Defensible space works because radiant heat falls with distance: a wide flame front at 1,000 C needs about 6 times its own height in clear ground to drop below the 12.5 kW/m² that ignites wood, and about 3 times at 800 C. The fire guide’s zones of 10 m, 30 m and 60 m therefore match flames of roughly 2 m, 5 m and 10 m at the hot figure, and they fail against taller flames such as burning tree crowns, which also throw embers that distance does not stop.
Part of Fire Prevention in Settlements
Defensible space is the managed band of vegetation around buildings, in zones of decreasing fuel toward the buildings, that keeps a wildfire’s heat and flame away from walls and roofs.
The Zone Widths Are A Radiant-Heat Distance For A Given Flame Height
The fire guide gives three zones. Zone 1, 0-10 m from buildings: no combustible vegetation, dead material removed, grass under 10 cm, no woodpiles or fuel storage, a non-combustible ground cover. Zone 2, 10-30 m: thinned vegetation, dead branches removed, crowns at least 3 m apart, ladder fuels removed. Zone 3, 30-60 m: reduced fuel load, dense forest thinned, dead standing trees removed, fuel breaks along approach routes. The guide gives the widths but not the reasoning. The spacing article’s radiant-heat method supplies it.
Worked Example: How Far Back Must A Flame Front Be
The spacing article uses a rectangular flame of width W and height H and the view factor from the centre of a facing target (Fire Breaks & Building Spacing): F = (2 ÷ π) × [X ÷ √(1+X²) × arctan(Y ÷ √(1+X²)) + Y ÷ √(1+Y²) × arctan(X ÷ √(1+Y²))], with X = W ÷ 2d and Y = H ÷ 2d. A wildfire front is much wider than it is tall, so let W grow without limit. Then X ÷ √(1+X²) tends to 1 and arctan(Y ÷ √(1+X²)) tends to 0, so the first term vanishes, and arctan(X ÷ √(1+Y²)) tends to π ÷ 2. What is left is
F = Y ÷ √(1+Y²) = H ÷ √(H² + 4d²)
and the flux on a wall at flame mid-height is q = F × E, with E = σT⁴: 149 kW/m² at 1,000 C (1,273 K) and 75 kW/m² at 800 C (1,073 K), the spacing article’s two flame temperatures.
To reach the wood ignition level q = 12.5 kW/m² (the spacing article’s figure for ignition with a flame present), F = 12.5 ÷ E. Solving for d: d = (H ÷ 2) × √((E ÷ q)² − 1).
- At 1,000 C: E ÷ q = 149 ÷ 12.5 = 11.9; (11.9)² − 1 = 141; √141 = 11.9; d = 5.9 × H.
- At 800 C: E ÷ q = 75 ÷ 12.5 = 6.0; 6.0² − 1 = 35; √35 = 5.9; d = 2.96 × H.
| Flame height H | d at 1,000 C | d at 800 C |
|---|---|---|
| 2 m | 12 m | 6 m |
| 6 m | 36 m | 18 m |
| 10 m | 59 m | 30 m |
| 20 m | 119 m | 59 m |
Read against the zones: 10 m of clear ground (Zone 1) matches a 2 m flame at either temperature; 30 m (Zones 1 and 2) matches a 5 m flame at the hot figure (5.9 × 5 = 30 m) and a 10 m flame at the cooler one; 60 m (all three zones) matches a 10 m flame at 1,000 C or a 20 m flame at 800 C. The flame heights are inputs, not measurements, and no flame-height source is given in this corpus.
Limits Of The Calculation
The sheet is taken as uniform, infinitely wide and fully radiating, which overstates the flux because real flames are partly transparent. Wind tilts flames toward the target and raises it. The calculation gives the flux, not the time to ignite; the fire guide says the front passes in 5-15 minutes, which is long enough for prolonged exposure to matter, since the spacing article notes that the 12.5 kW/m² figure applies to prolonged exposure. And it says nothing about embers, which ignore the zones (Ember Protection).
What You Need
- A measured or estimated flame height for the fuel beyond each zone: grass, shrub or tree crown
- A tape and pegs to mark the zone edges
- Tools and labour for the clearing (Fuel Management & Clearing)
- A non-combustible ground cover for Zone 1: gravel, stone or bare earth
Method: Fit The Zones To The Fuel
Step 1: Estimate the flame height of the unmanaged fuel beyond the outer zone: grass (low), shrub (medium), forest (tall crowns).
Step 2: Read the distance from the table for the hotter figure and take the larger.
Step 3: Compare with the guide’s 60 m. If crowns taller than about 10 m stand at the edge, widen the outer zone or thin the crowns.
Step 4: Manage each zone so its own flames stay short: bare in Zone 1, low in Zone 2, thinned in Zone 3.
Step 5: Keep the zones maintained, because a zone left to regrow is just fuel near the buildings.
Common Mistakes
- Clearing 10 m and stopping. It covers a grass flame, not a forest edge.
- Leaving ladder fuels. Brush carries a ground fire up into the crowns.
- A cleared zone covered in woodpiles. Zone 1 holds no fuel storage at all.
- Treating the zone as an ember defence. It is a heat and flame defence.
Safety
Heat injures before anything ignites
The 12.5 kW/m² level is about the wood-ignition figure; skin is hurt at far lower fluxes, so a person in Zone 1 or 2 when a front passes can be burned without any building burning. Do not stay outside or in a lightly built building. The guide’s rule that the front passes in 5-15 minutes does not mean a person can stand in the open for that time (Shelter-in-Place Preparation).
What’s Next
- Learn next: Fuel Management & Clearing — keeping the zones working
- Combine with: Wildfire Defense — the whole plan