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Reading the Fire: Building Factors

First of the critical building features is construction method. Building construction influences both fire behavior and structural stability under fire conditions. Combustible construction such as wood-frame, ordinary (masonry and wood), and heavy timber construction contribute to the fuel load, while non-combustible and fire resistive construction do not.Slideshow Images:

As discussed in the previous article, fire behavior indicators can be grouped into five general categories: Building, Smoke, Air Track, Heat, and Flame (Figure 1). A simple mnemonic for remembering the categories is B-SAHF ("be safe").

This article focuses on the only set of factors that is available before the fire starts; the building and its contents.

Building Factors Overview

Within the building category, it is important to consider structural elements (i.e., floor and roof support systems), non-structural elements (i.e., compartmentation, interior finish), contents, ventilation profile (i.e., building openings; heating, ventilation, and air conditioning (HVAC) systems), and the size of the building. The interrelationship of these factors is graphically illustrated in Figure 2.

This article provides an overview and examines a number of important building factors to consider as part of pre-incident planning and incident size-up. Practice reading the building under non-fire conditions to identify critical characteristics that will influence fire behavior and structural stability if a fire does occur.


First of the critical building features is construction method. Building construction influences both fire behavior and structural stability under fire conditions. Combustible construction such as wood-frame, ordinary (masonry and wood), and heavy timber construction contribute to the fuel load, while non-combustible and fire resistive construction do not. It is important to note that construction classification (i.e., fire resistive, non-combustible) refers to structurally supporting materials, not interior finish and roofing material that may contribute substantially to fire load.

In examining the influence of construction on fire development and spread it is also important to consider the structural void spaces as a path of fire travel and as compartments which may present different fire conditions than encountered in adjacent spaces.

Older wood-frame structures may be of balloon-frame design. This type of wood-frame construction provides a ready path of travel for fire that breaches compartment walls or ceilings, or that originates in an unfinished basement. Newer wood-frame construction is generally of platform frame design, and has less potential for fire spread from floor to floor through structural voids. However, structural voids can still present a significant hazard as evidenced by the smoke explosion in a Wyoming apartment that resulted from accumulation of flammable products of combustion and pyrolysis products in the trussloft (Hartin, 2006; NIOSH, 2005). Figure 3 illustrates the same type of "room in attic truss" that was involved in this incident.

Even voids in non-combustible construction can significantly influence fire behavior and present a hazard to firefighters. In 2003, two Memphis, TN, firefighters died in a collapse subsequent to a ventilation induced flashover or backdraft in a non-combustible commercial building (NIOSH, 2004). Extremely hot flammable products of combustion and vapors from asphalt roofing material accumulated in a void space between the ceiling and metal deck roof (Figure 4). This hot fuel ignited when firefighters opened the suspended ceiling during firefighting operations.

It is more difficult to recognize the presence of void spaces during firefighting operations than before the incident. Studying buildings and identifying potentially problematic construction features during informal or formal preplanning is important to understanding fire development and spread.

Non-structural elements such as interior finish can also have a significant effect on fire development. On November 28, 1942; a fire in Boston's Coconut Grove Nightclub claimed the lives of 492 people (Benzaquin, 1959). The extreme number of fatalities in this incident resulted from rapid fire spread due to combustible interior finish and inadequate exits. Similarly, on February 20, 2003; 96 people died in the Station Nightclub fire in West Warwick, Rhode Island (NIST, 2005). In this incident, combustible acoustical foam ignited by pyrotechnics and combustible interior finish along with lack of an automatic sprinkler system contributed to the speed of fire growth and development. Figure 5 illustrates temperatures five feet above the floor in the Station Nightclub 90 seconds after ignition.

When examining non-structural elements as fuel, it is important to not only look up and look around. Look down at the floor. While hot gases rise and convection will generally result in vertical and lateral heat transfer, radiant heat from the fire and the hot gas layer also transfers heat energy to the floor. Like other hydrocarbon based synthetic materials, carpet and carpet padding may have a much higher heat of combustion than wood and can be a significant source of fuel (Division of the State Fire Marshal, 2002; FBU, 1996).

As discussed in factors influencing fire development, insulation and energy efficiency of the structure will have an influence on fire behavior. However, like many other building factors, thermal characteristics of the structure may not be readily visible during firefighting operations. Insulation is designed to reduce heat transfer through the building shell or other structural elements. Insulation is normally intended to retain building heat when ambient temperatures are cold and slow heating when ambient temperatures are hot. Insulation performs the same way under fire conditions. A well-insulated compartment will retain more heat, increasing the speed of fire development (all other things being equal). Reduced building leakage and use of multi-pane glazing in windows also reduces leakage and potential increases in ventilation, speeding the transition to ventilation controlled conditions and increasing the probability of developing backdraft conditions.

Building Contents

A large percentage of compartment fires simply involve contents and non-structural elements such as interior finish of the ceiling, walls, and floor. Key characteristics influencing fuels' burning characteristics include state (solid, liquid, or gas), chemical composition, and distribution. Building contents can include gas, liquid and solid fuels. However, most ordinary contents are in the solid form.

The chemical composition of fuel influences its heat of combustion (the amount of heat released by a given mass of fuel) and the heat release rate (the speed with which that heat is released). Oxidation of a specific amount of fuel (i.e., kilogram) releases a given amount of heat energy (i.e., kilojoules). Kilojoules/kilogram (kJ/kg) is a standard unit of measure for heat of combustion. A fuel's heat of combustion is dependent on its chemical content. The heat of combustion for hydrocarbon fuels such as plastics, gasoline, propane and methane can be considerably higher than that of cellulose fuels such as wood (DiNenno et al., 2002) as illustrated in Figure 6.

The heat of combustion for wood and paper varies with the specific type of material that is burning. The heat of combustion for plastic materials also varies with the specific material involved. However, as noted in Figure 6, some types of plastic have a heat of combustion approaching that of hydrocarbon fuels such as gasoline, propane, and methane. These fuels release considerably more heat energy than ordinary combustibles such as wood and paper.

While the total heat energy released when fuel burns is important, the rate at which it is released is also significant. Heat release rate (HRR) is the energy release per unit of time and is usually expressed in kilowatts (kW). A kW is 1000 joules per second (J/s). Heat release rate is dependent on the type, quantity, and orientation of the fuel as well as the characteristics of the enclosure (if the fire occurs inside a compartment).

The minimum size fire (expressed in terms of heat release rate) that will cause a flashover in a given room is dependent on compartment size and ventilation. HRR varies over time, increasing as more fuel becomes involved and the temperature in the compartment increases (higher temperature increases the speed of the combustion reaction). HRR decreases as fuel is consumed and temperature in the compartment decreases.

As illustrated in Figure 7, the small wastebasket does not release sufficient heat energy to cause a 16' x 20' (4.88 m x 6.10 m) room to reach flashover. However, the rate of heat release from the wood and polyurethane sofa is more than sufficient to result in flashover.

A number of factors influence ignitability and heat release rate of solid fuels, in addition to the fuel type; thermal thickness, and configuration are also significant. When heated, the temperature of thermally thin materials will rise quickly. With thermally thick materials, surface temperature will increase, but the internal temperature will rise more slowly. Thermal thickness is dependent on physical thickness and thermal conductivity. Materials that are physically thin and/or have high thermal conductivity are thermally thin. Materials that are physically thick and/or have low thermal conductivity are thermally thick. Thermally thin materials heat and reach their ignition temperature more quickly than materials that are thermally thick. Regardless of the thermal characteristics of the fuel material, the more surface area exposed to heating the more rapidly it will reach its ignition temperature. As previously discussed in relation to building construction (e.g., heavy timbers vs. light weight trusses), surface to mass ratio can be a significant factor in fire development.

In the wildland environment, firefighters recognize the significance of horizontal and vertical continuity of fuel materials. When fuel consistently covers the ground, fire can spread more readily. When there is continuous fuel from the ground up to the level of trees fire can quickly spread vertically from the ground to aerial fuels. Structural firefighters face the same issues. However, in a compartment it is critical to examine not only fuel on the floor (the ground), but also fuel on the walls and ceiling (contents, interior finish, and structural materials).


At the most basic level, size involves area and height. However, it is important to consider the impact of compartmentation. For example, a building is 50 feet deep x 100 feet wide and 30 feet tall. Would the fire problem presented be different if this was a single compartment as compared to a three-story building? What if each floor was open as compared to being subdivided into small rooms?

Compartmentation (or a lack thereof) is often recognized as a factor in high rise buildings. However, many modern homes have considerably less compartmentation in living areas such as kitchen, dining room, living room, and family room etc. than older buildings. What impact might this have on fire development and fire control operations? When examining compartmentation within a building, don't limit your view to habitable spaces. Void spaces are compartments too!

Size is really about the volume of the compartment, not simply floor area. High ceilings increase compartment volume, providing increased oxygen for fire development and often masking developing fire conditions from firefighters operating at floor level. For example two Chicago, firefighters were killed by a backdraft in a high ceiling commercial building in 1998. Firefighters were operating in a large commercial building with a 20-foot ceiling height (bow truss roof). Firefighters inside the building observed smoke overhead, but conditions at floor level were dark (nighttime fire) but not smoky or hot. Unknown to the firefighters, backdraft conditions had developed in the hot smoke layer under the buildings bow truss roof. A change in ventilation resulted in a backdraft that killed two firefighters and injured three others (NIOSH, 1998).

Building and compartment size has a significant influence on both fire development and fire control requirements. Large compartments contain more oxygen and require more fuel and a higher rate of heat release to reach flashover. Fire development in a larger compartment may be slower (depending on the type and quantity of fuel available). When a fire in a large compartment progresses beyond the incipient stage, an increased flow rate will be required (fire flow requirements will be addressed in a subsequent article).

Ventilation Profile

For years, firefighters have learned that ventilation is "the planned and systematic removal of heat, smoke, and fire gases and their replacement with fresh air". However, from a building construction and indoor air quality perspective ventilation is simply the exchange of the atmosphere inside a building with the atmosphere on the outside to maintain a habitable and healthful environment.

Effective ventilation under normal (non-fire) conditions requires a regular exchange of air. Ventilation rates identified by building codes and related standards express the flow rate of outside air brought into the building and are typically expressed in terms of air changes per hour, floor area being ventilated (cubic feet per minute per square foot (cfm/ft2), or by the number of people being served (cfm/person). Ventilation may be accomplished using natural or mechanical means. Natural ventilation occurs primarily through open windows and doors and infiltration through cracks in the building envelope. On the other hand, mechanical ventilation involves delivery of outside air to the interior of a building through the use of fans and in many cases ductwork (e.g., heating, ventilation, and air conditioning (HVAC) system) (Lawrence Berkley National Laboratory, n.d.).

Ventilation profile is simply the existing ventilation and potential changes to ventilation that may occur due to fire effects or tactical action. Normal ventilation is designed to provide a healthful atmosphere for building occupants and is not sufficient to support fuel-controlled combustion, typically resulting in ventilation-controlled conditions at some point in fire development. Some building openings, such as windows may be more prone to failure under fire conditions than others. These potential changes are a critical building factor in predicting fire behavior. In addition, building construction has a significant influence on ventilation tactics. For example, consider the differences between wood, metal, and concrete roofs.

When a fire develops to the point where it becomes ventilation controlled, available ventilation will determine the speed and extent of fire development and in many cases the direction of fire travel. Under fire conditions current ventilation is based on the actual exchange of products of combustion inside the building or compartment with outside air. However, it is essential to recognize the potential for changing ventilation conditions during firefighting operations. Firefighters must consider the size, number, and arrangement of existing and potential ventilation openings (see Figure 8)

The photo series in Figure 8 illustrates changing ventilation profile due to fire effects. In the first photo the window is intact. The second photo illustrates increasing discharge of hot smoke. As the window begins to fail; hot, rich smoke ignites outside the window. After the window failed, the compartment flashed over, resulting in a fully developed fire in the compartment.

Fire Protection Systems

Fire detection systems such as smoke detectors increase the probability that firefighters will arrive early enough in fire development to encounter pre-flashover conditions (and that flashover may occur after the initiation of interior firefighting operations). While early detection and intervention is critical to occupant safety and an important step in reducing property loss, firefighters must recognize and mitigate the hazards presented by rapid fire development.

Automatic sprinkler systems have a tremendous impact on fire development and life safety. "When sprinklers are present, the chances of dying in a fire and property loss per fire are cut by one half to two thirds, compared to fires reported to fire departments where sprinklers are not present" (Hall & Cote, 2003, p. 2-21). The Station Nightclub fire in Rhode Island provides an excellent example of the impact of fire suppression systems. The National Institute for Standards and Technology (NIST) modeled this fire under two sets of conditions, first without automatic sprinklers (actual incident conditions) as previously illustrated in Figure 5. Second, fire development was modeled as if the building had been equipped with automatic sprinklers. Figure 9 illustrates temperatures five feet above the floor 90 seconds after ignition. Compare this to the temperatures modeled in the unsprinklered condition illustrated in Figure 5.

Fire Behavior Preplanning

Examining buildings and predicting potential fire behavior can be approached with varying degrees of formality. At the simplest level, firefighters should examine their surroundings on an ongoing basis. Look at buildings you visit for other purposes and develop the habit of identifying critical building features that will impact fire development. Think about how hot smoke and flames will move through the structure and how the fire will develop. On a more formalized basis, individuals (or even better) fire companies can develop fire behavior pre-plans as part of their study of fire behavior. This practice can and should also be integrated into development of target hazard pre-plans.

The first step in the fire behavior preplanning process is to gather information about the building. This will generally include the address, general description, construction type, occupancy, type of contents, building configuration (including extent of compartmentation), ventilation profile, and fire protection systems. Draw a simple floor plan and (if possible) take photos to help communicate your fire behavior preplan to others.

Basic information about the building and its contents provides a basis to predict anticipated fire development. Think about how fires would develop given different points of origin. Visualize fire development and the spread of heat and smoke inside the building. What are the likely avenues of fire travel? How might the ventilation profile change (is failure of window glazing likely?). Don't limit yourself to just one scenario; consider a variety of alternatives to build a more comprehensive picture.

Calculate the required fire flow (more on this in the next article). How might this influence your tactical operations?

Study and Discussion Questions

Use the information presented in this article to answer the following questions:

  1. What are the five categories of fire behavior indicators?
  2. What are the major building factors that impact on fire behavior?
  3. How do void spaces influence fire development and spread?
  4. How do efforts to increase energy efficiency impact fire development?
  5. Given that hot gases rise, how is it that carpet and carpet padding can be a significant contributor to fuel load?
  6. What is the difference between heat of combustion and heat release rate?
  7. What fuel characteristics influence heat release rate?
  8. What size related factors should you consider when thinking about fire development in a building?
  9. How does fire development differ when a compartment has a high ceiling?
  10. Briefly describe what is meant by the term "ventilation profile".
  11. How does the ventilation profile influence fire development?
  12. How do fire suppression systems such as automatic sprinklers impact fire development?

Fire Behavior Preplan Activity

Develop a fire behavior preplan for a building in your response area and share the information with other members of your department. Use the preplan to facilitate a discussion of how the building factors presented in this article apply to this specific building and occupancy.

Slideshow Images:


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  • Hall, J. & Cote, A. (2003) An overview of the fire problem and fire protection, in Cote, A. (Ed.) Fire protection handbook, (19th ed.). Quincy, MA: National Fire Protection Association.
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  • National Institute of Standards and Technology (NIST). (2005). Report on the technical investigation of the station nightclub fire, Report NCSTAR-2, Volume I. Gaithersburg, MD: Author.

