External Fire Spread Calculator

AD B Vol 1 paragraph 11.16 gives two simple methods for walls at least 1m (1000mm) from the relevant boundary. Method 1 (Diagram 11.7) applies to small residential buildings — blocks of flats and dwellinghouses up to 3 storeys (excluding basements) or 24m per side — and sets an absolute maximum unprotected AREA in m²: 5.6 m² at 1m, 12 m² at 2m, 18 m² at 3m, 24 m² at 4m, 30 m² at 5m, and no limit at 6m or more. Method 2 (Table 11.1) is for other small buildings or compartments up to 10m high and uses a PERCENTAGE of the enclosing rectangle: 4% under 1m, 8% at 1m, 20% at 2.5m, 40% at 5m, rising to 100% at 12.5m. This calculator picks Method 1 for flats and dwellinghouses (purpose group 1, the only buildings para 11.17 assigns to Diagram 11.7) and Method 2 for all other buildings, including institutional and residential-other (purpose group 2). For taller buildings or precise design, use the full BR 187 enclosing-rectangle procedure.

For Method 2 (Table 11.1) you draw the smallest rectangle that encloses all unprotected areas on a facade, divide the total unprotected area by the area of that rectangle and multiply by 100 to get the unprotected percentage, then compare it against the maximum permitted percentage for the distance to the relevant boundary (4% under 1m rising linearly to 100% at 12.5m, interpolating between table values). The boundary distance is measured perpendicular from the facade to the site boundary or the centre line of an adjoining road, railway or canal. Method 2 is used where the residential Method 1 (Diagram 11.7) is not appropriate, and applies up to 10m building height.

An unprotected area is any part of an external wall that does not achieve the required fire resistance for the building. This includes windows (unless fire-rated glazing), doors, openings, and wall panels that do not meet the fire resistance standard. Areas of wall with less than the required fire resistance from the inside (EI rating) are also unprotected. Small unprotected areas under 1m² and more than 1.5m from any other unprotected area may be disregarded.

Where a building is fitted throughout with an automatic sprinkler system, AD B Vol 1 paragraph 11.21 allows you to either double the permitted unprotected area, or halve the boundary distance (to a minimum of 1m). In practice this means roughly double the unprotected area at the same distance, or the same area at a shorter distance. The system must be designed and installed in accordance with AD B Appendix E — BS 9251 for residential buildings, or BS EN 12845 for non-residential buildings (or residential buildings outside the scope of BS 9251).

The two simple methods in AD B Vol 1 only apply to walls a minimum of 1m (1000mm) from any point on the relevant boundary (paragraph 11.16). Below 1m, neither Method 1 nor Method 2 applies: the wall must be fire-resisting from both sides and only the small unprotected areas allowed by Diagram 11.5 are permitted (paragraph 11.8). At 1m without sprinklers, Method 2 permits about 8% unprotected area, or Method 1 permits 5.6 m² for a small residential building. The relevant boundary includes the site boundary, the centre line of a road, railway or canal, or a notional boundary between two buildings on the same site (AD B treats the distance to a notional boundary the same as to a relevant boundary).

Yes, fire-resistant glazing that achieves the required EI (integrity and insulation) rating for the building is treated as protected area and does not count as unprotected. E-only rated glazing (integrity only, such as Georgian wired glass) still counts as unprotected area because it does not prevent radiant heat transfer. Using EI-rated glazing is a common solution when boundary distances are tight.