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Phases of Fire, Flashover & Backdraft: How a Compartment Fire Develops
The four stages of fire growth, why ventilation changes everything, and how to recognize flashover, backdraft and smoke explosion.
- Fires move through ignition, growth, fully developed and decay stages.
- A ventilation-limited fire holds its heat and fuel-rich smoke and responds fast to any new air.
- Flashover is the transition to a fully developed fire: about 600°C (1,100°F) in the upper gas layer and 20 kW/m² at the floor.
- UL research found modern rooms reach flashover in under five minutes; legacy rooms took at least seven times longer.
- Backdraft needs a starved, hot, fuel-rich space and then air. Control the openings and have a charged line before you make any.
Four stages of a compartment fire
A fire inside a room does not burn at one steady intensity. It moves through recognizable stages, and the stage you are facing changes what water, air and a door will do. Today’s fire science describes four: ignition, growth, fully developed and decay. Older texts use three (incipient, free-burning and smoldering). They describe the same process in different words.
- Ignition. The fire starts and is small. Oxygen is plentiful, flame temperatures can already be high, but the room itself is barely warmer.
- Growth. The fire spreads to nearby fuel. A plume of hot gases rises, hits the ceiling and spreads out, forming a growing hot gas layer that banks down from the top. Fire gases and smoke are being produced faster and faster.
- Fully developed. Everything combustible in the room is burning and heat release is at its maximum. In a room with enough air, temperatures can climb well above 1,300°F (700°C).
- Decay. The fire runs out of fuel or, more often in a closed building, out of oxygen. Flame diminishes and temperatures drop, but the room may stay full of hot, unburned gases.
Fuel-controlled or ventilation-limited?
This is the single most useful idea in modern fire behavior. A fire is either fuel-controlled, with plenty of air and burning as fast as the fuel allows, or ventilation-limited, with plenty of fuel but not enough air, so oxygen sets the pace.
Modern homes are more airtight and full of synthetic fuel. A fire can grow quickly, use up the oxygen in a closed room, and be ventilation-limited before the first engine arrives. Such a fire does not go out. It holds its heat and its fuel-rich smoke and waits for air. Research by UL’s Firefighter Safety Research Institute found that even with an opening vented, the fire in their experiments was still ventilation-limited and responded as fast or faster to any added air.
The path air takes from where it enters to where hot gases leave is the flow path. If you are standing in it, you are standing in the exhaust. Every new opening, whether it is a door, a window or a hole in the roof, creates a new flow path.
UL FSRI tests showed that a closed door reduces the oxygen reaching a fire, lowers temperatures and improves survivability for both victims and firefighters. Many departments teach door control, closing a door behind you or limiting how long an entry door stays open, to keep from feeding the fire. Follow your department’s tactics.
Reading smoke and conditions
Smoke is a gauge of what the fire is doing inside. Many departments teach firefighters to read it by volume, velocity, density and color, together with what it is doing at doors and windows. Rapid, pressurized movement, thick dark smoke, and smoke that pulses or “breathes” can all indicate a ventilation-limited fire. No single sign is a guarantee. Color alone can mislead, so treat each as a clue and combine them with what you see, feel and hear.
Flashover
Flashover is the transition from a growing fire to a fully developed one. As the hot gas layer grows, it radiates heat down onto everything in the room. Contents heat up, pyrolyze, and release flammable gases until nearly all exposed surfaces ignite at about the same time. A common engineering definition is an upper gas layer at roughly 600°C (1,100°F) and a heat flux at floor level above about 20 kW/m².
The classic warning signs are rollover (flames dancing in the smoke layer overhead), rapidly rising heat, thick smoke banking down toward the floor, and heat that becomes unbearable even low to the floor. But flashover can come with little warning, and no one survives it in a room unprotected. The best defense is early: cool the hot gas layer, control the air, and keep your escape route behind you.
Rollover vs. flashover. Rollover is ignition of the hot gases in the upper layer only. Flashover is the whole room, contents included, becoming involved.
Modern fuels give you less time
UL FSRI’s side-by-side room tests are the clearest picture of how much has changed. In three sets of experiments, all of the modern-furnished rooms went to flashover in under five minutes. The fastest legacy room took just over 29 minutes. Legacy rooms took at least seven times longer. The takeaway for the fireground: the fire you are looking at may be much further along than the time since dispatch suggests, and the window to act before flashover is short.
Backdraft and smoke explosion
A backdraft is a rapid, sometimes explosive burn that happens when air reaches a ventilation-limited compartment holding hot, fuel-rich gases. The fire had been starved, but the gases are above their ignition temperature. Add oxygen and the stalled combustion resumes all at once.
A smoke explosion is the ignition of accumulated smoke and gases in a space away from the fire, such as a void or an adjoining room, where the mixture has reached the flammable range.
| Where | What you may see or hear |
|---|---|
| Smoke | Under pressure, pulsing in and out, or dense gray-yellow after turning from black |
| Windows and doors | Soot-stained glass; glass or door that is hot; little visible flame |
| Sounds | Muffled sounds; air whistling in through cracks |
| When an opening is made | A sudden inward rush of air |
Older teaching says to vent at the highest point so hot gases leave before anyone enters. More recent research puts the emphasis on controlling air: having a charged hose line in place and managing every opening before creating more of them. Many departments teach both, in different situations. Your department’s procedures and your officer decide.
Charleston Sofa Super Store (2007): a fire waiting for air
On the evening of June 18, 2007, a fire in the Sofa Super Store in Charleston, South Carolina, trapped and killed nine Charleston firefighters. It was the largest single loss of firefighters since September 11.
NIST later modeled the fire to reconstruct how it grew. It found that, early on, the fire could not get enough air and its growth was slowed. When windows at the front of the building were broken, air flowed in along the floor toward the rear and became available to the fire. The fire then swept from the back of the main showroom to the front extremely quickly and spread into the neighboring showrooms. NIST named the major factors: large open spaces filled with furniture, which supplied a high fuel load; the inward rush of air after the windows were broken; and no sprinklers.
NIST was careful to say its fire science could not explain how or why individual firefighters became trapped. The lesson for this page: the fire you are fighting may be holding its energy until you give it air. Ventilation is a tactic that changes fire behavior, and its timing has to be deliberate.
Fireground talk
- Flow path
- The route air and hot gases take through a structure, from inlet to exhaust.
- Black fire
- Dense, dark smoke with little visible flame; hot, fuel-rich and ready to ignite.
- Rollover
- Flames rolling across the ceiling in the smoke layer, often a warning of flashover.
- Getting in the flow path
- Standing between the inlet and the fire where air and heat are moving fast.
- Cool the ceiling
- Short bursts of water fog into the hot gas layer to reduce temperature and lower flashover risk.
Practice it in the simulators
Study questions
What is flashover?
The transition from a growing fire to a fully developed fire, when radiant heat from the hot gas layer ignites exposed surfaces across a room at about the same time. A common benchmark is an upper layer near 600°C (1,100°F) with about 20 kW/m² at floor level.
What is a ventilation-limited fire?
A fire with plenty of fuel but not enough oxygen, so the available air sets how fast it burns. It holds hot, unburned gases and can grow rapidly when air is added.
What is the difference between rollover and flashover?
Rollover is ignition of hot gases in the upper layer, with flames moving across the ceiling. Flashover is the whole room, contents included, becoming involved.
What is the difference between backdraft and smoke explosion?
A backdraft happens in a starved, ventilation-limited compartment when new air arrives. A smoke explosion is the ignition of accumulated smoke and gases in a space away from the fire, such as a void or adjoining room.
Sources and further reading
- Modern Residential Fires (Fire Safety Journal, Issue 1) — UL Firefighter Safety Research Institute
- Charleston, SC Furniture Store: Technical Study of the Sofa Super Store Fire — NIST
- 41 CFR 102-80.145: What is meant by flashover? — U.S. Code of Federal Regulations (Cornell LII)
- Ventilation-Limited Fire: Keeping It Rich and Other Tactics Based on Science — Fire Engineering
- UL FSRI Introduces Close Your Door — UL
- NFPA 921, Guide for Fire and Explosion Investigations — NFPA (print standard)
About this guide. The Fire Behavior & Chemistry series was originally informed by LBFD Training Division recruit training material. This edition has been independently rewritten and expanded using the sources listed on each page, and its diagrams were drawn for this site. Free Fire Simulator is not affiliated with or endorsed by LBFD. Spotted an error or have a better source? Use the Contact Us button.