Building spacing is a radiant-heat calculation: a fully burning wall 5 m long and 3 m high, at a flame temperature of 1,000 C, delivers about 23 kW/m² to a facing wall 5 m away, nearly twice the roughly 12.5 kW/m² at which wood ignites when a spark or flame is present, and it falls below that level only beyond about 7 m. A longer burning wall needs more room, 10 m for a 10 m wall and about 15 m for a 30 m wall, which is why a spacing of 5 m is only safe against non-combustible walls and long buildings need wider breaks.
Part of Settlement Layout for 150 People
Fire breaks and building spacing are the cleared distances between buildings and building groups that keep the radiant heat of one burning building below the level that ignites the next.
Distance Beats Fire-Fighting Because Radiation Falls Roughly With The Square Of Distance
The fire guide sets 5 m minimum (8 m preferred) between homes, doubled for combustible walls, and breaks of 5-10 m (Fire Prevention in Settlements). Those numbers can be checked with a short calculation, which also shows when they fail.
A fully burning wall acts as a radiating surface. The heat flux q on a facing target is
q = F × ε × σ × T⁴
where σ = 5.67 × 10⁻⁸ W/m²K⁴ (the Stefan-Boltzmann constant), ε is the emissivity (taken as 1, which overstates slightly), T is the absolute temperature of the flame in kelvin, and F is the configuration (view) factor, the fraction of the radiation leaving the emitter that reaches the target. For a target facing the centre of a rectangle of width W and height H at distance d, with X = W/(2d) and Y = H/(2d):
F = 4 × (1/2π) × [ X/√(1+X²) × arctan(Y/√(1+X²)) + Y/√(1+Y²) × arctan(X/√(1+Y²)) ]
Worked Example: A 5 m By 3 m Burning Wall
Inputs: W = 5 m, H = 3 m, target distance d = 5 m, flame T = 1,273 K (1,000 C).
- X = 2.5 ÷ 5 = 0.5; Y = 1.5 ÷ 5 = 0.3.
- √(1+X²) = 1.118; arctan(0.3 ÷ 1.118) = arctan(0.268) = 0.262; first term = 0.5 ÷ 1.118 × 0.262 = 0.117.
- √(1+Y²) = 1.044; arctan(0.5 ÷ 1.044) = arctan(0.479) = 0.447; second term = 0.3 ÷ 1.044 × 0.447 = 0.128.
- F = 4 × (0.117 + 0.128) ÷ 6.283 = 0.156.
- Emitted flux: σT⁴ = 5.67 × 10⁻⁸ × (1,273)⁴ = 5.67 × 10⁻⁸ × 2.626 × 10¹² ≈ 148,900 W/m², about 149 kW/m².
- q = 0.156 × 149 ≈ 23 kW/m².
The same calculation at other distances and at a cooler 800 C flame (1,073 K, emitted flux about 75 kW/m²):
| Distance | q, flame 1,000 C | q, flame 800 C |
|---|---|---|
| 5 m | 23 kW/m² | 12 kW/m² |
| 8 m | 10 kW/m² | 5 kW/m² |
| 10 m | 6.7 kW/m² | 3.4 kW/m² |
| 15 m | 3.1 kW/m² | 1.6 kW/m² |
Solving for the distance at which q falls to 12.5 kW/m²:
| Burning wall | Flame 1,000 C | Flame 800 C |
|---|---|---|
| 5 m × 3 m | about 7 m | about 5 m |
| 10 m × 3 m | about 10 m | about 6 m |
| 30 m × 3 m | about 15 m | about 8 m |
What The Numbers Say About The Guide’s Distances
- The 5 m minimum sits at or below the ignition level for a wall of one house, so it protects only against non-combustible walls, which do not ignite, or a cool fire. That is the fire guide’s own condition.
- Doubling to 10 m for combustible walls covers a house-sized wall and a 10 m wall at the hotter flame, but not a 30 m longhouse wall, which needs about 15 m.
- The inverse-square statement in the fire guide holds well for distances beyond about the wall’s own size; between 10 and 20 m the factor falls from 0.045 to 0.012 for the 5 m wall, a ratio of about 3.8 against 4.
- A fire break of 5-10 m is a break in fuel, not a shield: its job is to stop ground fire and keep clearing, while the building-to-building distance does the radiation job.
Limits Of This Calculation
The calculation assumes a uniform, fully involved, flat wall at constant temperature and a still atmosphere, and it describes the flux, not the time to ignite. Real flames are partly transparent and cooler than 1,000 C, which lowers the figures; wind tilts flames toward the next building and raises them. The 12.5 kW/m² level is the commonly cited threshold for piloted ignition of wood under prolonged exposure; a higher flux ignites sooner, about 28 kW/m² is the level cited for ignition without a pilot, and a lower flux may still ignite thatch or dry grass over a long exposure. The calculation says nothing about embers, which the fire guide notes can travel hundreds of metres on wind and ignite combustible roofs far outside any spacing distance.
Sources And Honest Limits
The Stefan-Boltzmann constant and the view-factor formula are standard results found in any heat-transfer or fire-dynamics text; the numbers above were computed for this article. The 12.5 kW/m² and 28 kW/m² ignition levels and the 800-1,000 C flame range are the commonly cited figures from fire-engineering references (Drysdale, An Introduction to Fire Dynamics, is the usual source); they are not stated elsewhere in this corpus and should be checked against a text before they are used to set a binding rule.
Method: Setting A Separation Distance
Step 1: Measure the longest exposed wall of the building that might burn.
Step 2: Read the distance from the second table for that length, using the hotter column.
Step 3: Choose the wall of the neighbour. A non-combustible neighbour wall and roof allow the shorter spacing; a timber or thatch neighbour needs the full figure or a shield.
Step 4: Check the roof separately for embers, using metal, tile or sod rather than thatch.
Common Mistakes
- Using 5 m for timber homes — against a house-sized burning wall the flux at 5 m is about twice the level at which wood ignites.
- Ignoring wall length — the long wall of a longhouse is the dominant radiator.
- Filling the break with fuel — woodpiles and haystacks in the gap turn it into fuel.
Safety
A radiant-heat zone is also a burn zone
A flux of 12.5 kW/m² or more burns exposed skin quickly, so a fire-fighter standing within a few metres of a burning wall is in the danger zone. Fight fire from behind a non-combustible wall and wet the neighbour’s wall rather than approaching the burning one.
What’s Next
- Learn next: Water Points for Fire Fighting — wetting the neighbour
- Combine with: Longhouse — the building whose long wall dominates the calculation