Photo: Blåljusbilder A facade fire in Sweden last year led to an […]
Photo: Blåljusbilder
A facade fire in Sweden last year led to an investigation of the building and several fire tests of the facade materials. A Swedish report now concludes that the tested materials did not perform as expected, but that the cause of the failure cannot be established. The report therefore recommends further research to obtain better documentation of the durability of the fire performance of fire-retardant-treated timber facades.
Timber has been a widely used building material in the Nordic countries for many years and is also gaining ground in Denmark as part of the green transition. A new Swedish report, ’Brandskyddets beständighet hos en brandskyddsimpregnerad träfasad – resultat från tester enligt SP FIRE 105’, published by Lund University, once again draws attention to the extent to which fire-retardant-treated timber facades retain their fire performance over time.
Two full-scale tests showed that material from a timber facade that was just under three years old did not meet all the criteria of the Swedish SP FIRE 105 test method. In both tests, there was greater vertical fire spread along the facade than permitted. However, the facade system met the criteria for temperature at the eaves and heat radiation towards the window above.
The results therefore demonstrate a specific problem with the tested material at the time of testing, but the report cannot establish why the material did not have the expected fire performance. The report therefore recommends further research to document the long-term effects of fire-retardant treatment in timber facade cladding when the facade is exposed to weather.
Swedish facade fire led to investigation
The background to the two fire tests was a facade fire that occurred in the summer of 2025 in a four-storey building in the Swedish municipality of Upplands-Bro. The fire started in an apartment on the second floor and spread externally via the facade to several apartments in just 24 minutes. The building was constructed from solid timber with non-combustible insulation, the external wall was clad with timber panels, and the associated balconies had timber decking.
The fact that the fire developed so rapidly and ultimately destroyed the building led to an investigation of the building. Uppsala Fire & Rescue Service, which conducted the technical analysis of the fire development and the building’s fire safety measures, concluded that the fire had spread significantly faster than could be expected. The investigation also indicated that the fire-retardant-treated facade had lost some of its fire resistance and that the combination of the timber facade and the balcony construction was a significant factor in the rapid fire spread.
Unresolved cause of fire development
In the two SP FIRE 105 fire tests, facade cladding from two different areas of the facade was tested. The tested materials had been naturally exposed to weather during the nearly three years that the building had been standing and were assessed not to have been damaged by fire. Both fire tests produced the same result.
Earlier SBI tests of material from the same building, carried out as part of Uppsala Fire & Rescue Service’s investigation of the building, likewise showed lower fire performance than expected and therefore support the concern. In these tests, samples from both directly weather-exposed and more protected parts of the facade performed at a level corresponding to reaction to fire Class D, rather than the expected Class B. The directly exposed samples generally showed the poorest performance, suggesting that weather exposure may have played a role.
However, in connection with the report, there was no access to production control records for the specific batch, the original test report for the facade system, or documented fire testing of the material’s properties at the time of delivery. It therefore cannot be established whether the identified deficiencies were caused by ageing of the timber due to weather exposure, variations in the impregnation, production quality, deviations between the classified and delivered material, the design and installation of the facade, or a combination of several of these factors.
“The Swedish study reveals a serious problem in this specific case, but it does not demonstrate that all fire-retardant-treated timber facades are inadequate,” says Anders Dragsted, Head of Advanced Building Solutions at DBI, and continues:
“However, the report sends a warning signal that the fire performance of a fire-retardant-treated timber facade may differ from the performance established through classification and subsequently assumed during design, and that a classification or system approval does not necessarily mean that the fire properties will remain intact throughout the facade’s service life.”
Fire-retardant treatment can vary
Fire-retardant treatment can change the reaction to fire of timber, meaning that a product can, for example, achieve a Class B classification rather than the Class D classification of untreated timber. According to Maja Katharina Fjalland, who is affiliated with DBI and is working on a PhD project on the leaching of fire-retardant treatment from timber, there can nevertheless be differences in how thoroughly timber products are impregnated.
“Two timber elements from the same batch may have absorbed different amounts of treatment agent. It is therefore important that testing is based on a representative sample and that there is traceability between the classified product and the products delivered to the construction project,” she says and explains:
“Fire-retardant treatment involves adding fire-retardant substances to the timber, which reduce its ignitability and limit fire development. Some manufacturers also add resins, which act as a binder to help retain the fire-retardant substances in the timber. In laboratory experiments, we have seen that some fire-retardant agents dissolve at high humidity and that they leach out of the timber when exposed to water. We suspect that the same may apply to fire-retardant-treated timber used outdoors.”
“The Swedish report is consistent with the hypothesis that weather exposure may have reduced the effectiveness of the fire-retardant treatment of the facade cladding in this specific case. However, since no measurement was taken of the amount of fire retardants present at the time of delivery, it is not possible to establish how much may have leached out between delivery and the examination of the timber,” says Maja Katharina Fjalland.
EN 16755 does not document a specific service life
The new report also raises the question of exactly what existing standards document.
EN 16755 is not a general standard for the ageing of timber or the outdoor durability of timber. Rather, it is a standardised method for assessing whether a fire-retardant-treated timber facade product is ’suitable for external use’. A classification according to EN 16755 should therefore not, in itself, be regarded as documentation of a specific service life.
EN 16755 is currently being revised, with the revised version expected to be ready later this year.
“In the revised standard, the fire documentation will presumably be based on two SBI tests of aged timber, with the final result calculated as an average of the two tests. The timber must either have undergone 42 days of accelerated ageing or at least one year of natural ageing,” explains Martin Pauner, Master of Engineering in DBI’s fire testing department.
But the report also raises a broader question: To what extent do standardised exposures represent the many different influences – such as climate, construction details and maintenance conditions – that a facade is subjected to throughout its service life?
“Standardised methods are necessary if products are to be compared and marketed. But the methods need to be linked to knowledge about actual use, representative sampling and production control. Otherwise, we risk a gap between the documented properties and the actual performance of materials over time,” says Martin Pauner.
What does the report mean for Denmark?
The SP FIRE 105 method is not directly used as the general pre-accepted documentation route in Denmark. A failure in SP FIRE 105 therefore does not automatically mean that a Danish building with a similar timber facade fails to comply with the building regulations or is unsafe from a fire perspective.
“Nevertheless, the report is relevant to Denmark. In Danish construction projects, fire-retardant treatment may be a crucial prerequisite for a timber cladding to achieve the reaction to fire class on which the design is based,” says Anders Dragsted and concludes:
“If the fire properties deteriorate significantly during use, a key prerequisite for the building’s fire safety may change.”
The industry needs more knowledge
Bio-based materials are important for the green transition in construction. The Swedish report should therefore neither be used to reject timber as a building material nor as an argument against fire-retardant treatment of timber. However, the uncertainty should not be downplayed either.
“The report leaves us with several possible explanations. The responsible response must therefore be to reduce the uncertainty through better traceability, field studies and documentation of the actual long-term performance of fire-retardant-treated timber products,” says Anders Dragsted and continues:
“The goal should be a robust system in which we not only document the fire properties of timber products at the time of testing and the quality of production, but also have a credible basis for assessing whether these properties are maintained over time and under the expected conditions of use. This will strengthen both fire safety and confidence in bio-based solutions in construction.”
DBI therefore recommends that the Swedish report be used as an opportunity for a targeted Nordic knowledge initiative. Only when data are available from several products, buildings and exposure conditions will it be possible to determine the true extent of the challenge and what changes are needed in testing, product control, design and building operation.
Key findings from the Swedish report:
See the full report
Facades without fire-retardant treatment
Uncertainty about the long-term performance of fire-retardant-treated timber facades was one of the reasons behind DBI’s BioFacades:UpHigh project, which was completed in 2025.
The project investigated how bio-based facade constructions can be designed without fire-retardant treatment and without sprinklers. 10 full-scale facade tests were conducted, including fire barriers, flame deflectors, projections and non-combustible facade elements.
The results provide the industry with a basis for developing fire-safe bio-based facades in which safety is achieved through the overall design of the facade rather than relying solely on fire-retardant treatment of the timber.
Read more about BioFacades:UpHigh, and see all test material.
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