Fire classes from A to K: everything you need to know

Fire classes comprise a system for categorising fires according to the type of material that burns and the fuel for combustion, which determines the extinguishing strategies and protection solutions required, such as fire curtains and smoke control barriers. They are usually identified by letters according to the fire, but these differ in some territories.

What will you learn in this article?

• What are fire classes?
• Fire classes: a complete description
• The importance of fire classification in fire protection
• General considerations for fire curtains and smoke control barriers

What are fire classes?

According to Spanish standard UNE-EN 2-1991/A:2005, fires are grouped into different classes depending on the nature of the material that is burning. Fires are classified as follows:

• Class A: fires involving common solid materials, such as wood, paper and textiles.
• Class B: fires caused by flammable liquids, such as oils, petrol and solvents.
• Class C: fires involving flammable gases, such as natural gas or propane.
• Class D: fires involving combustible metals, such as magnesium or sodium.
• Class F (or K): fires caused by cooking oils and fats.
• Class E (only in the US): electrical fires, although this class is not officially used in other countries.

This classification makes it easier to identify the specific risks in each type of fire and allows the most effective measures to be taken to control them.

Types of fire: a complete description

The classification of fires is based on criteria related to the nature of the fuel, its behaviour during combustion, the thermal energy released and the products generated (flames, radiation and smoke).

Class A: fires involving common materials

Class A fires involve solid materials of organic origin that generate embers during combustion. This group includes materials such as wood, paper, cardboard, textiles, and certain polymers. These fires tend to develop gradually, beginning with a pyrolysis phase of the material and evolving into sustained combustion.

This type of fire is characterised by continuous heat release and significant production of hot smoke, which can encourage its spread to other nearby elements as the fire progresses.

Class B: flammable liquid fires

Class B covers fires caused by flammable liquids and liquefiable solids, such as hydrocarbons, mineral oils, alcohols, solvents, or paints. Unlike Class A fires, combustion occurs mainly on the surface of the liquid, resulting in rapid fire development.

These fires are characterised by high thermal intensity in the early stages, intense flames and significant generation of dense, toxic smoke. The absence of embers means that the energy is released in a more concentrated form, increasing the initial danger of the fire.

Class C: flammable gas fires

Class C fires are associated with flammable gases such as natural gas, propane, butane, or hydrogen. Their behaviour is marked by a very high combustion rate, especially when there is a prior accumulation of gas in enclosed spaces.

This type of fire generates high-energy flames and combustion products at very high temperatures, which can lead to deflagration or explosion. They tend to develop rapidly and intensely, significantly increasing the risk to people and nearby facilities.

Class D: combustible metal fires

Class D covers fires involving combustible metals such as magnesium, sodium, potassium, titanium or aluminium in the form of powder, shavings or granules. These fires are extremely aggressive, reaching very high temperatures and reacting to conventional extinguishing agents. Although they are uncommon in general construction, they are relevant in specific industrial processes.

Class F (or K): cooking oil and grease fires

Class F, known as class K in other markets, corresponds to fires caused by cooking oils and greases, especially in industrial and commercial kitchens. Due to their high heat load and smoke generation, these fires develop at very high temperatures and have a high potential for reignition.

Class E (only in the US): electrical fires

Class E is used in some countries to describe fires affecting live electrical equipment, although it is not recognised as a separate class in European regulations. Its use is mainly operational, aimed at identifying risks associated with energised installations.

In this case, the determining factor is not the type of fuel, but the presence of active electrical voltage, which conditions intervention and extinguishing strategies. Once the power supply has been cut off, the fire is no longer considered an electrical hazard and is classified according to the material that is burning.

The importance of fire classification in fire protection

Fire classification directly determines how fire protection should be approached in a building. Not all fires develop in the same way or generate the same risks, so correctly identifying the type of fuel allows us to anticipate its behaviour and define the most appropriate protective measures.

Each class of fire has significant differences in terms of intensity, speed of spread, temperature reached and volume of smoke generated. These factors influence how fire can spread between spaces, affect the structure or compromise evacuation routes. For this reason, the compartmentalisation of the building and the control of the spread of fire and smoke must be designed in line with the actual risks present in each use.

Fire classification also has a direct impact on the definition of fire sectors and the fire resistance requirements of the elements that delimit them. In public buildings, critical infrastructure, or industrial environments, proper classification is key to limiting damage, facilitating the evacuation of people, and enabling safer intervention by emergency services.

Furthermore, regulations on compartmentalisation and smoke control are based on this classification, as the risks associated with the materials present and the activity carried out determine the solutions that must be incorporated into the project. Correct interpretation allows systems to be designed that are tailored to the actual needs of the building, avoiding ineffective or disproportionate solutions.

General considerations for fire curtains and smoke control barriers

Compartmentalisation is one of the fundamental pillars of passive fire protection. Its purpose is to confine fire and combustion products within a specific compartment, limiting their spread and maintaining acceptable conditions for evacuation and emergency response teams. In this context, smoke control barriers and fire curtains perform complementary but clearly differentiated functions.

Smoke control barriers are designed to manage the behaviour of hot gases generated during a fire. They can act as delimiting elements, creating smoke deposits under the roof, or as channelling systems, directing smoke towards natural or mechanical extraction areas. Fire curtains, on the other hand, are designed to contain the spread of flames and thermal radiation between fire sectors.

Fire curtains offer a number of clear advantages over traditional solutions such as fire doors:

• Versatile architectural integration: their design allows them to be integrated into a wide variety of spaces without altering the functionality of the premises or compromising the architectural aesthetics of the project.
• Retractable and non-invasive system: they remain hidden during normal use of the building and are only deployed in the event of a fire. This guarantees fire protection without interfering with daily functionality or the visual perception of open-plan spaces, double-height spaces or atriums.
• Maintenance of spatial fluidity: as there are no permanent elements in the closed position, the visual and functional continuity of passageways and large open spaces is preserved, something that is difficult to achieve with rigid solutions such as fire doors or walls.
• Compartmentalisation without loss of usable space: they allow for sectorisation without the need for fixed enclosures, avoiding permanent occupation of space and keeping the functionality of the available space intact.
• Lightness and minimal structural load: manufactured with high-performance technical fabrics — such as fibreglass with special coatings — they offer high fire resistance with low weight, without adding significant loads to the building structure.
• Adaptation to complex geometries: they are particularly effective in projects with unique architectural forms, large spans or irregular configurations where traditional compartmentalisation solutions are not viable.

Do you need to incorporate compartmentalisation or smoke control solutions into your next project?

At Tecnitex, we design and implement textile systems, from the concept phase to commissioning and subsequent maintenance, ensuring that each solution is integrated effectively and in accordance with current regulations. Contact us to assess your project’s needs.