What are the building height thresholds for external wall fire requirements?

UK Building Regulations set three height thresholds for external wall materials. Above 18m: all materials in the external wall (cladding, insulation, fillers) must be non-combustible, achieving Class A1 or A2-s1,d0 per Regulation 7(2). Between 11m and 18m: materials must achieve Class B-s3,d2 or better (limited combustibility). Below 11m: no specific reaction to fire class is mandated, though general fire safety requirements still apply. These thresholds were strengthened after the Grenfell Tower fire.

Can PIR insulation be used in buildings over 18m?

No, PIR (polyisocyanurate) insulation cannot be used in external walls of buildings over 18m in height. PIR typically achieves Class C or D reaction to fire, which does not meet the A1/A2-s1,d0 requirement. The only common insulation materials that achieve Class A1 or A2 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.

What is a BS 8414 fire test?

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. For buildings 11-18m, a BS 8414 test can demonstrate compliance as an alternative to material classification. For buildings over 18m, BS 8414 is no longer an alternative — non-combustible materials are mandatory.

How is building height measured for these regulations?

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.

Do these requirements apply to existing buildings?

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.

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External Wall Fire Calculator

Determine external wall and cladding fire requirements based on building height per Approved Document B, Regulation 7 and the Building Safety Act 2022.

Building Height

Top floor surface to lowest ground level

Building Type

Building use classification

Cladding Type

External cladding system proposed

Insulation Type

External wall insulation material

How We Calculate This

This calculator determines the fire classification requirements for external wall materials based on building height, using the thresholds set by Approved Document B and the Building Safety Act 2022.

Height thresholds

Above 18m: All materials Class A1/A2-s1,d0 (non-combustible) — no exceptions.
11m to 18m: Minimum Class B-s3,d2 or BS 8414 tested system.
Below 11m: Standard Building Regulations — no mandatory classification.

Key standards

BS EN 13501-1: Reaction to fire classification
BS 8414: Large-scale external fire spread test
Regulation 7(2): Combustibility restrictions

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.