Ember attack | Chalkline

Ember attack is the wind-blown shower of burning embers landing on or entering a building during a bushfire event. Embers can travel kilometres ahead of the fire front and arrive long before flame contact, often when the occupants haven’t yet recognised the bushfire as a direct threat. Ember attack is the dominant ignition mechanism in Australian bushfire residential losses. Black Saturday, Black Summer, and earlier major fire events all show ember ignition is responsible for the majority of houses lost.

The two ember-ignition pathways:

External ignition: embers land on combustible elements (deck timber, eaves linings, garden mulch, doormat, window frames, fence) and ignite the structure from the outside.
Internal ignition: embers enter the building cavity through gaps (eaves, gable vents, sub-floor, ridge vents, unsealed roof tiles, gutter debris) and ignite material inside (insulation, accumulated dust, stored items) where occupants cannot see or extinguish.

Internal ignition is the more insidious mode because the fire is established inside the building before anyone notices.

Why ember attack matters for design. AS 3959 construction requirements scale with BAL rating, but all BAL levels from BAL-Low up to BAL-FZ include ember-attack controls. Even BAL-12.5 (the lowest non-trivial rating) requires:

Non-combustible roofing materials (metal, tile, or rated composite).
Sealed eaves with no open vents to the roof space, or vents fitted with non-combustible mesh.
Ember-proof gutters (gutter guard or design that doesn’t accumulate debris).
Window openings with non-combustible flyscreen mesh (aperture ≤2 mm).
Sub-floor enclosures screened with non-combustible mesh.
Door seals and weather strips rated for bushfire conditions.

Higher BALs add radiant heat and flame controls on top of these baseline ember-attack measures.

Common ember-failure paths on existing houses:

Open eaves vents in pre-AS 3959 builds.
Plastic ridge vents that melt and admit embers.
Gutters full of leaves acting as ember accelerant.
Door under-cuts wider than the threshold seal.
Window flyscreens with degraded or wrong-aperture mesh.
Deck timber and fence acting as flame paths to the building.

For builders.

Specify ember-resistant detailing on every BAL job. The ember controls are cheap relative to the radiant-heat controls; getting them right is the highest-value spend.
Detail door and window thresholds with attention. Most BAL-12.5 building element specs pass, but on-site fit-out often introduces ember pathways through under-cut doors or worn flyscreens.
Hand over an ember-attack maintenance plan: gutter cleaning frequency, screen mesh replacement, ember-storm preparation (close windows, retract awnings, clear mulch from foundations).

Also known as: ember storm, ember shower, ember ignition.

Category: Approvals / bushfire / building physics.

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