Several instances of a fire originating either in the building or externally, have led to rapid vertical fire spread via combustible cladding finishes, insulation materials and other significant items within the external wall build-up. The spread is also aided by the provision of vertical cavities and plentiful supply of combustion air.
Using combustible materials and extensive cavities in the building fabric can exacerbate fire within the external wall medium, particularly given the delamination and spalling nature of some heated components. Constructing the facade using materials with lower spread of flame, key components with restricted combustibility, and utilising robust cavity barriers, can reduce the rapid spread of fire.
The document BR135 – Fire Performance of External Thermal Insulation for Walls of Multi-storey Buildings considers the design of the facade materials and external wall construction, and explains the mechanisms of fire spread in these situations.
As part of the design of the cladding systems proposed for the facades, there are four suitable options to provide an acceptable system:
Use materials of limited combustibility (MOLC) for all elements of the cladding system. This includes the insulation, internal lining board and the external facing material. Smaller gasket parts and similar low-risk items can be excluded from this requirement but need to be considered on a case-by-case basis.
Submit evidence that the complete proposed external cladding system has been assessed according to the acceptance criteria in BR135. The preferred method of demonstrating compliance is via a fire test carried out in accordance with BS8414 Parts 1 & 2 or NFPA 285.
The test should be carried out by an independent accredited testing body. The BS8414 and NFPA 285 tests do not give a PASS / FAIL answer because the data obtained is used by different bodies with different minimum requirements.
Therefore, for code compliancy purposes, any test using this method needs to be supported with a Classification Report for the proposed specification confirming that the acceptance criteria of BR135 has been met. This acceptance criteria is listed in Annex A or Annex B of BR135 and includes external fire spread, internal fire spread and mechanical performance.
If no fire test data exists for a particular system, the client may instead submit a desktop study report from a qualified fire specialist stating whether, in their opinion, BR135 criteria would be met with the proposed system. The report should be supported by test data from an independent accredited testing body and so this option may not be of benefit if the products have not already been tested in multiple situations / arrangements. The report should also specifically reference the tests, which have been carried out on the product.
If none of the above options are suitable, the builder may consider a holistic fire engineered approach for the entire building, which is recognized in NFPA and the British Standards as an acceptable alternative approach.
Fire Safety Engineering not only considers the performance of structures, systems, products and materials when exposed to fire, it also includes fire prevention and active and passive fire protection measures e.g. effective means of egress and adequate measures for alarm, detection, control and extinguishment.
Furthermore, it can facilitate innovation in building design without compromising fire safety, particularly in some large and complex buildings, as well as multi–purpose buildings, where it may be the only practical way to achieve a satisfactory level of fire safety.
If taking this advanced route to compliance, it is recommended that the guidance given in BS7974 and IFEG (International Fire Engineering Guidelines) should be followed.