Bushfire Attack Level (BAL)

A Bushfire Attack Level (BAL) is a rating used in Australia to measure how severely a building site could be exposed to ember attack, radiant heat and direct flame during a bushfire. The BAL determines which construction requirements of the Australian Standard AS 3959, Construction of buildings in bushfire-prone areas, apply to the building, including its roof.

The levels

Ratings run from BAL-LOW to BAL-FZ (Flame Zone):

• BAL-LOW: very low risk; no specific bushfire construction requirements.
• BAL-12.5, BAL-19, BAL-29 and BAL-40: increasing exposure; the number refers to the expected radiant heat flux in kilowatts per square metre.
• BAL-FZ: the site is likely to be exposed to direct contact with the fire front, very high heat flux and ember attack.

How a BAL is assessed

The assessment considers the regional fire danger index, the type of surrounding vegetation, the distance from the building to that vegetation, and the slope of the land beneath it. After the February 2009 Victorian bushfires, which destroyed more than 2,000 houses, Standards Australia fast-tracked revisions and bushfire attack assessments became central to building approvals in bushfire-prone areas. The Australian Building Codes Board adopted a design flame temperature of 1,090 K (about 820 °C) for the standard.

Roofing implications

As the BAL rises, AS 3959 requires progressively stricter roof details. Typical concerns include:

• Non-combustible roof coverings such as metal sheeting or tile at higher levels;
• Sarking (underlay) with a low flammability index beneath tiles;
• Sealing gaps at eaves, ridges and between the covering and fascia, often with mineral wool or non-combustible closures;
• Ember guards on vents, gutters and evaporative cooler openings;
• Restrictions on combustible fascias, eaves linings and skylights.

In New South Wales, the Rural Fire Service's Planning for Bush Fire Protection 2019 document sets out additional statutory requirements for building on bushfire-prone land.

Comparison with other systems

BAL is a site exposure rating, not a product rating. It differs from North American systems, which rate roof coverings (for example Class A under ASTM E108) and apply wildland-urban interface code zones.