What This Requirement Covers
Approved Document B, Requirements B3 and B4 address the external fire spread from a building to adjacent buildings and from one building to another across a boundary. The aim is to limit the risk of fire spreading via external walls and the roof.
Following the Grenfell Tower fire in 2017, significant amendments were made to restrict the use of combustible materials in the external walls of residential buildings over 18 metres in height.
Key Requirements
Boundary Distance and Unprotected Areas
- The permitted extent of unprotected areas (windows, doors, and areas of wall that do not achieve the required fire resistance) depends on the distance between the external wall and the relevant boundary
- Calculations use the enclosing rectangle method (Diagram 13.1, Approved Document B Volume 2) or the aggregate notional area method
- At 1 metre from the boundary, only a very limited percentage of the wall can be unprotected
- At distances greater than 6 metres, restrictions on unprotected areas are significantly relaxed
- A wall on or within 1 metre of the boundary must achieve the full fire resistance period from both sides and have no unprotected areas (with limited exceptions)
External Wall Materials
- Buildings over 18 metres in height (Regulation 7(2) as amended by SI 2018/1230): All materials in the external wall (including insulation, filler materials, and any component forming part of the wall) must achieve European Classification A2-s1,d0 or better (limited combustibility) or A1 (non-combustible)
- This applies to residential buildings, hospitals, care homes, dormitories, and student accommodation over 18 metres
- Buildings 11-18 metres: Materials in the external wall must be Class A2-s1,d0 or Class B-s3,d2 minimum, or the wall system must pass a BS 8414 large-scale fire test with BR 135 criteria
- Buildings under 11 metres: The external surface of walls must achieve a minimum Class B-s3,d2 rating (or index not exceeding 20 per BS 476: Part 6)
Roof Coverings
- Roof coverings are classified by their performance under BS EN 13501-5
- Buildings within 6 metres of a boundary require a roof designation of at least B-roof(t4) or national class AA, AB, or AC
- Thatch and wood shingle roofs have specific distance restrictions
Residential vs Commercial
The boundary distance requirements and unprotected area calculations apply equally to residential and commercial buildings, but the fire resistance period differs by building type, height, and use. Commercial buildings may require higher fire resistance periods and more onerous material restrictions depending on the Approved Document B Volume 2 tables.
Post-Grenfell Changes
- The ban on combustible cladding materials for buildings over 18 metres came into force on 21 December 2018
- The Building Safety Act 2022 further strengthened requirements for higher-risk buildings
- The Government has established remediation programmes for existing buildings with unsafe cladding
- The EWS1 (External Wall System) form process is used to assess fire risk in existing buildings for mortgage and sale purposes
Practical Compliance Tips
- Commission an external wall fire assessment early in the design process for buildings over 11 metres
- Obtain test certificates and classification reports for all external wall materials before procurement
- The boundary distance is measured to the centre line of a road, railway, or canal, and to the actual boundary with adjacent land
- Where a building is set back from the boundary, use the most onerous distance for each section of the external wall
- Keep records of all material certifications for the building's Golden Thread (higher-risk buildings)
- For existing buildings undergoing re-cladding, obtain an independent fire engineer's assessment
- Consider the impact of balconies, solar panels, and other attachments on external fire spread