Editor’s note: The science of firefighting and flow path management has consumed much of our educational discussions for the past 10 years or so. This article, originally published in 2009, is a good reminder to not lose focus on the dangers of flashover, which of course can be managed somewhat through good flow path management.
Flashover remains a critical hazard, regardless of a firefighter’s experience or training level. Over the years, extensive education, training props, and live-fire demonstrations have been developed to teach the warning signs and survival techniques. However, despite these efforts, flashover continues to pose a significant risk. To effectively recognize and respond to it, we must have a clear understanding of what flashover is.
What is a flashover?
When a fire starts, it releases heat and light through convection, conduction, and radiation. Conducted heat from direct flame contact can cause rapid fire spread, especially with modern synthetic materials that burn hotter and faster. These fuels also create thick smoke—essentially unburned fuel—that spreads heat through the structure. Radiated heat reflects off walls, ceilings, and floors, intensifying the fire. As the fire grows, more surfaces reach ignition temperature. When everything ignites almost at once, it’s called a flashover—the transition from a growing fire to a fully developed one. It’s not a surprise event; it’s a predictable shift driven by heat and fuel.
NFPA 921 defines flashover as,
“A transitional phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach its ignition temperature more or less simultaneously and fire spreads rapidly throughout the space resulting in full room involvement or total involvement of the compartment or enclosed area.”
What factors are involved in a flashover?
Here are some, compiled from various sources:
- Location of fire
- Size of compartment
- Shape, area and volume of the compartment
- Heat gain and heat loss
- Contents and their relative surfaces, materials, etc.
- Ambient temperature at the beginning of the fire
- Location and height of doors, windows and openings
- Fire growth rate
In reading “Flashover and Fire Analysis” by Patrick and Kathryn Kennedy, we also learn that just because a fire leads to full-room involvement, it does not always mean the room flashed over. Reports point out that ventilation is one of the most significant factors in whether flashover ever occurs, with Vincent Dunn describing it as one of his three recommended ways to delay such an event.
The Kennedys also state: “The indicators of flashover do not define flashover. Rather, flashover is defined by its nature (rapid transition to a full room involvement.”
Another misconception among fire investigators is the rate of speed for a flashover. Kennedy points out that investigators make subjective decisions based on the rate of “full room involvement” and often deem a fire to have been started with some form of accelerant. In fact, NFPA 921 states the contrary, saying, “Research has shown that time to flashover from open flame can be as short as 1 1/2 minutes in a residential fire.”
What are the common signs of a flashover?
The most common signs of a flashover are high heat and rollover. The heat will be extreme so be on the alert for a severe and sudden change in temperature. This heat will be so intense that it will be felt through your gear.
Rollover is often confused with flashover. Rollover is usually observed by flames “rolling” or “dancing” across the ceiling. Rollover is the ignition of gases present in the room, not the contents.
How do you prevent a flashover?
By proper ventilation, we can eliminate superheated air and gases from the compartment. Depending on the fire, you can call for a horizontal or vertical vent. Just be conscious of the venting and location of the vent to not get other portions of the uninvolved structure involved. This method also reduces heat in the compartment and increases visibility for potential search operations. Vincent Dunn mentions in his report that non-ventilation is also another option: “By not venting and instead closing the door to the burning room, you can also delay flashover.”
He adds: “By not venting, you starve the fire of oxygen, which slows down the combustion rate, which slows down the build-up of heat in the room. This may be done when there is a delay in stretching a hose line and all persons are out of the burning room.”
If ventilation is not immediately possible, or a hoseline is not in place, you can also consider a portable extinguisher to cool down the heat in the burning compartment to delay the potential flashover. The odds of escaping a flashover alive are slim, and escaping a flashover without injury is even rarer.
Knowing the warning signs is key in giving you the signal to get out immediately. Multiple sources average your time for a flashover to be from 7 to 10 seconds.
So, the best-case scenario is you have 10 seconds to react and get to safety. This is usually about 5' of distance for the average firefighter.
How do you avoid being caught in a flashover?
Full PPE is key but not always a lifesaver. Temperatures during a flashover often exceed the PPE limitations. Also, try to always consider alternate egress points or areas of potential refuge, which could include doors, windows or adjacent rooms. Be proactive, read the smoke, study the fire and if you see something, report it. Do not perform PPV until a charged hoseline is in place. The key for me is that we need to put more emphasis on staying out of trouble in the first place and less on the last few seconds when we have to save our life.
This article, originally published on June 10, 2009, has been updated with a video and additional resources.