Bushfire Attack Level (BAL) Determination: AS 3959 Site Assessment for Australian Builds

Bushfire Attack Level is the rating that drives every fire related construction decision on a bushfire prone block. Get the BAL wrong at design stage and the build either over engineers and blows the budget or under engineers and fails approval.

What it is

Bushfire Attack Level or BAL is a numerical rating set under Australian Standard AS 3959 Construction of buildings in bushfire prone areas. The rating measures the heat flux a building is expected to experience during a bushfire event in kilowatts per square metre. There are six BAL categories that drive construction requirements for new homes in declared bushfire prone areas.

The six BAL ratings

BAL LOW indicates a low risk of ember attack and no specific construction requirements beyond the standard NCC provisions. BAL 12.5 indicates ember attack risk with a heat flux up to 12.5 kW per square metre. BAL 19 indicates ember attack and burning debris with heat flux to 19 kW per square metre. BAL 29 indicates increased ember and burning debris with heat flux to 29 kW per square metre. BAL 40 covers heat flux to 40 kW per square metre with risk of flame contact, and BAL FZ or Flame Zone covers direct flame contact at the highest rating.

When a BAL assessment is required

A BAL assessment is required whenever a new dwelling, substantial alteration or addition is proposed on land mapped as bushfire prone by the local council or state planning authority. The trigger is mapping, not the builder's judgement of risk. NSW maps bushfire prone land through the Rural Fire Service. Victoria designates Bushfire Prone Areas and Bushfire Management Overlays through the planning system. Queensland uses bushfire hazard area mapping under local government planning schemes.

Where to check bushfire mapping

The fastest first check is the local council's planning portal or the state planning mapping tool. In NSW, the Planning Portal hosts the bushfire prone land layer. In Victoria, the Planning Maps Online tool shows BPA and BMO layers. Queensland mapping sits in the state planning interactive mapping system or the local council overlay.

How a BAL rating is calculated

AS 3959 sets out two methods, a simplified method and a detailed method using Method 2. The simplified method is what most residential designers use.

The four inputs

The simplified method needs four inputs. Fire Danger Index for the region, which is set by state authorities. Vegetation classification within 100 metres of the building, sorted into seven types such as forest, woodland, shrubland, grassland and scrub. Distance from the classified vegetation to the building. And effective slope under the classified vegetation measured downward from the building.

Vegetation classification

Vegetation is classified using AS 3959 Table 2.3. The seven types each have a typical fire behaviour profile. Plantation forest and forest are the highest fuel load categories. Grassland and rainforest are typically lower. Managed garden, isolated trees and low threat vegetation can be excluded from the assessment when they meet the AS 3959 exclusion criteria.

Slope measurement

Slope is measured downward from the building under the classified vegetation. Upslope and flat ground are treated as zero. The downslope is expressed as a percentage or in degree bands such as zero to five, five to ten, ten to fifteen, fifteen to twenty and over twenty. Steeper downslopes push BAL higher because flames travel faster up a slope toward the building.

Distance from vegetation

Once vegetation and slope are known, AS 3959 Table 2.4 gives the BAL based on distance from the classified vegetation. The further the build sits from the vegetation, the lower the rating. This is why driveway and clearing decisions often shift a build from BAL 29 to BAL 19.

Construction requirements by BAL rating

Each BAL rating triggers different construction requirements in AS 3959 Sections 5 through 9.

BAL 12.5 and BAL 19

These ratings focus on ember protection. Subfloor mesh, sealed roof and wall penetrations, ember screens to vents and gutter guard are typical requirements. External glazing must meet specified ratings, often toughened glass.

BAL 29

Construction tightens further. External walls require non combustible cladding or fire rated systems. Window frames must be tested non combustible or timber of specified species. Decking material is restricted.

BAL 40 and FZ

These ratings require non combustible external construction throughout, fire shutters or specialised glazing, sealed roof systems and detailed flame zone detailing. BAL FZ builds often need a bushfire consultant report and may require a fire engineer for the BCA performance solution pathway.

Who can complete a BAL assessment

In NSW, BAL assessments for residential development are completed by an accredited Bushfire Practitioner accredited by the Fire Protection Association Australia or a council qualifying officer. In Victoria, the assessment is completed by a building surveyor or accredited bushfire consultant. In Queensland, the assessment is part of the development application and is usually completed by a planning consultant or bushfire specialist.

Common BAL errors that fail approval

Underestimating vegetation density at the site boundary is the most common error. A neighbouring block with retained scrub can push BAL higher than expected. Ignoring downslope past the property boundary is the second common error since AS 3959 considers vegetation within 100 metres regardless of who owns it. Misclassifying a managed garden as low threat when it has unmanaged regrowth is the third.

Documenting the BAL on the build

The BAL report should be attached to the development application and the construction certificate. Builders should keep a copy on the site documentation set and use it to brief the framing, glazing and roofing trades on the relevant AS 3959 sections. A mid build BAL change because vegetation was misclassified at design stage is one of the most expensive variations in residential work.