BAL ratings explained

Bushfire Attack Level is a measure of the potential exposure of a building to ember attack, radiant heat and direct flame contact. It is set out in the Australian Standard for construction of buildings in bushfire-prone areas, AS 3959, which the National Construction Code calls up. A rating is assessed for the specific site, not for the suburb.

The levels

From lowest exposure to highest: BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40 and BAL-FZ (flame zone). The numbered levels refer to radiant heat exposure in kilowatts per square metre. Each step up adds construction requirements rather than replacing them.

What drives the rating

Four things: the vegetation classification around the site, the distance from that vegetation to the building, the slope of the land under the vegetation, and the fire danger index applying in that area. A block can sit next to an identical block and rate differently because of slope or because of where the house is placed on it.

What changes as the rating rises

Broadly: glazing and window frames, external wall construction, roof and eave detailing, decking material, screening of openings, and the sealing of gaps that embers can enter. At the upper levels the requirements extend to essentially the whole external envelope, and at flame zone they are onerous enough to change whether a design is viable at all.

Why it matters before you choose a design

A project-home design priced for an unrated block does not simply cost a little more at BAL-29 — some elements of it may need to be substituted. Finding that out after you have fallen in love with a floor plan is the expensive order to do this in. Ask for the rating, or for the assessment to be done, before design is locked.

Suburb mapping is not a site assessment

Our area pages report whether bushfire-prone land is mapped in the suburb, because that is what the state data supports. It tells you the question applies here. It does not tell you your block is affected or at what level — that requires an assessment against the actual site. We say this on every page rather than implying more precision than the data has.

Who does the assessment

An accredited bushfire practitioner, and in some states the relevant fire authority has a role in referral or approval. Your builder or certifier will know the path in your state. Ask early: it can sit on the critical path for approval.