BAL rating requirements: BAL 12.5 to BAL FZ explained

TL;DR: A BAL (Bushfire Attack Level) is a measure of how much radiant heat and ember attack a building is likely to face in a bushfire, set out in AS 3959 Construction of buildings in bushfire-prone areas and called up by the National Construction Code. There are six levels: BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40 and BAL-FZ (Flame Zone). The BAL is worked out from four things: the fire danger index for your region, the vegetation class within 100 m, the distance to that vegetation, and the slope under it. Every step up changes glazing, screens, decking, subfloor, cladding and roof detailing. BAL-12.5 to BAL-29 is mostly tighter detailing and better glass. BAL-40 brings shutters and non-combustible walls. BAL-FZ means tested systems and usually a performance solution. Get the BAL report before you price the job, not after.

If you quote residential work anywhere near trees in this country, BAL rating requirements will eventually decide whether your job makes money. The frustrating part is that the BAL is not something you can eyeball. Two blocks on the same street, same vegetation, same setback, can land on different levels because one sits below the bush and one sits above it. And because the BAL is set at planning stage by someone who is not you, builders regularly find out what they have signed up for after the contract is priced.

This is the plain English version: how a BAL gets assessed, what each level actually changes on site, and what it does to a quote.

What a BAL actually measures

A BAL is a radiant heat figure, expressed in kilowatts per square metre, representing the heat a building element is expected to be hit with at the peak of a bushfire front. It is not a probability, it is not a hazard rating for the suburb, and it is not a judgement about whether your client's house will burn down. It is an exposure number used to pick construction details.

The levels, and the sections of AS 3959 that carry their requirements:

BAL	Radiant heat exposure	AS 3959 section	Main threat
BAL-LOW	Risk considered insufficient to warrant specific construction	Section 4	Nil specified
BAL-12.5	Up to 12.5 kW/m²	Section 5	Ember attack, some radiant heat
BAL-19	Over 12.5, up to 19 kW/m²	Section 6	Embers plus increasing radiant heat
BAL-29	Over 19, up to 29 kW/m²	Section 7	Embers plus significant radiant heat
BAL-40	Over 29, up to 40 kW/m²	Section 8	The above plus flame contact
BAL-FZ	Over 40 kW/m²	Section 9	Direct flame contact

Worth saying clearly: the jump from BAL-29 to BAL-40 is where the standard stops assuming flames stay away from the building. That is the point where the cost curve bends, and it is the single most useful thing to know before you price anything.

How a BAL rating is assessed

AS 3959 gives two routes. Method 1 is the simplified procedure in Section 2, and it is what most assessors use. Method 2 is the detailed procedure in the standard's appendix, which models the fire more closely and sometimes returns a lower level for the same site.

Method 1 needs four inputs:

Fire Danger Index for the region. The standard assigns this by state and area, and it is not the same across the country. Victoria sits at the top of the range at 100, Tasmania considerably lower at 50. Check the FDI table in the edition your certifier is working from, because the number drives everything downstream.
Vegetation classification. Forest, woodland, shrubland, scrub, mallee or mulga, rainforest, or grassland, each assessed within 100 m of the site. Forest is the harshest, managed grassland the mildest. The standard also has exclusions: small isolated patches, low threat vegetation, maintained lawn, and single trees generally do not count as classified vegetation.
Distance from the site to that vegetation. Measured from the building, not the title boundary. Beyond 100 m, the standard does not consider the vegetation at all.
Effective slope under the vegetation. Flat or upslope is the best case. A downslope is the worst, because fire runs uphill at you and the flame front leans in. A 10 degree downslope can be the difference between BAL-19 and BAL-40 on an otherwise identical site.

Those four inputs feed a lookup table that spits out the level. That is the whole mechanism. It is closer in spirit to a wind rating than to anything discretionary, which is why the same logic trips builders up the same way: a single input moving one band shifts the whole construction spec. If you have been through that with wind, our wind ratings explained: N1 to C4 guide covers the identical trap from the other direction.

Who sets it, and where it appears in your paperwork

The BAL is normally determined by a bushfire consultant or an accredited assessor, and in most states it arrives attached to the planning approval rather than the building permit. Where you find out the site is in scope depends on the jurisdiction:

Victoria: Bushfire Prone Area mapping, plus a Bushfire Management Overlay on higher risk land. Victoria also requires a minimum of BAL-12.5 for new homes in a designated BPA, even where the calculation would otherwise come out lower.
New South Wales: the section 10.7 planning certificate shows bush fire prone land, and the RFS document Planning for Bush Fire Protection governs the assessment. A Flame Zone result on an infill site generally means a performance solution rather than a straight refusal.
Queensland, South Australia, Western Australia and Tasmania each have their own overlay or mapping instrument that triggers AS 3959. Confirm it against the current planning instrument rather than a memory of last year's job.

Do not accept a verbal BAL. Get the written report, with the vegetation classes, distances and slope it was based on, because those are the things that change and the things you will need if you ever have to defend the spec.

BAL rating requirements at a glance, level by level

Here is what changes as you climb. Treat this as the shape of the problem, not as a substitute for the glazing and cladding schedule your certifier signs off.

BAL-LOW. No specific bushfire construction requirements under AS 3959. Note that in some states you can still be in a mapped area and still be required to build to BAL-12.5 as a floor.

BAL-12.5. Ember attack is the whole story at this level. Gaps in the building envelope are closed to no more than 3 mm. Vents, weepholes and openings get corrosion-resistant mesh with an aperture no larger than 2 mm. Openable window sections get screens. Tiled roofs get sarking. Evaporative coolers need ember protection. Timber framing and most common claddings are still on the table. This level is mostly a detailing job, and it is the one most builders underprice because it looks like nothing changed.

BAL-19. Same ember logic, more radiant heat. Toughened glass starts appearing in the schedule, typically on glazing within 400 mm of the ground or of a deck surface, and the cladding and subfloor options narrow.

BAL-29. Glazing thickness steps up again. Decking is the one that bites here: boards near the building have to be non-combustible or made from one of the bushfire-resisting timbers listed in the standard's appendix. That list is short, roughly seven species including blackbutt, spotted gum, red ironbark, river red gum, silvertop ash, turpentine and kwila. If your client has chosen a decking board that is not on it, you are having a conversation.

BAL-40 and BAL-FZ: where the quote changes shape

BAL-40. The standard now assumes flame contact is possible. Windows and doors need bushfire shutters or screens that meet the standard. All glazing is toughened. External walls in practice go non-combustible. Mesh at this level must be steel or bronze, not aluminium, because aluminium gives up at the temperatures involved. Elements can also be demonstrated by test to AS 1530.8.1.

BAL-FZ. This is a different exercise. At Flame Zone you are not selecting products from a catalogue, you are assembling tested systems. Walls, windows and doors are either built to the prescriptive Flame Zone provisions of Section 9 or tested to AS 1530.8.2, and most Flame Zone homes end up with a performance solution and a fire engineer attached. It is buildable. It is not cheap, and it is not the kind of job to take on at a fixed price with a vague allowance.

For steel-framed homes there is also the NASH Standard for steel framed construction in bushfire areas, which the NCC references as an alternative path to AS 3959. Worth knowing if you frame in steel, because the detailing differs.

The general requirements that apply at every level

Section 3 of AS 3959 sets requirements that apply regardless of BAL, and they are where most non-compliances are picked up at inspection. Roof and wall junctions. Gutters and valleys. Roof penetrations and flues. Subfloor enclosure. Screening of every opening that an ember can get through, including the ones nobody photographs, like the gap behind a fascia or under a garage door.

Also know the boundaries of the standard. AS 3959 does not deal with your defendable space, your water supply for firefighting, your fencing, your water tank, or the landscaping. Those sit in the planning approval, not the building code. Builders get caught assuming the BAL report covers the lot, then find an asset protection zone condition on the permit that nobody costed.

The official source for which edition and amendment of AS 3959 applies in your state is the NCC itself, available free at ncc.abcb.gov.au, and the standard is published through Standards Australia. Check the referenced edition rather than trusting the PDF on your hard drive. And keep bushfire construction separate in your head from internal fire safety, which is a different set of provisions again. We covered that side in NCC fire safety requirements.

What a BAL does to your quote

We are not going to publish a cost-per-BAL table, because any number given without seeing the site, the design and the current supply situation is a guess dressed up as data. What we can tell you is where the money moves.

The three line items that swing a quote hardest are bushfire shutters, tested or certified window and door systems, and non-combustible cladding. All three arrive together at BAL-40. Below that, the cost is spread thinly across a lot of small items: mesh, sarking, sealing, toughened glass upgrades, a decking substitution, longer labour hours on detailing that used to be a nail gun and a caulk tube.

How to price it so you do not eat it:

Separate line items, named by BAL. "BAL-29 compliance package" on its own line, not absorbed into cladding. If the BAL changes after a re-assessment, you have something to vary.
Price the assessor's fee as a pass-through. Get the fee in writing, show it as a disbursement, and include GST properly on your invoice so there is no argument later.
Scope the glazing schedule before you quote, not after. Toughened and tested glazing has lead times, and a glazing substitution three weeks out is where margin disappears.
Put the slope assumption in writing. If the report assumed an upslope and the client then cuts a batter, the BAL can move and that is a variation, not goodwill.
Carry a stated allowance for the Section 3 detailing, because it is labour, not materials, and labour is what gets forgotten.

If you are pricing a smaller build on a bush block, the baseline numbers in our cost to build a granny flat in Australia breakdown are a useful starting point to add the BAL package on top of, rather than trying to hold a single all-in rate.

Where builders get caught

Four patterns, over and over.

The BAL report is older than the vegetation. Bush grows back, neighbours clear, councils re-map. A report from four years ago may not describe the site you are standing on.

Method 1 was used when Method 2 would have helped. The detailed procedure costs more in assessor time and occasionally returns a lower level. On a borderline BAL-29 or BAL-40 site, that is worth asking about before you accept a shutter package.

The spec was priced at one BAL and built at another. Any change to the site that moves distance or slope moves the BAL. Treat the four inputs as live variables.

The client was never told what the level means. "BAL-40" is meaningless to a homeowner until somebody explains that it buys shutters and non-combustible walls. The way the industry talks about compliance has a real effect on what clients accept in a price, which is something we dug into in sustainable building requirements and what the industry's language does to your quotes.

Frequently asked questions

Do sheds and carports need to comply?
Generally yes where a Class 10a structure sits close to the house, because a shed that catches is a fire source against the dwelling. The exact trigger distance and the scope of what applies comes from the NCC and your state's requirements, so confirm it on the permit rather than assuming the shed is exempt.

Can a BAL be reduced?
Sometimes. Increasing the distance to classified vegetation, managing vegetation so it falls under an exclusion, or moving the building footprint can all drop a level. The detailed procedure in AS 3959 can also return a lower figure than the simplified one. What you cannot do is clear your way out of it unilaterally, because vegetation removal has its own approvals and some states have specific allowances with their own conditions.

Does a BAL rating expire?
The rating itself is not time-stamped the way a certificate is, but it describes site conditions on the day it was assessed. If the vegetation, slope or mapping has changed, or if the approval has lapsed, expect to re-assess. Treat anything more than a couple of years old as needing a second look before you price from it.

Is BAL-FZ worth building, or should the client move the house?
Often the cheapest Flame Zone solution is to not be in the Flame Zone. Before committing to tested systems and a fire engineer, get the assessor to model what moving the footprint, or changing the orientation, does to the number. A ten metre shift can be worth a very large amount of shutter.

Does the BAL cover my water tank, fence and landscaping?
No. AS 3959 is about the building. Water supply for firefighting, asset protection zones, fencing and planting come through the planning approval and state bushfire planning requirements. Read the permit conditions as a separate cost item.

Price it once, properly

The pattern with BAL rating requirements is the same as with wind ratings and energy provisions: the compliance level is set by someone else, it lands as a schedule of products and details, and the builder who reads it properly before quoting keeps the margin. The builder who treats it as "a bit of extra mesh" does not.

If you are putting these packages into quotes, the free Built Simple app has all 45 of our calculators on your phone with no signup, so you can do take-offs for decking, cladding and glazing on the block while the assessor is still walking the vegetation line. Have a play with it, and if you want the quote to invoice side in one place too, the full app starts from $0.