Research and analysis

Fire safety: Fire hazards of timber cladding (Summary and recommendations)

Published 22 December 2025

Applies to England

1. Introduction

The research project sets out OFR consultant’s analysis, view and recommendations that relate to the experimental campaign delivered by BRE Global. The report considers the timber cladding from three perspectives:

• The potential hazards associated with timber cladding, how these hazards might vary in function of the cladding form and what role reaction-to-fire classifications have in supporting compliance with Part B of Schedule 1 of the Building Regulations.

• Changes in the hazard associated with timber cladding that might arise due to thermal modification.

• Changes in the hazard arising because of aging of thermally treated wood.

• The role that fire-retardant treatments could have in mitigating timber cladding fire hazards.

2. Background

On 9th June 2019, a fire at Samuel Garside House, Barking, spread across balconies and façade composed of timber, affecting eight flats. BRE Global investigated the fire and produced a report on 21st February 2020. The report determined that the timber was thermally modified Scots pine and examined the balcony construction’s contribution to unprotected areas. This lead to an experimental work programme that sought to better understand the fire characteristics of thermally modified timber. The project aimed to measure and characterise the fire behaviour of Scots pine timber in four different states: i) aged and thermally modified (from Barking), ii) a virgin state, iii) a thermally modified state, and iv) a thermally modified and fire-retardant treated state. For the purposes of reporting, this initial series of experiments is denoted as Phase 1.

Phase 2 of the work compared (a) the Barking cladding in a flat configuration (19 mm Scots Pine) and (b) the implications of reducing the thickness of cladding to align with ADB recommendations, that is 9 mm Cedar cladding.

3. Findings and recommendations

The study that has aimed to better understand the hazards associated with timber cladding through experimentation and testing across several scales. This study was motivated by the fire at Samuel Garside House and is intended to inform future technical policy (Approved Document B guidance), on the matter of timber cladding. Conclusions are structured to speak to the specific research questions identified, the potential implications for fire safety technical policy and what further work might be warranted.

4. Response to research questions

The study identified four core research questions. These are reproduced below along with relevant associated conclusions:

1. Does thermal modification of timber result in a greater external fire spread hazard than virgin timber, and how does this change with ageing / weathering?
   • Contrasting PS-Virgin and PS-TM-New samples at small-scale, i.e., the mass loss cone, there is no appreciable difference in the burning or ignition characteristics.
   • When ageing was introduced, i.e., PS-TM-Aged, the critical heat flux for piloted ignition reduced, indicating that it would typically be easier to ignite compared to PS-TM-New. Given aged virgin timber was not subject to investigation, it has not been possible to establish if thermally modified timber is unduly affected by ageing in terms of its burning rate or ignition characteristics.
   • At the scale of the SBI and BRE intermediate rig, thermally modified timber (both new and aged) resulted in slightly increased peak heat release rates, which would indicate a marginally greater extent of fire spread. However, this heat release rate was substantially below that of PE-ACM.
2. Does the slatted configuration of timber cladding, the like of which was observed at Samuel Garside House, result in a greater external fire spread hazard than more traditional flat / lapped timber cladding?
   • EU Commission Decision 2006/213/EC notes that flat timber cladding configurations would be considered, without further testing, to achieve Euroclass D in terms of the expected fire spread index (FIGRA). This is subject to field of application limits, such as minimum thicknesses and timber density. The slatted configurations without fire-retardant treatment, be that thermally modified or virgin, achieved Euroclass E, suggesting a greater rate and extent of fire spread at the scale of the SBI.
   • The benchmarking of the tests undertaken in the BRE intermediate scale rig against flat cedar cladding reference configurations identifies that the peak heat release rate of slatted configurations was substantially higher. This was the case whether using virgin or thermally modified timber. This increase in heat release rate is attributed to a greater extent of fire spread in the test.
   • Slatted timber cladding results in a greater external fire spread hazard when compared to traditional flat / lapped timber cladding.
3. To what extent can the external fire spread hazard associated with timber cladding be mitigated through the application of fire-retardant treatments?
   • The results presented in Section 7 at the scale of the SBI demonstrate that, with the application of a fire-retardant treatment, a configuration like that adopted at Samuel Garside House can achieve Euroclass B in terms of fire growth when new. This study has not evaluated how the FIGRA might be influenced by ageing and weathering. However, the current classification system does not require explicit consideration of the durability of products and systems, be that in terms of reaction to fire or fire resistance.
   • At the intermediate rig scale, Section 8 demonstrates that thermally modified timber with a fire-retardant treatment results in a heat release rate much below that of an untreated equivalent. It is also less than a flat cedar cladding of 19 mm thickness. Again, the implications of ageing / weathering on the treatment were not investigated.
   • The application of a fire-retardant treatment reduces the external fire spread hazard from new timber cladding. No statement can be made on the likely performance of aged / weathered timber cladding.
4. Is the guidance in ADB able to capture and differentiate any such hazards that might arise from material changes (e.g., thermal modification) or configurational changes (e.g., slats)?
   • Despite the relatively small heating source in the SBI test, the impact of the configuration is apparent in the classification, with all slatted cases (virgin or thermally modified) demonstrating high FIGRA values. Therefore, the SBI can seemingly differentiate the hazard of a slatted timber cladding surface versus that of a flat homogeneous timber cladding surface, despite the heat source not being of intensity reflective of façade fire exposure.
   • Table 12.1 of ADB expresses external surface performance expectations in terms of Euroclasses. Except for EU Committee Decisions, a Euroclass would typically be expected to be evidenced for a particular application / system and not generalised to materials.

   • Whilst Euroclass-based guidance exists in Table 12.1, it is potentially undermined by three things: (i) footnote (3) and the imprecise definition of “timber cladding”, (ii) the lack of any provision for external surfaces within certain purpose groups, heights or for portions of buildings, and (iii) the limitation of balcony guidance to residential cases. Items (i) to (iii) may lead to a situation where timber cladding that supports rapid fire spread can be present and, within some of these building situations, could undermine the fire safety strategy by, for example, allowing compartmentation to be easily circumvented.

   • The guidance in ADB only partially captures the hazards from material and / or configurational changes to timber cladding.
5. Can generalised guidance for timber cladding justifiably be included in ADB, such as Note 3 to Table 12.1 of Approved Document B?
   • Phase 2 of this study has shown that for flat continuous timber cladding surfaces, thickness is a variable that influences the fire spread characteristics by influencing the time taken for burn-through to occur and for the cladding to ignite on the ‘cavity side’.
   • For the same thickness of cladding, it is also shown that the configuration of the joints between cladding elements is a factor influencing if burn-through occurs and, thus, the fire spread characteristics.
   • This suggests that thickness should not be the only determinant of fire performance for timber cladding, even for flat continuous timber surfaces. The configuration has a substantial influence and should be considered when products are specified that comply with the requirements of the Building Regulations.
   • Generalised guidance for timber cladding in ADB is not adequately justified by the inclusion of Note 3 to Table 12.1.

5. Recommendations for fire safety technical policy

Based on this study, OFR would make the following recommendations:

• Footnote (3) to Table 12.1 should be removed, leaving the external surface classifications presented only in terms of Euroclasses. This would prevent thickness being used as the sole determinant of performance and would include the effect of configuration as the SBI requires the testing of more ‘end-use’ representative samples.

• The relationship between Table 12.1 and its role in potentially facilitating rapid circumvention of compartmentation in some situations should be considered. This may require some separate guidance in the context of Regulation B3(3), given the historical emphasis of B4(1), and the associated guidance, has been on space separation.

• Where rapid circumvention of compartmentation has the potential to undermine the fire safety strategy for a building, consideration should be given to the inclusion of a minimum reaction-to-fire classification in cases where there is currently no provision. A similar classification could be extended to elements forming balconies.

• Whilst, except for EU Committee decisions, general material reaction-to-fire classifications do not exist, it is observed that actors within the construction industry can assume they do. This can extend to building control bodies. To this end, the BSR should consider issuing a clarification within ADB that reaction-to-fire classes should not be generalised and reflect the behaviour of products used in the as tested configuration.

6. Potential areas of further investigation

Three areas have been identified for potential further investigation:

• It is highlighted that aged thermally modified timber supported fire spread more rapidly than new thermally modified timber. It is not known whether this is an artefact of the thermal modification process or the aging process. Therefore, consideration should be given to exploring the impact of aging on the reaction-tofire characteristics of unmodified timber cladding.

• It has been established that fire retardants are effective in mitigating the burning rate of timber cladding systems, even under challenging configurations, such as those present at Samuel Garside House. However, this conclusion is drawn based upon fire retardant treated timber that is both new and has not been subject to an external environment or aging. If reliance were to be placed on such coatings to improve the reaction-to-fire performance of timber cladding, it would be prudent to undertake research with the aim of establishing its long-term efficacy.

• The study has not been able to explore other important variables that could impact the burning characteristics of timber cladding, including: the impact of species, moisture content and seasonal variations, other treatments such as varnishes or weatherproofing, nor the impact of different fire-retardant treatments.