BAL Ratings Explained: Bushfire Attack Levels from BAL-LOW to BAL-FZ (2025 Guide)
The six Bushfire Attack Levels in AS 3959:2018, the radiant heat each one represents, how an assessor derives a BAL from fire danger index, vegetation, distance and slope, what each level requires for walls, windows, roofs and decks, and where the NCC and the state schedules change the rules.
A BAL rating, or Bushfire Attack Level, is the number that decides how a building on bushfire prone land has to be built: which glass goes in the windows, whether the deck can be timber, how the roof is sealed against embers, and whether a Performance Solution is needed at all. There are six levels, BAL-LOW to BAL-FZ, each a band of radiant heat in kilowatts per square metre. This guide explains what the levels mean, how an assessor arrives at one under AS 3959:2018, what each level requires, and where the National Construction Code and the states hook in.
The short answer
AS 3959:2018 defines a Bushfire Attack Level as "a means of measuring the severity of a building's potential exposure to ember attack, radiant heat and direct flame contact, using increments of radiant heat expressed in kilowatts per metre squared" (Clause 1.5.4). Table 3.1 sets the six bands: BAL-LOW, BAL-12.5 (up to 12.5 kW/m²), BAL-19 (more than 12.5 up to 19), BAL-29 (more than 19 up to 29), BAL-40 (more than 29 up to 40) and BAL-FZ, the Flame Zone, above 40 kW/m². Each band has its own construction section, 5 for BAL-12.5 through to 9 for BAL-FZ, with Section 3 applying to all.
The BAL only matters on land that has been designated bushfire prone, which the NCC glossary defines as "land which has been designated under a power of legislation as being subject, or likely to be subject, to bushfires". For a house, NCC 2025 Volume Two H7D4 is satisfied by AS 3959 or the NASH Standard for steel-framed construction in bushfire areas; for apartments and hotels, Volume One G5D3 requires AS 3959. Both clauses are word for word the same in NCC 2022.
1. What the numbers mean physically
Bushfire attack is "attack by wind, burning embers, radiant heat or flame generated by a bushfire" (Clause 1.5.2), and the scale ranks sites by the radiant heat the building is expected to receive. Appendix G, Table G1 gives the numbers scale: pain to a person after 10 to 20 seconds at 4 kW/m², pain after 3 seconds at 10 kW/m², unpiloted ignition of timber over a long exposure at 25 kW/m², and ignition of timber in 20 seconds at 45 kW/m². So at BAL-12.5 the real enemy is embers, BAL-29 sits above the long-exposure ignition point of timber, BAL-40 approaches the point where exposed timber lights in seconds, and BAL-FZ means the flames reach the building. Even BAL-LOW, in Appendix G's words, is "insufficient risk to warrant any specific construction requirements but there is still some risk".
2. The six levels and what each one requires
Three rules frame everything below. The BAL applies to the entire building and any structure within 6 m of it (Clause 2.1). Higher construction is always acceptable for a lower level (Clause 3.4). And an elevation fully shielded by another part of the same building may drop one level, never below BAL-12.5, and never for roofs or subfloors (Clause 3.5).
BAL-LOW
Section 4 is one paragraph: the standard "does not provide construction requirements" at BAL-LOW. LOW is the result when everything within 100 m is excluded or low-threat vegetation under Clause 2.2.3.2.
BAL-12.5 (Section 5)
Keeping embers out. Every gap, vent and weephole is screened with 2 mm mesh (Clause 3.6.1) and the openable part of every window is screened (Clause 5.5.3). Roof coverings are non-combustible, tiled roofs fully sarked, roof vents fitted with ember guards (Clause 5.6). Wall and window rules bite only in the lowest 400 mm above ground or a deck: cladding there must be non-combustible, 6 mm fibre-cement, steel or bushfire-resisting timber (Clause 5.4.1), and glazing there Grade A safety glass at least 4 mm thick (Clause 5.5.3). Above 400 mm the wall is unrestricted. Decking may be spaced and is only restricted within 300 mm of glazing less than 400 mm above the deck (Clause 5.7).
BAL-19 (Section 6)
Almost identical, with one step up: glazing within 400 mm of the ground or a deck must be 5 mm toughened, and any window with annealed glass must be screened externally over both fixed and openable portions, because the mesh is now doing radiant-heat duty as well as ember duty (Clause 6.5.3).
BAL-29 (Section 7)
The 400 mm concession disappears. The whole external wall must be 90 mm masonry, concrete or earth, dense timber logs, or a framed wall sarked outside and clad in 6 mm fibre-cement, steel or bushfire-resisting timber (Clause 7.4.1). Window frames must be bushfire-resisting timber, metal or metal-reinforced uPVC, and all glazing 5 mm toughened (Clause 7.5.3). Decking, framing and posts on an unenclosed deck must be non-combustible or bushfire-resisting timber (Clause 7.7.3), as must balustrades within 125 mm of glazing or a combustible wall (Clause 7.7.4).
BAL-40 (Section 8)
Timber leaves the envelope. Framed walls need 9 mm fibre-cement or steel over external sarking (Clause 8.4.1). Window frames must be metal, glazing 6 mm toughened, and every window is screened externally, fixed and openable, with steel or bronze mesh (Clauses 8.5.2 and 8.5.3). Decking may not be spaced and must be non-combustible or tested to AS 1530.8.1 (Clause 8.7). Roof-mounted evaporative coolers are not permitted (Clause 8.6.1).
BAL-FZ (Section 9)
The Flame Zone works differently. The building needs a minimum setback of 10 m from the classified vegetation, and any element closer than that must be tested to AS 1530.8.2 (Clause 9.1). Walls are 90 mm masonry or concrete, a system tested to AS 1530.8.2, or a system with an FRL of 30/30/30 (Clause 9.4.1). Windows are shuttered, or screened and either rated -/30/- or tested to AS 1530.8.2 (Clause 9.5.3). Roofs follow the generic systems in Appendix H or a tested system (Clause 9.6). The standard's own note says BAL-FZ "may require reliance on measures other than construction" and may be separately regulated by the state.
3. How an assessor derives the BAL
Section 2 gives two methods. Method 1 is the table lookup in Clause 2.2, used in almost every BAL report. Method 2 is the calculation in Appendix B, for "where a more specific result is sought or where the site conditions are outside of the scope of Method 1" (Commentary C2.1); Method 1 cannot be used where the effective slope is more than 20 degrees downslope (Clause 2.2.1). The five steps:
Fire Danger Index (Table 2.1). Fixed by jurisdiction. ACT 100. NSW 100 in the Greater Hunter, Greater Sydney, Illawarra/Shoalhaven, Far South Coast and Southern Ranges fire weather districts, 80 elsewhere, 50 in alpine areas. Victoria 100, alpine areas 50. SA and WA 80. Tasmania 50. Queensland and the Northern Territory 40. A higher FDI means a higher BAL for the same vegetation at the same distance.
Vegetation classification (Table 2.3). Seven classes, A to G: forest, woodland, shrubland, scrub, mallee/mulga, rainforest and grassland. Clause 2.2.3.2 excludes vegetation more than 100 m away, single patches under 1 ha, multiple patches under 0.25 ha more than 20 m away, strips under 20 m wide, non-vegetated ground, and low-threat vegetation such as maintained lawns, golf fairways, vineyards, orchards, cultivated gardens and grassland kept short-cropped to a nominal 100 mm. If everything within 100 m is excluded, the answer is BAL-LOW.
Distance (Clause 2.2.4). Measured horizontally from the edge of the vegetation to the nearest external wall, or to the posts of a deck or carport. Eaves, tanks, chimneys, unroofed pergolas and sun blinds are ignored.
Effective slope (Clause 2.2.5). The slope under the vegetation, "not the slope between the vegetation and the building". Fire runs faster uphill, so downslope vegetation is banded at 0 to 5, 5 to 10, 10 to 15 and 15 to 20 degrees, and all upslope vegetation counts as flat.
Read the table (Clause 2.2.6). Tables 2.4 to 2.7 are one per FDI. Each vegetation type and slope band is assessed separately and the highest BAL applies to the whole building. The worked example in Appendix A, a house 40 m from forest on a 0 to 5 degree downslope at FDI 80, reads as BAL-19. At FDI 100 on flat ground, forest gives BAL-FZ inside 19 m, BAL-40 to 25 m, BAL-29 to 35 m, BAL-19 to 48 m and BAL-12.5 out to 100 m (Table 2.4).
A BAL is a property of the site geometry, so moving the footprint 10 m can change the level. It is also only as reliable as the vegetation call: recent fire or clearing "does not of itself warrant any reduction in the classification" (Commentary C2.2.3.1).
4. BAL-FZ and Performance Solutions
The Performance Requirement sits in the NCC. For a house, H7P5 requires the building to "reduce the risk of ignition from a design bushfire with an annual exceedance probability not more than 1:50 years", prevent internal ignition, and keep its structural integrity for the duration of the design bushfire. For Class 2 and 3 buildings, G5P1 sets the same tests at 1:100 years, and 1:200 for a Class 9 building. AS 3959 is the Deemed-to-Satisfy route, not the only route.
Volume One adds a Verification Method, G5V1: compliance is verified if the ignition probability of the building exposed to the design bushfire "does not exceed 10%", assessed by an event tree analysis of ember attack, accumulated debris, radiant heat and flame contact. That is bushfire engineering, and it is where a Flame Zone building usually ends up when the prescriptive Section 9 details are impractical.
Class 9 buildings are a special case. Under G5D2, a health-care building, early childhood centre, school or residential care building has a Deemed-to-Satisfy path (Specification 43, via G5D4) only at BAL-12.5 or lower. Above that, "There are no Deemed-to-Satisfy Provisions for these buildings": G5P2 must be met by a Performance Solution covering tenability, vehicular access and firefighting water.
5. Who does the assessment, and when
Before design. Confirm whether the lot is on a designated bushfire prone map. Designation is a state function. In NSW the maps are "prepared by local council and certified by the Commissioner of the NSW RFS" under section 10.3 of the Environmental Planning and Assessment Act 1979. In Western Australia the land "is designated by the Fire and Emergency Services Commissioner" on the Map of Bush Fire Prone Areas, and DFES validates any BAL calculated by a method other than Method 1.
At concept stage. Commission the BAL report while the siting is still flexible; it records the FDI, each vegetation class within 100 m, distance, effective slope and the resulting BAL. Check whether the state or consent authority requires an accredited assessor.
At development approval. In NSW, Planning for Bush Fire Protection governs the planning side and the NCC defers to it: NSW H7D4 applies AS 3959 "except as amended by Planning for Bush Fire Protection", or as modified by the development consent or a bushfire safety authority under section 100B of the Rural Fires Act 1997.
At building approval. Put the BAL and the AS 3959 section on the drawings and in the specification; the building surveyor or certifier checks glazing, screening, sarking, cladding and decking against it. At BAL-FZ, expect the approval to turn on a tested system or a Performance Solution.
In an AEC Assistant project, the site planning data fetched for the address lists the planning zone, overlays and constraints, the first place to look for a designation.
6. State by state: where the NCC text changes
NCC 2025 applies in Victoria, Tasmania, Western Australia and the ACT from 1 May 2026; New South Wales, Queensland and South Australia stay on NCC 2022 until 1 May 2027, and the Northern Territory stays on 2022. The bushfire clauses read the same in both editions unless noted; see the adoption dates guide for the full picture.
New South Wales. Planning for Bush Fire Protection overrides AS 3959 wherever the two differ (NSW H7D4 and G5D3), and Class 4 parts of buildings are added to the scope. For Class 9 buildings used as a special fire protection purpose, the Deemed-to-Satisfy threshold is radiant heat "not greater than 10kW/m² (calculated at 1200K)" rather than BAL-12.5 (NSW G5D2). One edition change: NCC 2022's NSW clauses carved Section 9 out of the AS 3959 call-up and said BAL-FZ buildings "must comply with specific conditions of development consent"; the NCC 2025 NSW text no longer carries that carve-out.
Queensland. The bushfire Deemed-to-Satisfy Provisions do not apply where the classified vegetation is Group F rainforest (excluding wet sclerophyll), mangrove communities, or grassland under 300 mm high (H7D4(3) QLD, G5D2 QLD).
South Australia. The BAL is read from the Planning and Design Code. General bushfire risk areas are BAL-LOW; medium risk areas are BAL-12.5; high risk areas are assessed under AS 3959; urban interface land is BAL-LOW within 500 m of a high risk area but assessed within 100 m of one (H7D4(3) SA, G5D5 SA).
Tasmania. TAS H7P5 drops the 1:50 year design bushfire, requires ignition risk to be reduced "to the degree necessary", and adds vehicular access for firefighting and "a sufficient supply of water for fire fighting purposes on the site". The NCC points to the Director's Determinations on Bushfire Hazard Areas at cbos.tas.gov.au.
Victoria, WA, ACT and NT. No state variation to H7D4 or G5D3 in NCC 2025; AS 3959 applies as written. Victoria's FDI of 100 outside alpine areas is the highest in Table 2.1.
Checklist
Confirm the lot is on the state's designated bushfire prone map first. No designation, no BAL requirement under the NCC.
Get the BAL assessed at concept stage, while the footprint can still move.
Apply the highest BAL to the whole building and everything within 6 m. At BAL-12.5 and BAL-19 concentrate on the lowest 400 mm and on 2 mm screening; at BAL-29 the whole wall and all glazing are governed; at BAL-40 expect metal frames, 6 mm toughened glass and unspaced non-combustible decking.
At BAL-FZ check the 10 m setback first; if it cannot be met, the design rests on tested systems or a Performance Solution against G5V1 or H7P5.
Read the state schedule: NSW defers to Planning for Bush Fire Protection, SA reads the BAL off the Planning and Design Code, Queensland excludes rainforest and short grassland, Tasmania adds access and water.
Put the BAL and the AS 3959 section on the drawings. See also the guides on NCC climate zones and NCC stair requirements.
Check it against the source
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Sources: AS 3959:2018 Construction of buildings in bushfire-prone areas (incorporating Amendments 1 and 2), Sections 1 to 9 and Appendices A, B and G; NCC 2025 Volume Two H7D4 and H7P5 with their NSW, QLD, SA and TAS variations; NCC 2025 Volume One Part G5 (G5D2 to G5D5, G5P1, G5P2, G5V1) with its NSW, QLD, SA and TAS variations; NCC 2025 Housing Provisions 12.1.1; NCC 2022 H7D4 and G5D3 for the edition comparison; NSW Rural Fire Service, Bush Fire Prone Land; WA Department of Fire and Emergency Services, Map of Bush Fire Prone Areas. Figures verified against the ABCB's published text on 4 September 2026. Diagram by AEC Assistant. This article is general guidance; the building surveyor or certifier for your project determines compliance.
