Question 1

What is the enclosing rectangle method for external fire spread?

Accepted Answer

The enclosing rectangle method (BR 187, reflected in Approved Document B Vol 1 Table 11.1) assesses external fire spread risk. You draw the smallest rectangle that encloses all unprotected areas on a facade, calculate the percentage of unprotected area within that rectangle, and compare it against the maximum permitted percentage based on the distance to the relevant boundary (4% at under 1m, rising linearly to 100% at 12.5m). The boundary distance is measured perpendicular from the facade to the site boundary or centre of a public road.

Question 2

What counts as an unprotected area?

Accepted Answer

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.

Question 3

How does sprinkler protection affect the boundary distance calculation?

Accepted Answer

Where a building is fitted with an automatic sprinkler system, AD B allows the unprotected area to be effectively halved for the purposes of the boundary distance calculation. In practice, this means you can have approximately double the unprotected area at the same boundary distance, or achieve the same unprotected area at a shorter boundary distance. The sprinkler system must comply with the relevant part of BS EN 12845 or BS 9251.

Question 4

What is the minimum distance to a boundary?

Accepted Answer

There is no absolute minimum distance specified in AD B, but at distances less than 1 metre, the maximum permitted unprotected area is very small (around 4-8%). At 1 metre, the maximum is approximately 8% without sprinklers. In practice, most buildings need at least 1 metre to the boundary. The relevant boundary includes the site boundary, the centre line of a road, railway or canal, or a notional boundary between buildings on the same site.

Question 5

Can fire-rated glazing reduce the unprotected area?

Accepted Answer

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.

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