Diminishing fire department budgets and cuts in manpower across the country are forcing today's fire companies to become leaner and meaner. In striving for efficiencies to maximize their impact on the fireground, companies often times look for shortcuts that are quick and easy, compared to more time consuming traditional methods or tactics. This is not necessarily a bad thing, but if the overall benefit of a shortcut is a compromised result, the shortcut may actually be less efficient than a more time consuming traditional tactic.
This is especially true when making decisions about tactical ventilation and how to apply it to the many different types of structures you might have in your first due area. Is it enough to take some windows? Maybe a gable or two? Can I hold off on the roof, or do I need to immediately commit initial truck crews there? Since the scope of building types is much too large to be covered in a particular article, the focus here will be on some of the typical types of peaked-roof private dwellings you may encounter.
Before committing any resources to vertical venting, you should have a firm grasp of exactly what you are hoping to achieve. To that end, revisiting the four effects a roof vent can have on a fire, while very basic, will help in understanding the potential interaction of ventilation vs. structure type.
The four effects you are trying to achieve with vertical venting are as follows:
Changing momentum - Creating a chimney effect; helping to centralize the fire and thus reducing horizontal spread or mushrooming of both fire and super heated gases.
Exhaust - Creating an opening which can vent both the products of combustion and extinguishment, allowing the interior crews to make their push.
Lifting effect - Alleviates untenable conditions within the structure allowing fire crews to advance, or buying time for victims. To realize an actual lifting of the super heated atmosphere - and subsequent heat layers - the hole needs to be very large and opened quickly once it is begun. The importance of this to interior crews cannot be overstated, as it will give them the extra time and the "headroom" needed to make even the most difficult of fires. The lifting effect is the most difficult to achieve and should be the ultimate goal of any truck crew going to the roof.
Controlling Explosive atmospheres - since this effect is relatively rare, it is not generally the main focus of a roof vent, but when it is, it is the most important effect. This includes smoke explosions and back drafts, and really highlights the advantages of vertical ventilation and its ability to allow violent and sudden releases of energy to be vented away from fire crews operating within a structure.
Different house types will lend themselves in different ways to vertical vent. Ranch homes will mostly benefit from the chimney effect of a vertical vent, it's not that the other effects won't take place, just that they can mostly be achieved by taking windows and not the more time consuming, manpower intensive act of opening the roof. Conversely, merely taking windows on a deep, steeply- peaked roof space will not provide adequate effect and relief; therefore, a greater efficiency can actually be realized by opening the roof, even though it will require more time and manpower.
You should only be going to the roof if the fire is on the top floor (this would include the first floor in a one story house), the attic or both. Going to the roof for basement (yes, even balloon frame), room and contents or rubbish fires is generally a waste of time. Basic criteria should include:
The fire has grown beyond the room of origin;
The fire is in the attic/crawl space or;
The fire is immediately below the attic/crawl space and has likely extended into the attic crawl space.
Vertical ventilation, when properly applied and executed, remains the Gold Standard of ventilation; there is simply no better way to predictably remove superheated gases and products of combustion than making an opening at the highest point directly above or closely adjacent to the fire.
With this in mind let's take a look at some basic home styles ranging from small ranch homes - with minimal attic spaces - to larger multi-story homes with large, deep,walk-up attics. Each type of home has been given a priority rating based on its construction characteristics, and the immediacy of conducting vertical ventilation.
Very small/small single story ranch house with minimal crawl spaces
Small ranch houses are prevalent throughout the Country, and account for a large number of rural homes. Barring backdraft-like conditions, there is very little advantage to committing crews to the roof. It is much more productive to utilize existing windows for ventilation -then getting inside and pulling ceilings. This strategy should enable engine crews to make their initial push and extinguish any remaining fire which has extended into the attic space.
Medium and larger ranch homes
As ranch houses increase in size, so does the probability that vertical ventilation will result in greater benefit. This is due mainly to the extensive, but shallow nature of the peak spaces on ranch homes. Fire which has flashed-over, and begun to breakout from the room of origin, will have most likely started to fill and pressurize the small attic space with superheated gases and other products of combustion. This will pre-heat the attic space making it vulnerable to rapid fire spread once it is exposed to the fire from below. While venting vertically will also ignite these built up fire gases, most of the energy will be vented safely away from crews on the interior. In addition, fire spread will be minimized in this space due to the chimney effect that a vertical vent hole provides. While the priority remains relatively low compared to large, deep peak-spaced structures, there is still value in getting truck crews to the roof and opening up.
One story with half attic
This occupancy, while similar to a one story ranch, differs in that it has a solid tongue-and-groove attic floor. This means pulling ceilings to expose the attic will no longer be an option. Access to the attic space will have to be ga1ned through a scuttle or a pull dov.11 stam::ase. This also means vertiCal vent becomes a higher priority, because crews will have to at least partially enter the crawl space to extingu1sh the fire.
1 1/2 story with small crawl space
MODERATE - HIGH
The sole factor keeping this type of building from always being a high priority is that they are generally smaller homes with smaller peak spaces. Otherwise, the need for vertical ventilation on these types of structures increases over a ranch style house for a number of reasons:
1: Lack of windows. There are very few windows along the sides of these two buildings which means even a fire on the first floor will probably not be adequately ventilated merely by taking out windows. It is because of this: much more fire effluent will be forced into the second floor than normal with a subsequent increase in both heat and smoke volatility.
2: Sharing the roofline. Interior crews making the push on the second floor will be operating within the peak space. Operating at the same level as the roofline means that there may be no windows at all, and the funneling effect of the roof will ensure a rapid build-up of volatile, superheated atmospheres in the peak space and second floor.
3: Knee walls. When the roofline overlaps the floor level, as it does in these two structures, there will be often be knee walls that run the length of the building or a good portion of it. Sufficiently opening up knee walls from the interior is usually difficult and awkward, especially when there is active fire. Knee walls are notorious for fooling crews into thinking fire conditions are not as bad as they really are. This can lead to a false sense of security until the knee wall fails or is breached and rapidly changes conditions from light smoke and minimal heat to high heat and untenable conditions.
Two story with small crawl space
Even though this is a two story home, it can basically be treated as a one story ranch home in regards to roof ventilation. The small shallow attic space means fires on the second floor usually can be adequately ventilated and controlled by taking second floor windows and pulling ceilings.
2 1/2 Story with full walk-up attic
HIGH - VERY HIGH
O.K., who's going to the roof? Fire in the attic of these types of building will mean one thing: truck crews are going to have a long day. Huge attic and peak spaces, and minimal windows fed by big fire loads, require truck crews to act quickly and get to the roof before interior crews begin to make their push. The decision of whether or not to go to the roof is greatly simplified: If the engine is going in then you're going up. You will need to maximize all three effects (momentum, exhaust, lifting) if engine crews hope to be successful. The best way to do this is with good vent placement and big holes (forget the 4x4 holes, little holes=little results) Think BIG! 25-30 sq. ft. or more.
This article is not saying to never go to the roof on low priority structures. Rather, there may be better ways to deploy your initial manpower that can be faster and equally effective. Trying to achieve the highest efficiencies with available manpower should remain the goal of even the most well staffed departments. Knowing when you can skip the roof, or at least save it for later arriving companies, is a good start in attaining that goal and maximizing your efficiency. There are many variables involved in making decisions with regards to tactical ventilation: the first and most important step will be in recognizing the structure on fire and understanding how the attic and potential void spaces will influence these same tactical decisions. Hopefully this article has offered a good basis to formulate initial tactics and recognize when less is more or when only more will be adequate.