Building Standards technical handbook 2017: non-domestic buildings

There is a risk of fire spread on the external walls of a building. Fire could break-out through a window or door opening and spread onto the external walls. External walls close to the boundary are also at risk as they may be exposed to radiant heat flux from a fire in an adjoining building.

Entertainment and assembly buildings are given special consideration because of the higher risk of wilful fire-raising against the external walls (as in the Summerland fire, 1973).

Residential care buildings and hospitals also present a greater risk because the mobility, awareness and understanding of the occupants could be impaired and as a consequence, full evacuation immediately a fire is discovered may not be the most appropriate course of action.

In high rise buildings, there is a need to take further precautions as external fire spread could involve a large number of floors thus presenting greater risk both to the occupants of the building and to fire fighters. The reaction to fire characteristics of cladding materials are therefore more demanding the higher the building.

The guidance for fire spread on external walls of buildings should not be assessed in isolation and reference should be made to the guidance to Standard 2.4 for fire spread in cavities and the guidance to Standard 2.6 for fire spread to neighbouring buildings.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

External wall cladding includes non load-bearing external wall cladding systems attached to the structure, for example, clay or concrete tiles, slates, pre-cast concrete panels, stone panels, masonry, profiled metal sheeting including sandwich panels, weather boarding, thermally insulated external wall rendered systems, glazing systems and other ventilated cladding systems.

External wall cladding not more than 1m from a boundary should have a non-combustible classification.

Where the cladding is more than 1m from the boundary and is constructed from combustible products more than 1mm thick, that has a low, medium, high or very high risk (as described in annex 2.E), the cladding should be constructed from materials with a reaction to fire in accordance with the following table:

A cavity formed by external wall cladding should be protected in accordance with the guidance to Standard 2.4 and fire spread to neighbouring buildings in accordance with the guidance to Standard 2.6.

Alternative guidance - BR 135, ‘Fire Performance of external thermal insulation for walls of multi-storey buildings’ and BS 8414: Part 1: 2002 or BS 8414: Part 2: 2005 have been updated to include the most up-to-date research into fire spread on external wall cladding. The guidance provided in these publications may be used as an alternative to non-combustible or low risk classifications (as described in clauses 2.7.1 and 2.7.2) and for materials exposed in a cavity, as described in clause 2.4.6.

In a building with any storey at a height of more than 18m above the ground, any insulation material situated or exposed in a cavity formed by external wall cladding should be non-combustible.

However an insulation product need not achieve a non-combustible classification where: