Review of Approved Document B: Fire Safety

47. Timber is a traditional building material that has been utilised successfully for centuries to provide low-rise housing in England. Recent developments in mass timber solutions have provided structural timber configurations that could support the use of timber in increasingly tall buildings.  

48. High-rise buildings have traditionally been built out of systems that do not contribute as fuel, such as masonry, concrete and steel. In case of fire, these traditional systems achieve a high fire resistance rating which delivers for post-fire performance of elements of structure. The emergence of mass timber construction systems has challenged the fundamental assumptions underpinning the guidance.  

49. While Approved Document B and its guidance on fire resistance ratings is seen as suitable to support the use of timber structures in low-rise buildings, BSR is clear that some parts of the guidance cannot be used for high-rise timber buildings. Government published FAQ 23 in 2022 making clear that tall, large or complex buildings with combustible elements of structure are out of scope of Approved Document B. 

50. To clarify what BSR considers as ‘tall, large or complex’ when combustible elements of structure are utilised, we are seeking to introduce a clear threshold within the guidance. The threshold will make clear the height above which buildings utilising combustible structural elements (including timber) cannot solely apply the guidance within Approved Document B to demonstrate compliance. Where combustible structural elements are used, an alternative approach is expected.  

51. A threshold of 11 metres is proposed in consideration with the guidance on consequence classes in Approved Document A and published industry guidance and standards. This strengthens the connection between Approved Document B (Fire safety) and Approved Document A (Structure) by aligning fire safety guidance with the principles of preventing disproportionate collapse. This alignment enhances the coherence between the two documents and reinforces their shared intent.  

52. While the change to the guidance incorporates an FAQ which outlines the current intent of Approved Document B, we recognise that this clarification may have an impact on the use of combustible structural elements above the threshold.  

53. BSR seeks to be clear that we continue to support the use of timber in both residential and non-residential low-rise buildings. Our position aligns with the Timber in Construction Road map Timber in construction roadmap 2025 - GOV.UK that seeks to:  

“increase the safe use of timber in construction (TiC) in England, to help reduce embodied carbon in the built environment and drive investment into tree planting, forest management and domestic supply chains, creating new green jobs and industries.” [Executive summary] 

54. The lower risk profile of buildings below the threshold set out above, means that timber can continue to be utilised safely and where the environmental benefits are more significant, with sufficient consideration of fire safety, structural safety and durability. Timber in low-rise construction is where the associated environmental benefits are more significant. The new threshold still allows for timber to be used above 11m by ensuring a fully fire engineered approach is taken.  

55. In line with priority theme 5 of the Timber in Construction Roadmap, aimed at: “addressing fire safety and durability concerns to safely expand the use of engineered mass timber.”[Priority theme 5], we are committed to working with industry, academia, and the wider sector to further investigate outstanding fire safety, durability, and competence questions. 

56. To support ongoing development of research into the deployment of timber structures at height, BSR published research conducted as part of the technical review of Approved Document B that considered the fire safety risks associated with fully encapsulated mass timber. The research report and findings can be found here. 

57. BSR plans to incorporate the FAQ into the guidance within Approved Document B and state a clear threshold of 11m for both residential and non-residential buildings where Approved Document B is not suitable if the building utilises combustible structural elements.  

58. The options considered were: 

1. Option 1 (Preferred): Introduce a threshold of 11m above which Approved Document B cannot be used when combustible structural elements are utilised 

2. Option 2: Introduce a threshold of 11m for residential buildings and 18m for non-residential buildings above which Approved Document B cannot be used when combustible elements are utilised 

59. Our preferred approach has an estimated cost of £0.74 million over 10 years, with an estimated net annual cost to business (ENACB) of £0.08 million. The costs are associated with the additional design time necessary to develop a fire engineered solution over and above an Approved Document B-based solution. Given the low number of buildings above 18m designed using combustible elements of structure that aren’t already taking a fire engineered approach, we are confident in the expected impact.  

60. Splitting the threshold to 11m for residential buildings and 18m for non-residential buildings would have a reduced cost of £0.6 million over 10 years with an estimated ENACB of £0.07 million. In our view, the complexities in fire safety design in buildings above 11m+ extend beyond the guidance provided in Approved Document B to be considered sufficient on its own.   

61. We anticipate that with this increased clarity, any existing hesitation within the industry regarding the use of combustible elements of structure will diminish over time, particularly in light of ongoing research efforts and the sector’s commitment to advancing sustainable construction practices.