How a BAL is worked out (AS 3959 Method 1)
Under Method 1 of AS 3959, a Bushfire Attack Level comes from four things: the class of vegetation near the building, how far away it is, the slope of the ground under it, and the regional fire danger index. Each patch of vegetation gets a level, and the highest one sets the building's BAL.
Method 1 and Method 2
The Australian Standard AS 3959 has two ways of working out a BAL. Method 1 is the simplified procedure most sites use. Method 2 is a more detailed calculation, used for steep sites and other cases Method 1 doesn't cover, or where a more site-specific answer is wanted; it's normally done by an accredited bushfire practitioner. BAL Buddy uses Method 1 only. AS 3959 explained covers the standard itself.
1. The vegetation class
Method 1 sorts vegetation into a small number of classes, such as forest, woodland, shrubland, scrub and grassland. Some types burn far more intensely than others, which is why the class matters. The class is judged mostly by structure rather than species: how tall the plants are, how much of the ground their canopy covers, and what grows underneath. Two stands of the same trees can fall into different classes if one has a dense understorey and the other doesn't.
Method 1 also lets some vegetation be left out, such as certain small areas, narrow strips and low-threat or managed vegetation. In BAL Buddy you draw each patch and choose its class, with the vegetation guide to help, and before it leaves out a patch for its size or shape it asks you to confirm that the patch is all there is.
2. The distance
The distance is measured from the building to the vegetation: the closer the vegetation, the higher the BAL. Vegetation more than 100 m from the building isn't assessed. BAL Buddy measures the distance from the outline you draw to the edge of each patch, and reports it for each one.
3. The slope under the vegetation
What counts is the slope of the ground under the vegetation, not under the building. Fire runs faster uphill, so vegetation on ground falling away from the building is more dangerous than vegetation on flat or rising ground. BAL Buddy works out the slope from public elevation data, within 100 m of the building, and says which data it used and how detailed it is. Where the ground under the vegetation falls away more than 20°, the site is outside Method 1, and BAL Buddy says so instead of giving a result.
4. The fire danger index
Each part of Australia has a fire danger index, reflecting its climate, that Method 1 uses to pick the right set of distances. Most states and territories have one index for the whole jurisdiction. In NSW it depends on the fire weather district, and alpine areas of NSW and Victoria have their own. BAL Buddy works out the index from where the building is, and asks about alpine areas when a site is high enough to be one. The BAL calculator by state says how it handles each jurisdiction.
Putting it together
For each patch, the class, distance, slope and fire danger index give a level from BAL-LOW to BAL-FZ (see BAL levels explained). The building takes the highest of them. BAL Buddy shows the result for each patch, so you can see which one sets the BAL and how far the building would need to move for a lower level.
BAL Buddy gives an indicative Bushfire Attack Level (BAL) only. It isn't a determination under any Act, and no council or building surveyor has to accept it. For a BAL you can rely on, use an accredited bushfire practitioner.
Questions people ask
Does BAL Buddy do Method 2?
No. It applies Method 1 only. Sites outside Method 1, such as very steep ground under the vegetation, need an accredited bushfire practitioner.
Why does the slope under the vegetation matter more than the slope under the house?
Because fire runs faster uphill. Vegetation on ground falling away from the building can bring a fire towards it faster and with more intensity.
Do I need to know my fire danger index?
Not to use BAL Buddy. It works it out from the building's location and shows how it did so; in NSW and Victoria it may ask whether the site is in an alpine area.
Can I see how the result was worked out?
Yes. The summary shows the class, distance and slope for each patch, the fire danger index and the data the slope came from.