AS 3959 Bushfire Attack Levels Overview & Guide

What is AS 3959?

AS 3959 (Construction of Buildings in Bushfire-Prone Areas) is an Australian Standard designed to improve the resilience of buildings against bushfires. It was developed by Standards Australia and has undergone several revisions, with the latest being AS 3959:2018. The Standard was adopted to address the increasing threat of bushfires and to ensure that buildings in designated bushfire-prone areas are constructed to withstand bushfire attacks. By setting out specific construction requirements, AS 3959 aims to reduce property damage and enhance the safety of occupants during bushfire events.

AS 3959:2018 Updated

The 2018 update of AS 3959 introduced several key changes to improve the Standard's effectiveness. One significant update was the refinement of the method for determining Bushfire Attack Levels (BAL), which now includes six categories based on heat flux exposure thresholds. The update also reorganised construction requirements by BAL categories rather than building components, making it easier for builders to understand and comply with the Standard. These improvements ensure that the AS 3959 bushfire Standard remains relevant and effective in mitigating bushfire risks in modern building practices.

Understanding Bushfire Attack Levels (BAL)

Bushfire Attack Levels (BAL) are a key component of AS 3959, representing the severity of a building's potential exposure to bushfire hazards. BALs are determined based on factors such as the Fire Danger Index, the slope of the land, the type of surrounding vegetation, and the proximity of the vegetation to the building. The BAL assessment quantifies the risk of bushfire exposure in kilowatts per square meter (kW/m²) of radiant heat. These levels guide the construction requirements needed to protect buildings from ember attack, radiant heat, and direct flame contact.

Check out this interactive BAL Assessment tool by the CSIRO.

Breakdown of BAL Categories

AS 3959 defines six BAL categories, each representing a different level of bushfire risk:

• BAL-LOW: Insignificant risk, no specific construction requirements.
• BAL-12.5: Low risk, primarily from ember attack and radiant heat up to 12.5 kW/m².
• BAL-19: Moderate risk, with increasing ember attack and radiant heat between 12.5 and 19 kW/m².
• BAL-29: High risk, with significant ember attack and radiant heat between 19 and 29 kW/m².
• BAL-40: Very high risk, with exposure to radiant heat between 29 and 40 kW/m² and potential flame contact.
• BAL-FZ (Flame Zone): Extreme risk, with direct exposure to flames and radiant heat exceeding 40 kW/m².

What Does a BAL Number Mean?

A BAL number quantifies the risk level of bushfire exposure to a building. It is determined by assessing various factors, including the type of vegetation, the slope of the land, and the building's distance from the bushfire-prone area. Higher BAL numbers indicate greater risk and require more stringent construction measures to protect the building from bushfire attacks. For example, a BAL-29 rating means the building must withstand radiant heat up to 29 kW/m² and increased ember attack, necessitating specific construction techniques and materials.

AS 3959 Construction Standards

AS 3959 outlines specific construction Standards for buildings in bushfire-prone areas. These Standards include the use of non-combustible materials, bushfire-resisting timber, and specific design principles to enhance building resilience. The construction requirements vary based on the assessed BAL. For instance, buildings in BAL-FZ zones must incorporate materials and designs that can endure direct flame contact.

What Materials & Design Principles Must I Follow?

To comply with AS 3959, buildings must use materials and design principles that enhance their resistance to bushfires. Recommended materials include non-combustible cladding, bushfire-resisting timber, and metal water and gas supply pipes. Design principles focus on minimising ember entry points, such as using ember-proof screens on vents and ensuring that roof spaces are sealed. Additionally, landscaping around the building should be managed to reduce fuel loads and create defensible spaces.

What Are Bushfire Construction Requirements?

The construction requirements mandated by AS 3959 for buildings in various BAL categories include:

• BAL-12.5: Use of ember-proof screens, non-combustible materials for roofs, and bushfire-resisting timber for exposed elements.
• BAL-19: Enhanced protection for windows and doors, and additional ember protection measures.
• BAL-29: Increased use of non-combustible materials, protection for subfloor spaces, and specific design features to withstand higher radiant heat.
• BAL-40: Comprehensive ember protection, use of fire-resistant materials, and design elements to prevent flame contact.
• BAL-FZ: Highest level of protection, with materials and designs capable of withstanding direct flame exposure and extreme radiant heat.

Why Was AS 3959 Adopted?

AS 3959 was adopted to address the growing threat of bushfires in Australia and to ensure that buildings in bushfire-prone areas are constructed to withstand such events. The Standard was developed in response to devastating bushfires that highlighted the need for improved building resilience. By setting out specific construction requirements, AS 3959 aims to reduce property damage, enhance occupant safety, and provide a framework for building in bushfire-prone areas.

Compliance & Implementation

Ensuring compliance with AS 3959 involves conducting a BAL assessment to determine the appropriate construction requirements for a property. Builders and property owners must incorporate these requirements into their building plans and obtain the necessary permits. Regular inspections and adherence to the Standard's guidelines are essential.

Certification & Inspections

Certification and regular inspections are crucial for ensuring ongoing compliance with AS 3959. Property owners should engage certified professionals to conduct BAL assessments and verify that construction meets the required Standards. Inspections should focus on key areas such as ember protection, material compliance, and adherence to design principles. Regular maintenance and updates to the building's bushfire protection measures are also important to maintain its resilience over time.