Boundary distance and unprotected areas (BR 187)
Space separation limits the radiant heat a burning building can deliver across a boundary. BR 187, External fire spread: building separation and boundary distances sets the accepted method, and Approved Document B adopts it.
This calculator uses the Appendix A radiation method directly rather than reading a table. It builds an enclosing rectangle around the unprotected areas on the elevation, computes the configuration factor for a receiver parallel to and centred on that rectangle, and solves for the separation at which the received intensity falls to 12.6 kW/m², the level BR 187 takes as the threshold for piloted ignition at the relevant boundary.
φ = (2/π)[ X/√(1+X²) · tan⁻¹(Y/√(1+X²)) + Y/√(1+Y²) · tan⁻¹(X/√(1+Y²)) ]
where X = W/2S and Y = H/2S (BR 187 equation A3)
13.86 m to the relevant boundary
A 12 m × 9 m enclosing rectangle at 100% unprotected needs about 13.86 m to the relevant boundary. If the boundary is a notional one between two buildings on the same site, the buildings need roughly twice that between them.
Working
| Step | Value |
|---|---|
| Emitter intensityfrom the purpose groupBR 187 | 84 kW/m² |
| Permitted configuration factor12.6 ÷ (84 × 100%)BR 187 equation A2, rearranged | 0.15 |
| Dimensionless parameters at the solutionX = 12 ÷ (2 × 13.86), Y = 9 ÷ (2 × 13.86)BR 187 equation A3 | X = 0.433, Y = 0.325 |
| Separation distancesolved so that received intensity = 12.6 kW/m² | 13.86 m |
What this means
- The enclosing rectangle is the smallest rectangle that contains all the unprotected areas on the elevation, with sides that are multiples of 3 m in the tabulated method.
- This is the parallel, centre-aligned case. Where the receiver is offset, or the boundary is at an angle to the elevation, the configuration factor is built up from corner-aligned components (BR 187 equations A4 and A5) and the answer will differ.
Sources
- BR 187 (2nd edition), Appendix A, equation A1Radiation intensity of the emitter, from the Stefan–Boltzmann law.
- BR 187 (2nd edition), Appendix A, equation A2Intensity received at the receiver, φ × I.
- BR 187 (2nd edition), Appendix A, equation A3Configuration factor for a parallel, centre-aligned receiver.
- Approved Document B, Volume 2, Section 13Space separation, unprotected areas and the relevant boundary.
- Approved Document B, Volume 2, Appendix A, Table A1Notional boundary and the assumption that the boundary lies on the centre line between two buildings on the same site.
This is an engineering aid, not engineering advice. The result above is produced from the inputs you gave and the guidance cited. It does not account for everything a competent fire engineer would consider on a real building, and it is not a substitute for reading the current published standard. Check every number before it goes into a design.
Questions people ask
What is the relevant boundary in Approved Document B?
The boundary the elevation faces, provided it coincides with, is parallel to, or is at no more than 80° to the elevation. Where two buildings sit on the same site, a notional boundary is taken between them, normally on the centre line, and each building is assessed to that notional boundary.
What counts as an unprotected area?
Any part of the external wall that is not fire resisting: windows, doors, openings, and any external wall with less than the required period of fire resistance. Combustible cladding more than 1 mm thick over an otherwise fire-resisting wall counts as an unprotected area equal to half its area.
Why 12.6 kW per square metre?
It is the radiant intensity BR 187 takes as the threshold for piloted ignition of the receiving surface (that is, ignition where a flame or spark is present). The separation is set so that received radiation at the relevant boundary does not exceed it.
Does this replace the BR 187 tables?
No. The tabulated distances in BR 187 and Approved Document B include rounding and conservatism, and they are what a building control body or the Building Safety Regulator will check against. Use this calculator to understand the sensitivity, to interpolate, and to sanity-check a tabulated answer; then cite the table.
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