The Neutral Plane and Fire Conditions


You are dispatched for an unknown type of fire in a large commercial vacant building. The building is well known to you and your department; you know that transients are living within. On arrival, companies find light gray smoke venting from roof vents, and the structure has some significant access issues. The building is so large that a responding chief is assigned to drive around it to get a full 360° look. As the engine sets up for an offensive attack, the chief officer advises the 360° is complete and there are no real hazards to report. Responders cut a large roll-up door to access the fire and put a 2½-inch hoseline [minimum 265 gallon-per-minute (gpm) flow] into service. As the initial hoseline is activated, the fire’s intensity significantly changes; the smoke changes from light gray to turbulent black. Your firefighters are facing high heat and zero visibility and are not making any progress on the fire. This happens extremely quickly, and the company officer and the chief officer decide to abandon the hoseline and move all fire apparatus away from the building.

What happened? Forcing the roll-up door gave the oxygen-starved fire a big breath of fresh air and transformed a small fire into a very large, intense fire. Moments after firefighters exited the structure, the smoke layer dropped to the floor, became extremely turbulent, and erupted into a ball of fire; this was a close call. The firefighters searching for fire in this building didn’t find the fire; in a sense, the fire found them. If the company officer and chief officer had not recognized the drop in the layer of smoke as a sign of deteriorating fire conditions, firefighters could have been badly burned.

Slowing Down to Speed Up

“Slowing down to speed up” on the fireground can benefit firefighters in many ways. Missing something because you are moving too fast can be detrimental to the mission’s outcome. But, of course, you must move quickly while making the calculated considerations to better understand what you are seeing in fire behavior and the building’s construction. The tactics and strategies implemented on the fireground are based on fire behavior and the building’s construction; building construction/size and fire behavior interact and influence our decisions and choices.

The Neutral Plane

The neutral plane can tell us a lot on arrival—the fire’s location, its intensity, why the fire is not progressing, and what heat we will encounter once we enter the building. Every fire has an intake and an exhaust. The low intake brings in the fresh air that sustains the fire’s growth; the high exhaust is the exit for the superheated gases. When a structure has only one opening, a door or a window, that opening serves as an intake and as an exhaust. When a fire is developing, the neutral plane—the dividing line between the superheated smoke exiting at the upper level and the cooler air being drawn into the seat of the fire—lowers and the color, velocity, and pressure of the smoke gases increase.

As a fire intensifies, its heat energy increases, which causes gases, including smoke, to expand. When contained, expanding gases increase in pressure, moving from areas of relatively high pressure to areas of lower pressure. You can assess a fire’s intensity by observing the color, density, and pressure of smoke venting from windows. Let’s consider two extremes: light gray smoke that lazily drifts out of a window and heavy black turbulent smoke that boils out of a window like the smoke from a steam locomotive.

The neutral plane is the balance between the exhaust the fire is creating and the fresh air the fire needs to progress and intensify. The color, velocity, and direction of the layering (the smoky upper layer) of the smoke at the ceiling tell us a lot about the fire’s intensity and location. Your ability to recognize this directional flow and read the smoke’s color, velocity, and density will give you a great deal of information before entry.

For the neutral plane to exist, there must be an opening in the structure; this is usually created by firefighters as they enter the building. Slowing down for a split second to get a better visual on what the fire is presenting is most important. The directional flow of the clean air drawn into the fire indicates in which direction you should start your push toward the seat of the fire. You need to apply water to the seat of the fire by following the path in the direction in which the fresh air is going.

Smoke Velocity/Density

Before making entry, observe the velocity at which the clean air is being drawn into the fire; it indicates the intensity or rate at which the fire is growing. Additionally, the distance from the floor to the neutral plane tells you the volume of air the fire is drawing. This information, combined with the velocity, will give you a good understanding of the conditions you will encounter when entering.

Inside Temperature

The smoke’s density at the exhaust point, which is typically your entry point, gives you some idea of what is burning and a read on the temperatures inside the fire building.

Smoke Color

The color of the smoke indicates what might be burning inside, how long it has been burning, and whether it has started to consume the building’s structural members. The smoke may exhibit different colors, indicating that several products are burning at the same time. The colors of the smoke they create combine. Slowing down will enable you to identify the fire structure’s type, the fire’s location, and the building’s contents.

Building Construction and Occupancy

The building’s occupancy and construction type greatly affect the fire’s behavior, especially if the building has been remodeled or altered. Remodeling can create void spaces within the construction that act like cocklofts and will allow superheated gases to accumulate and fire to travel throughout the building. In cases where the alteration involves the roof, the new roof may conceal the presence of the original one, and the void between the old roof and the new roof conceals a dangerous reservoir for carbon monoxide and other flammable fire gases (photo 1). This flow of oxygen throughout the building can lead to rapid changes in conditions within the structure.

Heat is transferred through conduction, radiation, and convection. In a building with void spaces, convection causes warmer areas of a liquid or gas to rise to cooler areas in the liquid or gas. This heat transfer creates a dangerous situation for firefighters during suppression or overhaul activities.

In many documented incidents, firefighters have experienced explosive fire events while opening up adjacent rooms, cocklofts, attics, and void spaces because fresh oxygen is introduced to an underventilated space containing superheated smoke that has reached its ignition temperature. Be very careful when opening up ceilings, walls, or adjacent buildings. Although these spaces are free of fire, they may be full of explosive fire gases. Introducing fresh oxygen will result in a very violent reaction. The building’s construction dictates how these gases move throughout a structure and also how they become trapped in certain areas.

Recent fire research indicates the following relative to operating at ventilation-controlled or ventilation-limited fires: When making an opening in an oxygen-deprived structure, it can take anywhere from 100 to 200 seconds for an oxygen-deficient fire to regain its intensity.1 The time depends on the building’s size and the fire’s location. A room-and-contents fire in a 1,500-square-foot home will not take as long to reintensify as a deep-seated, well-established fire in a large commercial building. The other factor affecting the fire reestablishing itself is the amount of oxygen in the fire building.

Initial Hoseline Setup

Set up the initial hoseline away from the front door and ensure that it is long enough and working. You can see a lot more from a distance than when nestled up to the front door. Things can change very quickly on the fireground. Having the hoseline set up so you can get a better look at the building will allow you to monitor the ever-changing conditions while setting up for the fire attack.

Photo courtesy of Miami-Dade (FL) Fire Rescue.

(1) Photo courtesy of Miami-Dade (FL) Fire Rescue.

Reporting Conditions to All

Immediately communicate the data gathered from your walk-around to the fire attack team and all fireground and incoming ventilation and rapid intervention units and chief officers, if possible. It will give them an accurate picture of the building’s construction and the fire’s location and intensity.

Flow Requirements

Setting a minimum gpm flow standard for local occupancy types in your community will ensure that you have the appropriate flow for the British thermal units you could encounter at specific occupancies. National Fire Protection Association (NFPA) 1710, Standard for the Organization and Deployment of Fire Suppression Operations, Emergency Medical Operations, and Special Operations to the Public by Career Fire Departments, sets our minimum gpm requirements (ability to flow 300 gpm total between the first two lines on a working fire), but you can set them higher to help ensure a successful fire attack at any given structure. Our department has a 150-gpm minimum for residential fires and a 250-gpm minimum for commercial fires. Our department’s engine companies far exceed that with our first lines on fires because of our nozzle complement. Most importantly, this standard gives them confidence in knowing that the gpm they are flowing meet and exceed the NFPA requirements and have minimal nozzle reaction and superior reach and penetration.

Hoseline Staffing

Although the gpm you are flowing will help with the heat you will encounter, if you cannot maneuver the hoseline to the seat of the fire efficiently, the fire will continue to grow and intensify. Setting a minimum for handline staffing allows your company officers to do their jobs, manage the hoseline, and move the line quickly and efficiently to the seat of the fire.

Size-up must be ongoing and is the responsibility of multiple people on the fireground at every fire—the fire attack officer, the incident commander, the ventilation group, and the rapid intervention team, among others. All firefighters must observe fire conditions such as the color, volume, and density of the smoke and the neutral plane. Any discrepancy observed at any time among any of these positions conducting ongoing size-up is a red flag because someone on the fireground has it wrong. Understanding what flow paths are and how the neutral plane is a visual indicator of the fire’s location and intensity, what the fire is lacking to progress, and the type of heat the fire is producing can ensure success on the fireground. Slowing down to speed up and understand not only what the smoke is telling you but also what the neutral plane is presenting will enhance your ability to find and extinguish fires more efficiently. Fireground speed is important, but letting the fire breathe when you first open the door prior to entry gives you advantages that pertain to the location and intensity of the fire and allows you to take a good look at the neutral plane prior to entry.

The building’s construction and the fire behavior should determine your fireground tactics and strategies. Doing your homework to ensure that you have the appropriate gpm for your handlines and ensuring that the handlines are stretched correctly and staffed for efficient movement will help to combat the fire behavior and construction issues that you may encounter on entry. You can overcome the obstacles you face on the fireground if you are well-prepared.


1. Kerber, S. (2013) “Study of the Effectiveness of Fire Service Vertical Ventilation and Suppression Tactics in Single Family Homes.” Underwriters Laboratories.

Dominic Magagnini is a captain with the Ceres (CA) Fire Department and a 15-year veteran of the fire service. He is the owner of the Engine Co., a fire service development company.

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