What is flashover?
Flashover is the moment at which a room suddenly becomes involved in fire in its entirety. In Dutch it is called vlamoverslag. A fire starts small, but the hot smoke layer under the ceiling radiates more and more heat onto everything in the room. At a certain point almost all combustible surfaces ignite at the same time. From then on the room is no longer survivable.
How does flashover occur?
- Growth phase: the fire heats the room and a hot smoke layer builds up under the ceiling.
- Tipping point: when the gas temperature reaches around 600 °C, heat release and smoke production rise sharply.
- Fully developed fire: all combustible materials in the room are burning; the temperature can then rise to above 1000 °C.
The time to flashover depends on the amount and type of fire load, ventilation, the size of the room and where the fire starts. A fire that begins by smouldering can take much longer than an open flame.
Flashover is not the same as fire spread. Flashover happens within a single room. Fire spread is fire jumping to another compartment or building, for example via the facade or by radiation.
Application
Flashover is the basis of the European reaction-to-fire classes for construction products (EN 13501-1). The classes were derived from the large-scale room corner test, which looked at how quickly a wall and ceiling lining leads to flashover. In practice, products are usually tested with the smaller SBI test.
- A1, A2 and B: no flashover expected.
- C: flashover after 10 minutes.
- D: flashover between 2 and 10 minutes.
- E: flashover within 2 minutes.
| A1, A2 and B | No flashover expected |
|---|---|
| C | Flashover after 10 minutes |
| D | Flashover between 2 and 10 minutes |
| E | Flashover within 2 minutes |
Tips
- Watch wall and ceiling linings: combustible linings speed up a fire. For panels, cladding boards and insulation, check the Euroclass on the declaration of performance.
- Smoke alarms: they give a warning well before a room flashes over. Fit them in the hallway and on the landing of every storey.
- Close the door: a closed door limits the air supply and keeps smoke and heat in the room of origin for longer.
Related terms
- Fire spread
- Fire penetration
- Fire classification
- Fire compartment
- Fire resistance
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.