Embers, not flames or radiant heat, are what reach buildings beyond the spacing gap: with a 10 m/s wind, a fragment lofted 30 m and falling at 2 m/s (illustrative values) lands about 150 m away, and a fragment lofted 100 m about 500 m. Defend against embers with a non-combustible roof, screens over every vent and gap, clean gutters and bare ground around the walls, because no distance between buildings is wide enough.
Part of Fire Prevention in Settlements
Ember protection is the set of measures that keep burning fragments, carried by wind from a fire, from lodging in a roof, a gap or a pile of fuel and starting a new fire.
An Ember Needs Only A Place To Lodge
The fire guide lists ember transport as the third way fire spreads, burning embers carried by wind landing on combustible roofs hundreds of metres away, and the building-spacing calculation says outright that it says nothing about embers (Fire Breaks & Building Spacing). The spacing rules answer radiant heat. A fragment that lands in a pocket of dry leaves in a gutter, a thatch roof, a vent or a woodpile needs no neighbour to be burning near it.
An ember lodges where fuel is fine and dry and air can reach it: thatch, shingle gaps, leaf litter in a valley or gutter, an open vent, a dry doormat, a woodpile against a wall, stored hay. Wildfire and a settlement fire in strong wind both send them ahead of the fire.
Worked Example: How Far An Ember Drifts
Drift distance is the wind speed times the time the ember is in the air, d = u × h ÷ v, where h is the height it is lofted to and v is its fall speed. The wind, height and fall speed below are illustrative inputs, not measurements; the fire guide’s own statement is only that embers travel hundreds of metres.
- Wind u = 10 m/s (36 km/h), lofted to h = 30 m, falling at v = 2 m/s: time in the air = 30 ÷ 2 = 15 s; drift = 10 × 15 = 150 m.
- Same wind and fall speed, lofted to h = 100 m: time = 50 s; drift = 10 × 50 = 500 m.
- Half the wind, 5 m/s, with h = 100 m: drift = 5 × 50 = 250 m.
The distance scales with wind and with height, and no sensible building gap approaches it. That is why the ember defence is the roof and the gaps, not the distance.
What You Need
- A non-combustible roof: metal, tile or sod, with its junctions closed
- Screens over vents and openings: the mesh must stop an ember and still pass air; as I recall from US wildland-urban-interface practice, about 3 mm openings on attic and crawl-space vents and no more than about 12 mm on chimney spark arrestors (recalled, not checked against a local source). A finer mesh blocks more embers and clogs more easily, so inspect it
- A spark arrestor on every chimney, as the fire guide asks, of salvaged hardware cloth (Chimney & Stove Safety)
- Closed eaves and gaps, with no pocket where an ember can lodge
- Shutters that close, for windows facing the fire
- Ground kept bare within the first zone (Defensible Space Zones)
Method: Close The Pockets
Step 1: Walk the roof line from the ground and list every gap, vent, valley and junction.
Step 2: Clear gutters, valleys and the roof surface of leaves, needles and straw before every dry season and after every windy spell.
Step 3: Screen each vent and cover each gap with metal mesh or plaster.
Step 4: Move the combustibles away: woodpiles, hay, brush and doormats out of the first zone (the fire guide asks 10 m for fuel storage).
Step 5: Close doors and shutters on the side facing the fire when embers are expected.
Step 6: After the front or the ember fall, patrol every roof line, attic and woodpile for smouldering spots, from the ground or a ladder, with water and a rake ready, and repeat for hours.
Common Mistakes
- Thatch as the roof of the building you are trying to save. It is the best ember catcher on the site.
- A spark arrestor never cleaned. Soot blocks it and the chimney stops drawing.
- Cleaning the roof once a year. Litter returns with every wind.
- Assuming the gap protects. A gap stops radiant heat, not drift.
Safety
Roofs are dangerous places to work in a fire
Do not climb a roof while embers are falling, smoke is heavy or the surface is hot or wet. Wet roofs and walls from the ground or from a ladder, and leave the building if it is well alight (Response Protocols & Training).
What’s Next
- Learn next: Wildfire Defense — where the embers come from
- Combine with: Chimney & Stove Safety — the arrestor and the hearth