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Building Spacing & Fire Breaks

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Prevention by Design
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Building Spacing & Fire Breaks

Fire spacing is a land cost as well as a safety rule: with houses 8 m wide, a gap of 5 m fits 8 houses along a 100 m frontage while 16 m, the preferred 8 m doubled for timber walls, fits only 4. A perimeter fire break 30-50 m wide around a settlement of 150 m radius covers 44% to 78% of the settlement’s own area, so plan breaks as paths, gardens and grazing from the start.

Part of Fire Prevention in Settlements

Building spacing and fire breaks are the cleared gaps between buildings and between building groups that keep one fire from reaching the next, planned as part of the settlement layout.

Every Metre Of Gap Costs Land, So Plan It As Multi-Use

The fire guide’s spacing is 5 m minimum (8 m preferred) between homes, 10 m (15 m preferred) between homes and workshops, 8 m between workshops that use fire and 10 m from any building to fuel storage, all doubled for combustible walls such as timber frame or thatch. Fire breaks inside the settlement are 5-10 m wide, and the perimeter break against forest or grassland is 30-50 m. The radiant-heat calculation behind these figures, and where they fail, is in Fire Breaks & Building Spacing; this article is about fitting them into a plan.

Worked Example: What The Gaps Cost

Inputs: houses 8 m wide (an assumed size); a frontage of 100 m; n houses need n − 1 gaps, so the length used is 8n + gap × (n − 1).

  1. Gap 5 m (minimum, non-combustible walls): 8n + 5(n − 1) ≤ 100, so 13n ≤ 105 and n = 8 houses, using 8 × 8 + 7 × 5 = 99 m.
  2. Gap 8 m (preferred): 16n ≤ 108, n = 6 houses, using 6 × 8 + 5 × 8 = 88 m.
  3. Gap 10 m (5 m doubled, timber walls): 18n ≤ 110, n = 6 houses, using 98 m.
  4. Gap 16 m (8 m doubled, timber walls): 24n ≤ 116, n = 4 houses, using 4 × 8 + 3 × 16 = 80 m.

So the same frontage holds 8 houses with the least gap and 4 with the preferred gap doubled for timber. Non-combustible walls and roofs (Fire-Resistant Building Materials) allow the shorter gap, so 8 houses fit instead of 6 at the minimum spacing and 6 instead of 4 at the preferred spacing.

Perimeter break: take a settlement of radius R = 150 m, area π × 150² = 70,700 m² (7.1 ha). A break of width w around it has area π × ((150 + w)² − 150²).

  1. w = 30 m: π × (180² − 150²) = π × 9,900 = 31,100 m², 3.1 ha, which is 31,100 ÷ 70,700 = 44% of the settlement’s area.
  2. w = 50 m: π × (200² − 150²) = π × 17,500 = 55,000 m², 5.5 ha, or 78%.

The radius is an illustrative size. The break is only useful if it is kept clear of dry fuel, so put it to use that keeps it mown or grazed: pasture, orchard, a path, a track (Fuel Management & Clearing).

What You Need

  • A scaled plan with every building drawn at its true footprint
  • The wall and roof material of each building
  • The wind rose for the dry season (Prevailing Wind Patterns)
  • A tape or paced distances to check gaps on the ground

Method: Fit The Gaps To The Plan

Step 1: Fix the fire sources first: forge, kiln, bakehouse and fuel stores go on the downwind edge, away from homes (Industrial Zone Workshops).

Step 2: Draw each gap from the table, doubling it for any timber or thatch wall.

Step 3: Check long walls. The long wall of a longhouse needs more than a house wall; the spacing article gives 10 m for a 10 m burning wall and about 15 m for a 30 m wall at the hotter flame.

Step 4: Break the layout into clusters of 4-8 houses, matching the water clusters, with a 5-10 m break between them (Water Points for Fire Fighting).

Step 5: Treat a terrace as one building. A party wall saves land only if it rises through the roof, or one fire runs the whole row (Row Houses).

Step 6: Give every gap a use and a person responsible for keeping it clear.

Common Mistakes

  • Spacing for the average wind. Wind tilts flames toward the next building and raises the heat on it.
  • Filling the gap. A woodpile, hay or a lean-to in the gap turns the break into fuel.
  • Using the minimum for everything. The 5 m is only safe against non-combustible walls.
  • A break nobody maintains. The fire guide: a break overgrown with dry grass is a fuse.

Safety

A gap is also a burn zone

A person within a few metres of a burning wall is in the danger zone for radiant heat. Fight a spreading fire from behind a non-combustible wall, wet the neighbour and keep the gap clear of people, carts and children while a building burns.

What’s Next