean that vertical ventilation should be abandoned. Rather, a large benefit can still be gained by opening the hole in reasonable proximity to the fire, as long as it does not draw the fire into or on top of the crews operating inside. In other words, make sure the crew is not in between or directly under the hole you are opening and the fire. This is why it is critical for the roof crew to coordinate operations with crews inside the fire building;
Coordinate: This includes communication, timing, and location;
Initiate: Begin making your opening;
Terminate: Complete the opening and get down.
The Math
Part I
A simple breakdown of some of the numbers shows just how critical ventilation can be for attic spaces. All numbers in Part I are based on the model pictured above:
Entire attic space – 13,500 cubic feet
Average sized room (15’ x 12’) ceilings – 1620 cubic feet
Peak-space/accumulation zone¹ of the attic – 2940 cubic feet
Dead-space/accumulation zone² of averaged size room – 240-360 cubic feet
Total volume of the attic is eight times that of the average sized room.
The peak-space/accumulation zone of the attic is ~eight to twelve times that of the dead-space/accumulation zone in the average sized room.
¹ Peak space/accumulation zone is the hard-to-ventilate area above windows in the attic.
² Dead space/accumulation zone is the hard-to-ventilate area above windows on all other floors.
THIS IS NOT JUST ANOTHER ROOM AND CONTENTS FIRE!!
Part I gives you a good idea of the volumes you may be dealing with in a walk-up attic vs. the average size room. Part II will address the size of windows vs. volume of the space to be ventilated; and note this as a ratio, which may be referred to as the potential vent ratio (PVR):
Part II
Modeling a house’s exterior and then deriving numbers from the model is relatively easy and accurate. Modeling an entire house’s interior and exterior, top to bottom, and then assigning dimensions to rooms and windows becomes a bit too complicated and hypothetical. Part II bases all measurements on an actual house. The house is pretty typical for the type (100 years old, full attic, balloon frame, with a twelve-twelve [12:12] pitched-roof) and should yield some reliable averages when determining the numbers and derived ratios.
• Only rooms were measured; hallways, closets, landings and stairways are not included.
Potential Vent Ratio Graphic:
The chart essentially reflects what you’d expect: The lowest PVR numbers (highest potential ventilation) are on the 1st floor, which is mainly occupied during waking hours when natural lighting (windows) can best be utilized. Furthermore, higher PVR numbers (less potential Ventilation) are found on the 2nd floor where the bedrooms are located; and the highest PVR number (least potential ventilation) is found in the attic space.
The Numbers’ Specifics:
The first floor of a structure will offer the best chance of affecting rapid and adequate ventilation due to numerous and accessible windows.
The second floor still presents good ventilation potential, albeit with a few more obstacles and restrictions due to less numerous and accessible windows.
The attic, even with windows vented, will make advancing hoselines and extinguishing the fire exceedingly difficult; adequate ventilation can only be achieved by creating new vent openings, preferably at the highest point: the roof.
The PVR of the attic is eight times the room average on the first floor, and six times that of rooms on the second floor. To achieve an equivalent PVR for the first floor would allow for only two, five by two foot windows!
The attic has thirty-two square feet of potential vent space (windows); we can double that number by making a good sized roof opening, which will essentially halve the PVR, and create an opening in the best possible location.
The main limitations of the PVR ratio are :
It does not address window location.
You could have similar PVRs, but also have very different ventilation profiles based on the location of the windows (i.e. a window located at the top of the wall will vent much more efficiently than a window located near the bottom) but for the most part, it is safe to assume that windows not in the attic will be located somewhere near the center portions of the wall. Therefore, with regards to floors located below the attic, there is a similar point of reference.
It cannot be determined when units pull up on-scene. As an initial size-up tool, it is relatively worthless. However, as a postmortem, “what went wrong” evaluator, it may be pretty enlightening.
Since this is an article on walk-up attic fires, I hesitate to read too much into the implications the PVR may have on other firefighting efforts, but the implications for attic fires are VERY CLEAR: additional venting is needed.
Summation:
While the assumptions of fire behaviors and phenomena described in this article come largely from observational experience, and not from specific scientific study or testing, it in fact becomes empirical when the same behaviors and results are witnessed time and again during fires involving large attic spaces. Henceforth, a strong case for early commitment to the roof during full attic fires has been made. Aggressive action and decision making by truck crews, properly executed, should payoff in: higher success rates, safer and faster knockdowns, reduced overhaul in both time and effort, and less time on-scene.
