By: Roy Little, Senior Associate and Francesca Ravenhill, Senior Engineer and Analyst.

Is fire spread, via the external wall, a B3 or B4 issue, and can the two even be considered separately? Maybe the answer to these questions comes down to the method of fire spread, and where it spreads to?

For reference requirements B3(3) and B4(1) from Part B of Schedule 1 to the Building Regulations 2010 state:

Requirement B3 – Internal Fire Spread (Structure)

(3) Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken, to an extent appropriate to the size and intended use of the building, comprising either or both of the following-

            (a) sub-division of the building with fire-resisting construction.

            (b) installation of suitable automatic fire suppression systems.

Requirement B4 – External Fire Spread

(1) The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another having regard to the height, use and position of the building.

Therefore, with consideration of the above requirements, fire which spreads between compartments via external flaming, could be considered a B3 issue, as this form of fire spread can occur regardless of the materials used in the external wall.

In January 2022, in the Martlet Homes Ltd vs Mulalley & Co Ltd case, in relation to the ‘key stages of external fire spread’, it states that ‘in a restricted fire spread case the fire is contained in the next level without rapidly spreading up successive levels.’

This seems to suggest that when considering fire spread, the design is not required to contain a fire to the compartment of origin but should restrict the speed at which the fire spreads between compartments.

The judgement in the case further notes that ‘restricting the fire spread allows time for the initial fire and any secondary fire to be brought under control before the fire can rapidly spread up the building’. This again supports the idea that the fire safety design intends to restrict the speed of fire spread, contrary to the common opinion that fire spread beyond the compartment of fire origin is a failure of the fire safety design.

The judgement notes that ‘rapid fire spread occurs when the cladding system (external wall) contributes to flame spread resulting in a risk of multiple simultaneous secondary fires’.

Hence whilst the speed with which fire spreads up the external wall is a B4 (external fire spread) issue, once a fire breaches compartmentation and re-enters the building, it also becomes a B3 (internal fire spread) issue.

Therefore, to achieve compliance with B3, you need to restrict the speed, and extent, of fire spread via the external wall i.e., by restricting the combustibility of the materials used. As such, in Approved Document B (ADB) should the recommendation relating to combustibility of the external wall be moved to B3, or at the very least be cross referenced?

Why is this important? 

The recommendations in ADB B4, relating to the combustibility of materials and products used in the external wall, are limited to buildings over 18m.

There is still a requirement to consider the external wall performance in buildings below this height, but in what regard? With specific reference to height, this leaves the door open for the potential use of combustible materials in buildings under 18m.

However, in a building under 18m that is provided with compartment floors to inhibit the spread of fire within the building (B3), external fire spread (B4) needs consideration to ensure that the compartmentation is maintained. More specifically, to prevent rapid fire spread (a risk of multiple simultaneous secondary fires) via the external wall.

For example, would a three storey block of flats, with compartment floors and combustible materials in the external wall, still comply with B3? With a defend in place strategy, my thoughts would be no. For other building types, I think the answer will depend on why you have included compartment floors in the design.

Therefore, to comply with the requirements of B3, the use of combustible materials in the external wall must be restricted. Referencing the recommendations in ADB B3 or B4 in isolation, could however result in this being overlooked.

Martlet Homes Ltd vs Mulalley & Co Ltd

• https://www.casemine.com/judgement/uk/62d06440b50db9afc3734f17
• https://www.judiciary.uk/judgments/mulalley-co-limited-v-martlet-homes-ltd/