BAL Ratings Explained: What Each Bushfire Attack Level Means, How a BAL Is Calculated under AS 3959, and a Free BAL Calculator (2025 Guide)

A BAL rating, or Bushfire Attack Level, measures how severely a building on bushfire prone land could be exposed to embers, radiant heat and flame, and it sets how that building has to be built under AS 3959. There are six levels, BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40 and BAL-FZ: the number is the top of a band of radiant heat in kilowatts per square metre, and FZ is the Flame Zone.

The level decides the glass in the windows, the cladding near the ground, how the roof is sealed against embers and whether the deck can be timber. This guide explains what each level means, how it is worked out under AS 3959:2018, the edition the National Construction Code (NCC) 2025 calls up, what each level requires, and where the states change the rules. The calculator in section 3 estimates the level for one vegetation type on a site and shows how far back the building would have to sit to drop a level.

In one minute

• Six levels, set by radiant heat. BAL-12.5 covers up to 12.5 kW/m², BAL-19 up to 19, BAL-29 up to 29, BAL-40 up to 40 and BAL-FZ anything above (AS 3959:2018 Table 3.1). BAL-LOW applies where the vegetation near the site is excluded or low threat (Clause 2.2.3.2).

• Four site facts decide it. The Fire Danger Index for the state, the class of vegetation, its horizontal distance from the building and the slope of the land under it (Clause 2.2). Tables 2.4 to 2.7 turn them into a level.

• The worst result governs. Each vegetation type is assessed separately, and the highest BAL applies to the whole building and to attached or adjacent structures within 6 m (Clause 2.1).

• It applies only on designated bushfire prone land. NCC 2025 calls up AS 3959 for houses in Volume Two H7D4 and for Class 2 and 3 buildings in Volume One G5D3.

• Four states change the rules. NSW applies AS 3959 as amended by Planning for Bush Fire Protection, South Australia reads the level from its Planning and Design Code in some areas, Queensland exempts rainforest and short grassland, and Tasmania adds access and water (section 7).

1. What a BAL rating measures

AS 3959:2018 defines the 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).

The number in each name is the top of its band of radiant heat at the building. BAL-19 means more than 12.5 and up to 19 kW/m², and BAL-FZ means more than 40 kW/m², with direct exposure to flames from the fire front (Table 3.1). BAL-LOW is not a band at all. It is the result when the vegetation around the site is excluded or low threat, and the Standard sets no construction requirements for it (Section 4).

For scale, Appendix G lists the effect of radiant heat in Table G1. A person feels pain after 10 to 20 seconds at 4 kW/m² and after 3 seconds at 10 kW/m². Timber ignites after a long exposure at 25 kW/m², and within 20 seconds at 45 kW/m². So BAL-12.5 is mostly about embers, BAL-29 reaches past the long-exposure ignition point of timber, and BAL-40 stops just short of the 45 kW/m² that lights timber within 20 seconds.

Even BAL-LOW is not "no risk". The Standard says the absence of construction requirements at BAL-LOW "does not imply these buildings are not at risk" (Clause 1.3, Note).

NCC 2025 calls AS 3959 up for a house, with the NASH Standard for steel-framed construction in bushfire areas as the alternative (Volume Two H7D4(2)), and for Class 2 and 3 buildings (Volume One G5D3). Both apply only in a designated bushfire prone area, which the NCC defines as "land which has been designated under a power of legislation as being subject, or likely to be subject, to bushfires".

2. How a BAL is calculated

Section 2 of AS 3959 gives two methods (Clause 2.1). Method 1 is the simplified procedure of Clause 2.2: five steps that end in a table. Method 2 is the detailed 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 land under the vegetation falls more than 20° downslope (Clause 2.2.1).

Step 1: the Fire Danger Index

The Fire Danger Index (FDI) is set by state or territory, and in New South Wales and Victoria by region, in Table 2.1. It ranges from 40 in Queensland and the Northern Territory to 100 in the ACT, in Victoria outside the alpine areas and in five New South Wales fire weather districts. The calculator in section 3 applies the full table.

Step 2: the vegetation

Table 2.3 sorts vegetation into eight classes: A forest, B woodland, C shrubland, D scrub, E mallee/mulga, F rainforest, G grassland, and H tussock moorland, which the table describes as significant only in Tasmania. Each vegetation type is classified and assessed separately (Clause 2.2.3.1). For forest, woodland and rainforest the classified vegetation is determined by the unmanaged understorey, not the canopy drip line or the trunks (Commentary C2.2.5). Recent fire or clearing "does not of itself warrant any reduction in the classification" (Commentary C2.2.3.1).

Clause 2.2.3.2 excludes:

• vegetation more than 100 m from the site;

• a single area under 1 ha that is not within 100 m of other classified vegetation;

• areas under 0.25 ha, and strips under 20 m wide, that are not within 20 m of the site, of each other or of other classified vegetation;

• non-vegetated areas such as roads, footpaths, buildings, waterways and rocky outcrops;

• low threat vegetation, including maintained lawns, cultivated gardens, orchards, vineyards, sporting fields, mangroves, and grassland in a minimal fuel condition, which the Standard pictures as grass short-cropped to a nominal 100 mm.

If everything within 100 m is excluded, the site is BAL-LOW.

Step 3: the distance

The distance is measured horizontally, in plan, from the edge of the vegetation to the nearest part of an external wall. Where a carport, veranda or deck has no walls, it is measured to the supporting posts or columns (Clause 2.2.4, Figure 2.1 notes). Eaves, tanks, chimneys and services, unroofed pergolas and sun blinds do not count.

Step 4: the effective slope

The slope that matters is the slope of the land under the vegetation, "not the slope between the vegetation and the building" (Clause 2.2.5). Land under the vegetation that falls away from the building is downslope, read in bands of more than 0° up to 5°, then 5° steps to 20°. Fire travels more slowly downhill, so all upslope vegetation is read as flat. Where the land under one vegetation type has more than one slope, each is assessed and the worst result applies.

Step 5: read the table

Tables 2.4 to 2.7 hold one table for each FDI: 100, 80, 50 and 40. Each row is a vegetation class on a slope band, and each column a BAL, given as a range of distances (Clause 2.2.6). Read the BAL for every vegetation type, and the highest applies to the entire building. Appendix A works an example: a house 40 m from forest on a downslope of up to 5° at FDI 80 reads BAL-19.

The chart below is one row. At FDI 100 on flat land, forest gives BAL-FZ inside 19 m, BAL-40 from 19 m, BAL-29 from 25 m, BAL-19 from 35 m and BAL-12.5 from 48 m to less than 100 m (Table 2.4).

Two features of the tables are worth knowing. Grassland's BAL-12.5 band ends at 50 m, not 100 m, so the tables give grassland further away than that no level above BAL-LOW. And shrubland, scrub and mallee/mulga have the same distances in all four tables: for them the FDI makes no difference, only the slope does.

3. Estimate a BAL for your site

Set the state, the vegetation class, its size, the slope of the land under it and the distance from its edge to the nearest external wall. The calculator reads the Method 1 table for that one vegetation type and gives the level, its heat band, the construction section and the main requirements that follow. It also shows the distance at which the same vegetation reads one level lower: the number to test a siting against.

Run it once for each vegetation type around the site and take the highest result. It gives no reading in New South Wales or where the land under the vegetation falls more than 20°, and it is not a BAL assessment.

4. What each level requires

Three rules frame every level. The BAL applies to the entire building and to attached or adjacent structures within 6 m (Clause 2.1). Construction for a higher level is always acceptable for a lower one (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 the roof or the subfloor (Clause 3.5).

From BAL-12.5 up, the general requirements of Section 3 apply alongside the level's own section (Table 3.1). Among them, every vent, weephole and gap is screened with mesh of 2 mm maximum aperture, except weepholes in window and door sills (Clause 3.6.1).

BAL-LOW (Section 4)

The Standard "does not provide construction requirements" for a building assessed as BAL-LOW.

BAL-12.5 (Section 5)

Keeping embers out. The openable part of every window is screened (Clause 5.5.3). Roof coverings are non-combustible, tiled roofs are fully sarked, sheet roofs are sarked or sealed at their gaps, and roof vents carry ember guards (Clauses 5.6.1 to 5.6.3).

Wall and window rules reach only the lowest 400 mm: within 400 mm of the ground, or of a deck, carport roof or awning flatter than 18° that projects more than 110 mm. Cladding there must be non-combustible, 6 mm fibre-cement, steel sheeting, bushfire-resisting timber or one of the listed timber species (Clause 5.4.1). Glazing there must be 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 restricted only within 300 mm of glazing that is less than 400 mm above the deck (Clause 5.7).

BAL-19 (Section 6)

The same pattern, one step up in the glass (Clause 6.5.3). Glazing in the lowest 400 mm is 5 mm toughened glass, and any window with annealed glass is screened externally over both its fixed and openable parts. The mesh now does radiant heat duty as well as ember duty.

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 frame sarked on the outside and clad in 6 mm fibre-cement, steel sheeting or bushfire-resisting timber (Clause 7.4.1). Unless a window is shuttered, its frame is bushfire-resisting timber, metal or metal-reinforced uPVC, its sash hardware metal and all its glazing 5 mm toughened glass, with the glazing in the lowest 400 mm screened externally (Clause 7.5.3).

Decking, and on an open deck its framing and posts, must be non-combustible or bushfire-resisting timber (Clause 7.7). So must the parts of handrails and balustrades within 125 mm of glazing or a combustible wall (Clause 7.7.4).

BAL-40 (Section 8)

Timber cladding and timber window frames are no longer accepted. Framed walls need 9 mm fibre-cement or steel over external sarking, unless the system meets AS 1530.8.1 (Clause 8.4.1). Unless shuttered, window frames are metal, glazing is 6 mm toughened glass, and every window is screened externally over its fixed and openable parts with steel or bronze mesh (Clauses 8.5.2 and 8.5.3). Decking may not be spaced and must be non-combustible or meet 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 setback of at least 10 m from the edge of the classified vegetation, and any element closer than that must meet AS 1530.8.2 (Clause 9.1). Walls are 90 mm masonry, concrete or earth, a system tested to AS 1530.8.2 from the outside, or a system with an FRL of 30/30/30 or –/30/30 tested from the outside (Clause 9.4.1). Windows are shuttered, or screened at their openable parts and either rated –/30/– or tested to AS 1530.8.2 (Clause 9.5.3). Roofs use one of the two generic systems in Appendix H or a system tested to AS 1530.8.2 (Clauses 9.1 and 9.6.1).

The Standard's own note says BAL-FZ "may require reliance on measures other than construction" and may be regulated by the building authorities of the states and territories (Clause 9.1, Note 2). The guide to FRLs explains what a rating such as 30/30/30 means.

5. BAL-FZ and Performance Solutions

AS 3959 is the Deemed-to-Satisfy route, not the only one. For a house, Performance Requirement H7P5 asks the building to "reduce the risk of ignition from a design bushfire with an annual exceedance probability not more than 1:50 years", to prevent internal ignition and to 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 years for a Class 9 building.

Volume One adds a Verification Method, G5V1. Compliance with G5P1 is verified if the ignition probability of the building exposed to a design bushfire "does not exceed 10%", assessed by an event tree analysis of ember attack, burning debris, radiant heat and flame contact. That is bushfire engineering, and it is a route for a Volume One building in the Flame Zone, other than a Class 9 building, where the Section 9 details are impractical.

Some Class 9 buildings are a special case. A Class 9a health-care building, an early childhood centre, a primary or secondary school or a Class 9c residential care building has a Deemed-to-Satisfy path only at BAL-12.5 or lower, through Specification 43 (G5D2, G5D4). Above that, in the clause's words, "There are no Deemed-to-Satisfy Provisions for these buildings." G5P2 must then be met by a Performance Solution covering occupant tenability, vehicular access, firefighting water and safe access. The NSW and Queensland variations to G5D2 differ. The guide to building classes sets out which buildings are Class 9.

6. Who assesses the BAL, and when

• Before design. Check whether the lot is designated bushfire prone: the NCC's bushfire provisions "only apply in a designated bushfire prone area" (H7D4(1)). Designation is a state function. In New South Wales, bush fire prone land maps "are prepared by local council and certified by the Commissioner of the NSW RFS" (NSW Rural Fire Service), and NCC 2025's NSW definition points to those maps. In Western Australia the land "is designated by the Fire and Emergency Services Commissioner" on the Map of Bush Fire Prone Areas (Department of Fire and Emergency Services, DFES).

• At concept stage. Commission the BAL assessment while the footprint can still move. It needs the FDI, each vegetation class within 100 m, its distance and its effective slope. Check whether the state or the consent authority requires an accredited assessor.

• At development approval. 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. In Western Australia, DFES says a BAL calculated by a method other than Method 1 will usually be referred to it where the decision maker requires validation of the approach.

• 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 tested systems or a Performance Solution.

7. State and territory variations in NCC 2025

• New South Wales. NSW H7D4 for houses, and NSW G5D3 for Class 2 and 3 buildings and Class 4 parts, apply AS 3959 except as amended by Planning for Bush Fire Protection, or as modified by the development consent or a bushfire safety authority. A Class 9 building used for a special fire protection purpose has Deemed-to-Satisfy Provisions only where radiant heat is no more than 10 kW/m² (calculated at 1200 K) under Planning for Bush Fire Protection, or where the consent or bushfire safety authority says they apply (NSW G5D2).

• Queensland. The bushfire Deemed-to-Satisfy Provisions do not apply where the classified vegetation is Group F rainforest (excluding wet sclerophyll forest types), mangrove communities or grassland under 300 mm high (QLD H7D4(3), QLD G5D2(2)). Queensland's G5D2 also lists Class 9a, 9b early childhood and school, and 9c buildings without the base clause's BAL-12.5 limit.

• South Australia. The level is read from the Planning and Design Code (SA H7D4(3), SA G5D5). General bushfire risk areas are BAL-LOW and medium risk areas 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 under AS 3959 within 100 m of one.

• Tasmania. TAS H7P5 drops the 1:50 year design bushfire. It asks for the risk of ignition 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". It points to the Director's Determinations on Bushfire Hazard Areas at cbos.tas.gov.au.

• Victoria, Western Australia, the ACT and the Northern Territory. No variation to H7D4, H7P5 or Part G5 in NCC 2025: AS 3959 applies as written.

8. Worked examples

Each row is one vegetation type, read from the table for the state's FDI. The first is the calculator's default.

Site	Table and band	BAL
Victoria, forest 35 m away on a downslope of up to 5°	Table 2.4 (FDI 100): 32 to less than 43 m	BAL-29; BAL-19 from 43 m
Western Australia, forest 40 m away on a downslope of up to 5° (the Appendix A example)	Table 2.5 (FDI 80): 37 to less than 50 m	BAL-19
ACT, forest 18 m away on flat land	Table 2.4 (FDI 100): less than 19 m	BAL-FZ; BAL-40 from 19 m
Queensland, grassland over 300 mm high, 10 m away on flat land	Table 2.7 (FDI 40): 8 to less than 12 m	BAL-19
Tasmania, tussock moorland 15 m away on flat land	Table 2.6 (FDI 50): 14 to less than 20 m	BAL-19
Victoria, grassland 55 m away on flat land	Table 2.4: the grassland row ends at 50 m	BAL-LOW
South Australia, medium bushfire risk area	SA H7D4(3): no table reading	BAL-12.5

In the first example, siting the house 8 m further from the forest drops it to BAL-19. The wall and glazing rules then shrink back to the lowest 400 mm, and a spaced timber deck is restricted only near low glazing.

9. Common mistakes

• Measuring to the eaves. The distance runs to the nearest external wall, or to the posts of a deck or carport. Eaves and roof overhangs do not count (Clause 2.2.4, Figure 2.1 notes).

• Using the slope between the house and the bush. Only the slope of the land under the vegetation counts, and upslope vegetation is read as flat (Clause 2.2.5).

• Assessing only the nearest vegetation. Every type is assessed separately and the highest result applies (Clauses 2.2.3.1, 2.2.6). At FDI 100 on flat land, grassland 10 m away reads BAL-29 while forest 60 m away reads BAL-12.5 (Table 2.4).

• Downgrading burnt or cleared bush. Recent fire or past clearing does not by itself reduce the vegetation class, and regrowth should be considered (Commentary C2.2.3.1).

• Using AS 3959 alone in New South Wales or South Australia. NSW applies it as amended by Planning for Bush Fire Protection, and South Australia reads the level from the Planning and Design Code in some areas (section 7). And attached or adjacent structures within 6 m take the building's BAL too, unless separated as Clause 3.2 allows (Clauses 2.1 and 3.2).

10. Where to read the source

AS 3959:2018 Construction of buildings in bushfire-prone areas, incorporating Amendments 1 (2019) and 2 (2020), is the edition NCC 2025 lists in its referenced documents. It is purchased from Standards Australia: Section 2 and Table 3.1 set the level, Sections 3 to 9 the construction. NCC 2025 Volume Two H7P5 and H7D4 and Volume One Part G5, with their state variations, are free on the NCC website.

Inside AEC Assistant the Code text is searchable by clause, and on a paid plan the Standards are too. Ask what BAL-29 means for the windows and deck of a house 35 m from forest and the answer comes back with the clause it was read from. Try it free. No credit card.

Figures checked against AS 3959:2018 (incorporating Amendments 1 and 2) and the ABCB's NCC 2025 text on 14 October 2026. Diagrams by AEC Assistant. This article is general guidance. The building surveyor or certifier for your project determines compliance.