The question being answered
Requirement B4 is about external fire spread. One half of it is the wall's own construction; the other half is geometry: how far the building sits from the boundary, and how much of its façade is unprotected.
The reasoning is straightforward. A building on fire radiates heat. If the neighbouring building is close enough, that radiation alone can ignite it without any flame contact. Space separation sets the distance at which the radiation received is low enough that this does not happen.
The terms, precisely
- Unprotected area
- Any part of the external wall that is not fire-resisting: windows, doors, openings, and any wall with less than the required fire resistance. Combustible cladding above a certain thickness counts as unprotected area too, at a reduced rate.
- Relevant boundary
- The boundary the building is being assessed against. It may be the site boundary, the centre line of a road, railway, canal or river, or a notional boundary between two buildings on the same site.
- Boundary distance
- The distance from the external wall to the relevant boundary.
- Enclosing rectangle
- The smallest rectangle that encloses all the unprotected areas of the wall being considered. The tabular method works from this rectangle and the percentage of it that is unprotected.
The 12.6 kW/m² criterion
The accepted criterion for space separation is that radiation at the relevant boundary should not exceed 12.6 kW/m². That figure is the intensity conventionally taken as capable of igniting timber in the presence of a pilot flame, and a real fire scenario supplies pilot flames in abundance, as burning debris and flying brands.
The convention is that the boundary is treated as a mirror line: if radiation at the boundary is limited to 12.6 kW/m², a building on the far side that is an equal distance away receives an acceptable level.
How the calculation works
BR 187 gives both a set of tables and the underlying analytical method. The analytical method is worth understanding even if you use the tables, because it explains what the tables are doing.
Step 1: radiation from the fire
The radiating surface is modelled as a black or grey body at a fire temperature. Its emissive power comes from the Stefan-Boltzmann relationship:
I = σ ε T4where σ is the Stefan-Boltzmann constant, ε is emissivity, and T is absolute temperature in kelvin. BR 187 Table 3 evaluates this at two compartment temperatures and rounds the result: a reduced fire load at 830 °C gives 84 kW/m², and a standard fire load at 1040 °C gives 168 kW/m². Which one applies is set by purpose group in the national building regulation guidance: 84 kW/m² for residential, office, assembly and recreation, shop and commercial; 168 kW/m² for industrial, storage and other non-residential. Higher fire loads therefore give correspondingly larger separation distances.
Step 2: the view factor
Only a fraction of that radiation reaches any given point. The fraction is the configuration factor, or view factor, φ, and it is pure geometry: how large the radiating rectangle appears from the receiving point, and at what angle.
BR 187 gives closed-form expressions for the standard cases. For a receiver on the centre line of a parallel rectangle, with dimensionless parameters X = W / 2S and Y = H / 2S where W and H are the width and height of the radiating rectangle and S is the separation:
φ = (2/π) [ X/√(1+X²) · tan-1( Y/√(1+X²) ) + Y/√(1+Y²) · tan-1( X/√(1+Y²) ) ]Related expressions cover a receiver aligned with the corner of a parallel rectangle, and a receiver on a plane perpendicular to the radiating rectangle. Real façades are handled by subdividing into rectangles and summing.
Step 3: the received intensity
Ireceived = φ × IsourceThen solve for the separation S that brings Ireceived down to 12.6 kW/m². Because φ is not invertible in closed form, this is done numerically, which is exactly what our boundary distance calculator does.
Why the answer is so sensitive to the rectangle. View factor falls off roughly with the square of distance but rises with the solid angle the emitter subtends. A tall narrow window at close range and a wide short window of the same area produce different view factors. Taking a generous enclosing rectangle to be safe can push the required distance out by metres, so define the rectangle honestly, and subdivide where the geometry warrants it.
The tabular method
For routine cases you do not need to compute anything. Approved Document B and BR 187 both give tables of permitted unprotected percentage against boundary distance for standard enclosing rectangles. The workflow is:
- Draw the smallest rectangle enclosing all unprotected areas on the wall.
- Express the unprotected area as a percentage of that rectangle.
- Read off the minimum boundary distance for that percentage and rectangle size.
The tables are conservative relative to a full calculation, which is the trade for their simplicity. Where the tabular answer does not work (a tight urban site, a boundary you cannot move), the analytical method will often give you a shorter distance for the same façade, and that is a legitimate route provided the calculation is presented properly.
Practical points that recur
- Both directions. Space separation is not only about protecting the neighbour. Check the radiation your building would receive from a fire on the adjoining site, particularly where the neighbour is close and the boundary is notional.
- Notional boundaries. Two buildings on the same site need a notional boundary between them, positioned so that both satisfy the criterion. It does not have to be the mid-point.
- Combustible cladding counts, and not at the same rate everywhere. In England and Wales, an external surface material worse than class B-s3, d2 and more than 1 mm thick makes that part of a fire resisting wall an unprotected area "equating to half its area", at Approved Document B Volume 2 paragraph 13.7 with Diagram 13.4, or Volume 1 paragraph 11.7 with Diagram 11.4. The halving is BR 187's engineering judgement that a cladding fire burns for far less time than a compartment fire. Northern Ireland takes the same approach at Technical Booklet E paragraph 5.10. Scotland does not. Technical Handbook clause 2.6.4 requires cladding more than 1 mm thick of European class B, C, D or E to be included in the unprotected area calculation, in full, with no half-area provision in the guidance. The trigger is stricter there too: class B or worse, rather than worse than class B-s3, d2. This catches people out on refurbishments where the wall was compliant before recladding, and it catches them again when an English precedent is carried over the border.
- Canopies, balconies and projections. Anything that changes the radiating geometry changes the answer. Balcony arrays in particular are not well represented by a single flat rectangle.
- The result is a design constraint, not a check. If space separation is going to drive the façade, it needs to be run at concept stage. Discovering it at technical design means changing window sizes late.
Last reviewed 25 July 2026 against the editions named above. Standards are revised; check the current published edition before relying on anything here in a design.