What is the enclosing rectangle method for external fire spread?

The enclosing rectangle method is the primary method in Approved Document B (Appendix B, Table B4) for assessing 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. The boundary distance is measured perpendicular from the facade to the site boundary or centre of a public road.

What counts as an unprotected area?

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.

How does sprinkler protection affect the boundary distance calculation?

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.

What is the minimum distance to a boundary?

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.

Can fire-rated glazing reduce the unprotected area?

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.

TradeCalculator

External Fire Spread Calculator

Check unprotected area percentages against boundary distance using the enclosing rectangle method per Approved Document B Table B4.

Boundary Distance

Distance from facade to relevant boundary

Boundary Condition

Site boundary or notional boundary

Facade Width

Width of enclosing rectangle

Facade Height

Height of enclosing rectangle

Number of Windows / Openings

Total unprotected openings in the facade

Average Window Area

Average area of each unprotected opening

How We Calculate This

This calculator uses the enclosing rectangle method from Approved Document B Table B4 to check whether the unprotected area on a facade complies with the boundary distance requirements.

How it works

Enclosing rectangle: Width x height of the smallest rectangle enclosing all unprotected areas.
Unprotected percentage: Total unprotected area divided by enclosing rectangle area.
Table B4 lookup: Maximum permitted percentage based on boundary distance.
Sprinkler factor: Effectively doubles the permitted unprotected area.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.