External Wall Fire Calculator

The two height-triggered combustibility rules apply to RESIDENTIAL "relevant buildings" only — those containing a dwelling, an institution (hospital, care home, student accommodation) or a room for residential purposes, plus hotels, hostels and boarding houses since 1 December 2022. For these uses: at 18m or more, every material in the external wall (cladding, insulation, filler/core material, spandrel panels) must be non-combustible — Class A1 or A2-s1,d0 — under Regulation 7(2). At 11m or more, any insulation product or filler material must be Class A2-s1,d0 or better under Approved Document B para 10.7. Below 11m there is no height-triggered class, but the boundary surface-spread rule still applies. Offices, assembly/public buildings and non-residential schools are NOT caught by Regulation 7(2) or para 10.7. However, ADB para 10.6 catches ANY building with a storey 18m or more, regardless of use: insulation and filler material must be Class A2-s3,d2 or better. Below 18m, these non-residential uses follow the general ADB external-fire-spread route. These rules were strengthened after the Grenfell Tower fire.

No, PIR (polyisocyanurate) insulation cannot be used in the external walls of residential relevant buildings 18m or more in height. PIR foam typically achieves Euroclass B (occasionally C), which does not meet the A1 or A2-s1,d0 requirement of Regulation 7(2). The only common insulation materials that achieve Class A1 or A2-s1,d0 are mineral wool (stone wool and glass wool), cellular glass and some calcium silicate products. This restriction applies to all materials within the external wall, not just the visible cladding. The same A2-s1,d0 limit applies to insulation and filler material at 11m and above in residential buildings (ADB para 10.7), so PIR is also not acceptable there.

BS 8414 is a large-scale fire test for external cladding systems. A full-size cladding system is built and subjected to a severe fire from a combustion chamber at the base. The test measures fire spread up the facade, internal temperatures within the cladding system and any falling debris. Results are assessed against BR 135 criteria. A BS 8414 / BR 135 tested system is a recognised route to demonstrate the general ADB external-fire-spread requirements. However, for a residential relevant building it cannot override the prescriptive material rules: the Regulation 7(2) A1/A2-s1,d0 ban at 18m and the ADB para 10.7 A2-s1,d0 insulation/filler requirement at 11m apply in their own right to the regulated components. BS 8414 evidence remains useful for components not caught by those prescriptive rules and for non-residential buildings following the general external-fire-spread route.

Building height is measured from the ground level on the lowest side of the building to the top of the floor surface of the topmost storey. It does not include the roof unless the roof space is a usable storey. For buildings on sloping ground, the measurement is taken from the lowest adjacent ground level. This is the definition used in Regulation 7 and Approved Document B. A building with its top floor at 18.0m is subject to the full non-combustible requirements.

The Building Safety Act 2022 and associated regulations have introduced requirements for existing buildings over 11m with specific cladding types to be remediated. Building owners of high-rise residential buildings (over 18m) with unsafe cladding must carry out remediation. The act established the Building Safety Regulator and requires building safety cases for high-rise residential buildings. Government funding schemes are available for leaseholders in buildings over 11m with unsafe cladding.