Expert Bushfire Site Classification

A Bushfire Attack Level (BAL) assessment determines the potential exposure of a building site to bushfire impact, including ember attack, radiant heat, and direct flame contact.

It informs design and construction measures under the Australian Standard AS 3959:2018 – Construction of buildings in bushfire-prone areas

Key Reference

AS 3959:2018 – Construction of buildings in bushfire-prone areas
(Referenced by the National Construction Code – Volume 2, and State/Territory planning requirements.)

Assessment Methodology

The assessment follows the six-step method set out in AS 3959:

1. Determine Fire Danger Index (FDI)

o Based on regional climatic zone (e.g., FDI = 100 for Victoria).

2. Identify Vegetation Type within 100 m of the site

o Forest, Woodland, Shrubland, Grassland, Scrub, Mallee, or Rainforest.

3. Measure the Distance to Vegetation (setback)

o Horizontal distance from the building site to classified vegetation.

4. Assess the Effective Slope under the vegetation

o Downslope or upslope affects fire spread intensity.

5. Determine the BAL Rating using AS 3959 tables (by vegetation type, slope, and distance).

6. Apply Construction Requirements matching the determined BAL.

Bushfire Attack Level (BAL) Ratings

Common Inputs Used

Vegetation mapping: From onsite survey or local GIS/DEW data.
Topography: Derived from LiDAR or contour survey to calculate slope under vegetation.
Site measurements: From nearest classified vegetation to the proposed building footprint.
Local fire weather index: From state fire authority or AS 3959 Table 2.1.

Outputs

Final BAL Rating (e.g., “BAL–19 – Moderate risk of ember attack”).
Construction controls: materials, windows, doors, roof, vents, decks, etc.
Management recommendations: vegetation maintenance, cleared zones, driveway access for fire trucks, and water supply for firefighting.

Example Summary for Report

A BAL assessment has been undertaken in accordance with AS 3959:2018.
The site at 80 Cobb St, Penshurst is surrounded predominantly by grassland within 100 m, with a gentle 2° downslope towards the north.
Based on an FDI of 100 (Victoria), grassland vegetation, and a separation distance of 25 m, the assessed Bushfire Attack Level is BAL–12.5.
Accordingly, construction measures to BAL–12.5 standard are required, including ember-resistant vents, mesh screening to openings, and non-combustible external cladding. The site is considered suitable for residential development with these mitigation measures applied.

Common Inputs Used

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