BAL Ratings and Construction Under AS 3959 in Australia

What it is

A Bushfire Attack Level, or BAL, is the rating system Australia uses to describe how exposed a building is to bushfire. The rating reflects radiant heat measured in kilowatts per square metre, the likelihood of ember attack and the possibility of direct flame contact. The higher the BAL, the more onerous the construction rules.

AS 3959 Construction of buildings in bushfire-prone areas is the Australian Standard that maps the BAL to the construction requirement. AS 3959:2018 is the current edition referenced by the NCC. The NCC 2022 calls up AS 3959 through Volume One Part G5 and Volume Two H7D4 for Class 1, Class 2, Class 3 and adjacent Class 10a structures in designated bushfire-prone areas.

The six BAL categories

BAL-LOW

Radiant heat under 12.5 kW per square metre is not credible at this site. There are no specific construction requirements under AS 3959. State variations can still apply, particularly in NSW where Planning for Bush Fire Protection sets non-construction requirements such as access and water supply.

BAL-12.5

Radiant heat up to 12.5 kW per square metre and ember attack. Construction requirements include ember screens to weep holes and vents, non-combustible external surfaces in some elements, and specific window and door treatments.

BAL-19

Radiant heat between 12.5 and 19 kW per square metre with increasing ember attack. External walls of non-combustible material or of a tested system. Toughened glass at least 5 mm in windows. Mesh aperture not exceeding 2 mm on vents and weep holes.

BAL-29

Radiant heat between 19 and 29 kW per square metre. External walls upgraded to non-combustible. Subfloor enclosed or protected. Roofing fully sarked. Glazing increased to toughened or specifically tested. Decking material non-combustible or one of the listed bushfire-resisting timbers.

BAL-40

Radiant heat between 29 and 40 kW per square metre with potential flame contact at gusts. All-non-combustible construction in most elements. Tested window and door systems. Sealed roof-to-wall junctions. Subfloor fully enclosed. The build cost premium over a BAL-LOW house starts at 20 percent and climbs.

BAL-FZ

Flame zone. Direct flame contact with radiant heat above 40 kW per square metre. AS 3959 covers BAL-FZ but most authorities require a performance-based solution backed by a fire engineer because the prescriptive path is so onerous. Cost premium routinely exceeds 50 percent over base.

How the BAL is determined

The assessment runs against five inputs. The fire danger index for the region as set in AS 3959. The vegetation type, classified into seven groups from forest to grassland. The distance from the site to the classified vegetation. The effective slope of the ground under that vegetation. The site itself.

The shortest distance from the building footprint to the worst combination of vegetation type and slope drives the result. A single tree line within 20 metres can move a site from BAL-12.5 to BAL-29. The assessment must be done by a competent person, typically a bushfire consultant accredited through FPA Australia, although in some states a building surveyor can do it for simple sites.

How it ties into the NCC

NCC 2022 Volume Two H7D4 mandates that a Class 1 building in a bushfire-prone area be constructed in accordance with AS 3959 or with the requirements of NSW H7D4, QLD H7D4 or SA H7D4 as applicable. Where the BAL is BAL-FZ the NCC requires a performance solution unless the relevant state has accepted the AS 3959 deemed-to-satisfy path.

The same provision applies to Class 10a structures that are immediately adjacent or connected to a Class 1, such as an attached carport, deck or pergola. A timber deck off a BAL-29 dwelling has to be built to BAL-29 even if it is detached from the boundary.

Common BAL defects TradeLens picks up

Build to BAL-LOW on a site that should be BAL-12.5

The bushfire report sits in the DA file but it was done early in the design and never reconciled with the as-built footprint. The house moved 4 metres closer to the boundary during design and the BAL was never re-run. Rectification means a fresh assessment and, if the BAL rises, a Performance Solution or retrofit work to the windows, vents and weep holes.

Ember screens missing

BAL-12.5 and above need a mesh aperture not exceeding 2 mm on weep holes, subfloor vents and roof ventilators. The classic miss is the kitchen rangehood vent or the bathroom exhaust through the eaves with no mesh.

Wrong glass

BAL-29 requires toughened glass at least 6 mm or a tested window system. A standard 4 mm float glass slider does not comply. Rectification means a unit replacement that runs from $1500 to $5000 per opening.

Combustible deck timber

A merbau deck off a BAL-29 dwelling. The species is not on the AS 3959 bushfire-resisting timber list. The deck needs to be re-sheeted with a compliant species or non-combustible alternative.

No certificate at handover

The bushfire consultant report exists but there is no statement that the build matches the report. The auditor cannot close out the defect without a final BAL assessment signed against the as-built drawings.

Why this matters at handover

A house that fails its bushfire requirements does not get a final inspection sign-off in any state. In NSW the certifier refuses the occupation certificate. In Victoria the building surveyor refuses the certificate of occupancy. Insurance underwriters increasingly require evidence of the BAL and the construction class before binding cover. The cleanest evidence pack is a BAL assessment dated after the final footprint, a marked-up plan showing each compliance item against the AS 3959 table, and supplier datasheets for windows, mesh, decking and roofing.