How is a BAL Rating Calculated?
In earlier posts, we explained what a BAL rating is and why it matters. If you missed it, you can catch up here. But how is your BAL rating actually determined?
A Bushfire Attack Level (BAL) rating is calculated using procedures outlined in AS 3959 – Construction of buildings in bushfire-prone areas. There are two methods available: the Simplified Procedure (Method 1) and the Detailed Procedure (Method 2). In most cases, Method 1 is sufficient, and that’s the focus of this article.
Here’s a breakdown of the five steps used to calculate your BAL.
Step 1: Determine the Fire Danger Index (FDI)
The Fire Danger Index (FDI) indicates how severe a fire could be under certain weather conditions. You’ve likely seen these on roadside signs during summer. While the sign changes daily, AS 3959 uses a fixed value depending on your region.
For Western Australia, the FDI is set at 80 under AS 3959.
Step 2: Classify Nearby Vegetation
All vegetation within 100 metres of the proposed building site is assessed. It must either be:
Excluded as low-threat, or
Classified into one of seven vegetation categories set out in AS 3959.
Correct classification is critical. Different types of vegetation—trees, shrubs, grasses, and scrub—affect fire behaviour in different ways, especially in terms of heat intensity and speed.
Step 3: Measure Distance to Vegetation
The next step is to measure the horizontal distance from the proposed building to the classified vegetation. This is done from the outermost wall or structural column—eaves and roof overhangs are excluded.
The closer the vegetation, the higher the potential bushfire impact.
Step 4: Determine the Slope Under the Vegetation
Slope plays a major role in fire behaviour. A fire burns much faster uphill, and the steeper the slope, the more rapidly the fire spreads.
As a rule of thumb, a fire's rate of spread doubles with every 10° increase in slope.
Step 5: Calculate the BAL Rating
Using the data gathered—FDI, vegetation type, distance, and slope—calculations are done using tables provided in AS 3959. Each vegetation plot is assessed, and the plot that poses the greatest risk determines your overall BAL rating.