Durability od reaction to fire performance

11 27 2019 | Fire protection | Fire regulations | Durability

Durability of reaction to fire performance of wood products according to classification standard EN 16755:2017

Fire protected wood products are subject to standards, classifications, testing methods, and requirements that even industry professionals need to read closely to understand and apply in practice. A correct understanding is important, as misunderstandings may result in solutions and wooden structures that will not provide the specified level of fire protection over the building's useful life.




Fire protection treatments can significantly improve the fire performance of wood. This protection of the wood product may be lessened substantially if the wood material is exposed to moisture or weathering

Moisture can cause the fire retardants, especially salt-based retardants, to crystallise on the surface of the wood material. The fire retardant may then be washed off the wood, which will compromise its fire performance. Flowing water, changes in the moisture content of the wood, and UV radiation may also reduce the amount of fire retardant in the treated wood material.

Considering the above, it is essential to verify the functionality of wood products and the long-term durability of their fire protection. The European Standard EN 16755 was developed for this purpose, based on Technical Specification CEN/TS 15912. The standard also makes use of research data from North America.




The tests and classes of the durability of reaction to fire performance standard EN 16755:2017 can unfortunately be confusing in some cases. The standard defines a classification system for verifying the durability of the fire protection of fire retardant-treated wood products for indoor and outdoor use. The standard defines three DRF (Durability of Reaction to Fire Performance) classes that describe the durability of fire protection:

  • INT1 – for permanent use in interior dry applications, service class 1.
  • INT2 – for permanent use in interior humid applications, service class 2.
  • EXT – for permanent use in exterior applications, service class 3.
The interior application classes require the hygroscopic properties of the fire protected wood to be tested. The exterior application class requires the verification of the hygroscopic properties and weathering performance (fire class after weathering).




The method of the hygroscopicity test is described in Annex A of EN 16755. It is based on the NT Build 504 and ASTM D3201-94 standards.

The test consists of submerging fire retardant-treated test specimens in water and then stabilising them in standard conditions. The moisture content is recorded once it has reached equilibrium. Each class – INT1, INT2, and EXT – has specific requirements.

The test aims to ensure that the moisture content of fire protected wood will not be significantly higher than that of untreated wood, as wood materials may become mouldy, rot, or corrode their fasteners if their moisture content constantly increases in a highly humid environment. Increased moisture content in the wood material may also affect the permanence of the fire retardant. For this reason, the standard sets class-specific maximums for the equilibrium moisture content of the test specimens.




The EXT class for fire protected wood products in exterior applications includes hygroscopic and weathering performance requirements.

Three steps are used to verify the impact of weathering on fire performance:

  • Step 1. The initial fire class is verified from the material's EN 13501-1 classification report.
  • Step 2. The fire protected test specimens are subjected to weathering by using one of the following standard test methods:
    • EN 16755 Annex B, method A (accelerated weathering and rain)
    • EN 16755 Annex B, method B (accelerated weathering, rain, and UV radiation)
    • EN 927-6 (accelerated weathering, rain, and UV radiation)
    • EN 927-3 (natural weathering).
  • Step 3. The test specimens undergo an EN 13823 or ISO 5660-1 compliant fire test to verify their fire class after weathering.




The durability of DRF EXT class fire protection in atmospheric conditions must satisfy the following requirements:

  1. The fire class of the fire protected wood before weathering must be declared in accordance with EN 13501-1 by using the SBI fire test of EN 13823.

  2. The fire class must remain the same in the post-weathering fire test, verified by the SBI test of EN 13823 or the ISO 5660-1 test.

  3. The equilibrium moisture content of the treated test specimens must be less than 28 per cent.

  4. Further fire test requirements are set for Euro class B (EN 13501-1 classification) products: The 30 second average of the Heat Release Rate (HRR) may not exceed 150 kW/m2 for 600 seconds after ignition, and the increase in Total Heat Release (THR) may not exceed 20 per cent for the first 600 seconds compared to the pre-weathering fire test.

  5. Euro class C (EN 13501-1 classification) products also have their own fire test requirements: The 30 second average of the Heat Release Rate (HRR) may not exceed 220 kW/m2 for 600 seconds after ignition, and the increase in Total Heat Release (THR) may not exceed 20 per cent for the first 600 seconds compared to the pre-weathering fire test.

Products in the DRF EXT class also meet the requirements for DRF classes INT1 and INT2, but not vice versa.





The general test uses Scots pine test specimens, and its classification applies to other wood species at the same retention. Tests carried out on other wood species only allow classification of the tested species.

The weathering methods accepted by the standard have major differences, and although the inclusion of multiple alternatives in the standard is partially due to the different technical properties of various fire retardants, it is also an artefact introduced by the political nature of standardisation, as industrial actors in different EU countries influence the content of standards. It is vital that the building control authorities understand the differences of the weathering methods and how they apply to buildings and construction.

For example, fire protected wood painted with ordinary paint only retains its fire class if the combination of fire retardant and paint was tested.

If the test was carried out on unpainted wood, the classification does apply to painted surfaces, but only if the topcoat does not compromise the fire class. Most ordinary exterior paints harm the fire protection, however, which is why we recommend that wood product manufacturers test their fire protected materials with different exterior paints before releasing them to the market.


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