Concepts - Fire Reaction and Fire Resistance

In modern construction, fire safety is one of the most important quality requirements, especially since wood and wood materials are being used more and more. However, confusion often arises between two important fire safety concepts: fire spread and fire resistance. Although these are related, they refer to completely different properties and requirements for building materials and structures. Below, we explain what each term means, how they are assessed in Europe, and why both are important in today's construction.

What is fire spread?

Fire spread describes a material's behavior when exposed to fire, i.e., how easily the material ignites, how quickly flames spread across its surface, and how much heat, smoke, and burning droplets the material releases when burning. This is essentially the material's reaction to fire, which determines the extent to which the material fuels the spread of fire before a general room fire (flashover) occurs. In Europe, the harmonized classification standard EN 13501-1 applies, which divides materials into seven Euro classes from a fire spread perspective: A1, A2, B, C, D, E, F. Ranging from non-combustible (A1) to easily ignitable (F). In addition to the main class, two sub-indices are added: s (smoke production) and d (formation of burning droplets).

The class B-s1,d0 is the highest fire safety class for wood, which means: B - the material is hard to ignite (contributes very little to the fire), s1 - very little smoke is produced, d0 - no burning droplets or particles are formed. In practical terms, this means that if wood material is treated with an appropriate fire retardant and achieves the B-s1,d0 class, the spread of fire along the surface is significantly slowed down, and smoke is produced in minimal amounts. This gives people in the building more time to evacuate and rescuers more time to respond to the fire. European construction rules often require this class for wooden surfaces to ensure people's safety. Without fire protection treatment, untreated wood is usually in the D or E class.

What is fire resistance? (REI classes)

Fire resistance characterizes a building's structural ability to maintain its load-bearing capacity, fire and smoke resistance, and thermal insulation for a specified period during a fire. Fire resistance is usually expressed with the combination of REI and a number of minutes, indicating the resistance time. The letters REI represent the main fire resistance indicators of the construction:

• R (Resistance) - maintenance of load-bearing capacity: the construction must maintain a given load without collapsing under the influence of fire.
• E (Integrity) - integrity (density): the construction's non-combustibility, where flames or hot gases must not penetrate from the fire side through the construction.
• I (Insulation) - insulation ability: thermal insulation, where the temperature on the non-fire side of the construction must not rise dangerously high, preventing excessive heat transfer.

For example, the REI 60 class means that the construction must withstand a standardized fire test for at least 60 minutes, maintaining the required load-bearing capacity (R), fire and smoke tightness (E), and thermal insulation (I) properties during this time. In other words, within 60 minutes of the start of a fire, a wall, ceiling, or beam construction must not collapse, crack, or allow heat to ignite adjacent rooms on the other side of the construction. Different buildings require different fire resistance times according to standards, usually these are 30, 60, 90 minutes or more, depending on the type and use of the building. The purpose of fire resistance is to ensure that the construction remains stable for long enough to allow evacuation and fire extinguishing during this time.

It is important to understand that fire resistance is a property of the entire construction, not of a single material (such as wood). Thus, several factors - material type, cross-section, protective layers - can affect how long a wall or beam system can withstand fire. For example, thick laminated timber or CLT (cross-laminated timber) wall elements can achieve surprisingly high fire resistance due to their massiveness: a three-layer solid CLT panel can achieve approximately REI 30 without a cover, as the top layer of wood chars and protects the inner layers to maintain load-bearing capacity. However, even such an element must eventually succumb to heat if the fire lasts longer than its natural fire resistance time.

What Are K-Classes (for example K1 10, K2 30)?

In addition to REI classifications, K-classes are often used in timber building fire protection. A K-class (sometimes also referred to as a protective covering class) indicates the fire protection performance of a building’s surface layer. The K-class shows how long a wall or ceiling surface layer can protect the underlying structure from ignition and heating. This is especially important for load-bearing timber structures. A covering with a K-class functions like a "sacrificial layer", which takes the initial impact of the fire, keeping the structural material untouched for a certain time.

Standard EN 13501-2 defines two categories: K1 and K2, and time intervals of 10, 30 or 60 minutes. Depending on the test conditions and the substrate, a covering material is classified as follows: K1 10 or K2 10, K2 30, K2 60, etc. Here the number refers to the minutes of protection, while K1 vs. K2 indicates the type of substrate and test conditions (K1 is a stricter class with a limited substrate and K2 more general in use).

Practical examples: In the case of a K2 10 rated covering, the surface layer must ensure that for at least 10 minutes after the start of the fire, the temperature behind the covering does not immediately rise to a dangerously high level or ignite the underlying material. K2 30 requires 30 minutes of protection, and K2 60 at least 60 minutes. For instance, if a wall is covered with a K2 60 rated protective layer (e.g., special fire-resistant gypsum board), the covering must stay in place without collapsing for 60 minutes and keep the temperature behind it under control. The average temperature on the structural surface behind the covering must not exceed the surrounding temperature by more than 250 °C. Also, at the end of the test, the structure behind the covering (e.g., wooden surface) must not be ignited or charred.

Importance of K-classes for timber: In wooden constructions, this means that a gypsum board or other fireproof cladding acts as a protective layer, buying time before the fire reaches and ignites the timber or reduces its load-bearing capacity. In multi-storey wooden buildings, it is often required that beams and walls be covered with K2 30 or K2 60 class linings to achieve the required fire resistance. Research and testing have shown that even thick enough wooden paneling can function as a fire protective layer: for example, 27 mm thick solid wood cladding meets class K2 30, and two layers of such boards (total thickness ~54 mm) can provide protection for 60 minutes, i.e., class K2 60. It has also been found that adding a single layer of gypsum board (fireboard) to a CLT wall can increase its fire resistance by approximately 1.5 times. For example, if an element without covering had REI 60, adding one layer of gypsum board achieved REI 90, which illustrates the effectiveness of K-class coverings.

How Do Fire Reaction and Fire Resistance Work Together in Fire Safety?

Fire reaction and fire resistance are two aspects of a building’s fire safety that complement each other. In a building with a high level of fire safety, materials and constructions must meet both criteria:

• A good fire reaction class means that the material does not ignite easily or spread fire quickly, which slows the development of the fire in the early stages, giving people time to escape and reducing damage.
• Good fire resistance means that even if the fire lasts, the building’s structure remains intact long enough to prevent the spread of fire to other areas and to avoid collapse.

It is important to understand that these two properties can vary across materials, and both must be ensured. A good example is the comparison between steel and wood. Steel as a material is non-combustible and therefore classified as Euroclass A1 (excellent in terms of fire reaction, generally not contributing to fire). However, a bare steel beam structure loses half of its strength at around 500°C and heats up very quickly in a fire, meaning that without additional protection, a steel structure can fail in a very short time, failing fire resistance requirements even if its reaction class is A1. Wood, on the other hand, is a combustible material (for example, spruce is Euroclass D, meaning it ignites relatively easily), but a massive wooden beam or CLT panel chars on the surface and can surprisingly retain load-bearing capacity in fire. Often, even longer than unprotected steel beams, before the panel is burned through. There are documented cases where, after a fire, the wooden beam remained intact while nearby unprotected steel beams warped or collapsed. This example highlights that a good fire reaction class does not automatically mean good fire resistance, and vice versa. Therefore, designers and engineers must always account for both.

In practice, this means that for timber buildings, special measures must be taken to reduce the ignitability of wood (e.g., impregnating it with a fire retardant, using non-combustible surface coatings) while also ensuring structural fire resistance (e.g., dimensioning beams with a larger cross-section to allow the formation of a charring protective layer or protecting load-bearing parts with gypsum boards). European building codes include requirements for both material fire classification and structural fire resistance, meaning a comprehensive solution must often be planned to satisfy both. For example, in the case of timber ceiling beams, a solution may be: the beams themselves are sufficiently large and, if needed, treated to achieve R60 load-bearing capacity, and they are also covered from below with a B-s1,d0 class fire protection coating or gypsum board ceiling, which meets the K2 30 requirement-thus both limiting fire spread and protecting the load-bearing structure for at least 30 minutes.

Experts emphasize that neither aspect should be underestimated. If a material ignites and spreads fire rapidly (poor fire reaction), the fire can spread before fire resistance even comes into play. Conversely, if the material is non-combustible but the structure is weak, the building may collapse before the fire spreads. Therefore, materials and construction solutions must always be chosen and tested for both reaction to fire and fire resistance, according to the specific situation.

Summary

Fire reaction and fire resistance are two different fire safety indicators: the former shows how much a material contributes to rapid fire spread, and the latter how long a structure withstands fire. With the growing use of wood materials, understanding these concepts is crucial, and achieving high fire safety requires limiting fire spread (good Euroclass) and maintaining structural stability (adequate REI). Additionally, K-classes must be considered in timber buildings, adding a third dimension-the duration a surface layer protects the structure before fire penetrates deeper. When all three aspects-fire reaction, fire resistance, and surface protection durability- are addressed, wood can be a very safe and durable building material, even in taller structures.

The positive news is that new technologies and products allow wood to become increasingly fire-resistant. For example, innovative flame retardants like SPFR100 enable wood to achieve the highest fire reaction class (B-s1,d0) and facilitate compliance with K-class requirements, while being environmentally friendly and preserving the natural aesthetics of the wood. With such solutions, Europe can successfully combine wood’s architectural appeal and sustainability with strict fire safety standards, which are increasingly important to both regulators and the public. Ultimately, fulfilling both fire reaction and fire resistance requirements ensures that a building provides time to save lives and keeps the structure stable long enough to control the fire before it can destroy the entire building.

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