Bushfire Attack Level or BAL is a method for rating the intensity of a locations potential exposure to bushfire. Identifying the Bushfire Attack Level around your property can help you site a new structure or better manage an existing one.
Following on from the Hazard identification tutorial, you should have the information you need to determine the BAL ratings around your property. BAL ratings are organised into five categories of increasing exposure, as shown in the table below.
Currently, the Australian Standards outlines two methods for calculating BAL. Our CSIRO BAL calculator is based on the most commonly used methodology, which is described in AS 3959:2009.
While important, BAL (calculated using any method) should be thought of as a relative estimate of the potential exposure on your property, rather than a precise measurement. It’s important to understand that for development in the Bushfire Management Overlay (BMO) your council’s Planning Scheme in many instance will set minimum requirements for you BAL and defendable space which will need to be incorporated into you planning permit application.
If you need help determining your BAL or with a building and planning proposal, we recommended contacting an accredited bushfire consultant. Licensed consultants can provide a range of services, including BAL assessments, BMO reports, and bushfire planning.
|BAL||1Heat flux threshold (kW/m2)||2Predicted bushfire attack and level of exposure|
|BAL-12.5||≤ 12.5||Significant ember attack, burning debris and radiant heat up to a level of 12.5 kW/m2.|
|BAL-19||12.5 – 19||Increasing levels of ember attack, burning debris and radiant heat up to a level of 19 kW/m2.|
|BAL-29||19 – 29||Increasing levels of ember attack, burning debris and radiant heat up to a level of 29 kW/m2.|
|BAL-40||29 – 40||Increasing levels of ember attack, burning debris and radiant heat up to a level of 40 kW/m2. Flames from the bushfire front may intermittently contact the house.|
|BAL-FZ||≥ 40||Increasing levels of ember attack, burning debris and radiant heat in excess of 40 kW/m2. Flames from the bushfire front are likely to engulf part or all of the house.|
1This is the average radiant heat (in kilowatts per square metre) that will be experienced at the house when under the specified Bushfire Attack Level. 2These are the bushfire conditions that will be experienced when under the specified Bushfire Attack Level. Check the BAL at your home or the site of your new build – either, your home should be built to resist these attacks, it should be sited at a location to achieve a lower BAL, or vegetation on your property should be managed to reduce your BAL (e.g., by clearing vegetation, installing firebreaks, earthworks or other barriers). For more information on BALs, see AS 3959:2018
Siting a new structure on your property
The BAL site assessment tool is used to calculate BAL contours for new structures.
BAL-based heat and exposure contours will help you comply with the building regulations at your address, but more importantly they will aid in siting your new home or structure. The inputs required for the tool would have been gathered during your Hazard identification assessment.
The BAL site assessment tool – note where you enter your sites slope, vegetation type, and FFDI
- If you haven’t already, identify your Forest Fire Danger Index, dominant Classified vegetation type, and Slope.
- Open the BAL site assessment tool
- Use the slider or text box to enter the relevant site slope and effective slope.
- Select the relevant Vegetation type and FFDI from the drop down boxes.
BAL contours reveal the level of potential exposure that your new home will experience. These values are calculated automatically in the BAL site assessment tool.
The BAL site assessment tool – note the results are automatically displayed at the top of the page
- After entering the information in Step 1, read the distances for each BAL category from the image at the top of the tool or in the table displayed at the bottom.
- If possible, site your new home in an area of your property with a low BAL rating. For example, the green (BAL-low) or yellow (BAL-12.5) zones, as in the example above.
Calculating BAL contours for existing structures
The BAL calculator is used to calculate BAL contours around existing structures or known location of future dwelling.
The nature of your property and the location of your house will influence the building regulations that apply at your address. The inputs required for the tool would have been gathered during your Hazard identification assessment.
The BAL calculator – note where you enter your sites slope, vegetation type, FFDI, and its distance to vegetation
- If you haven’t already, collect information on your Fire Danger Index, dominant Classified vegetation type, and Slope.
- Open the BAL calculator
- Use the slider or text box to enter the site slope and the effective slope.
- Select the relevant vegetation type and Forest Fire Danger Index from the drop down boxes.
BAL-based contours can reveal the potential exposure that your house may experience. The values for these contours are calculated automatically in the BAL calculator.
The BAL calculator – note the results are automatically displayed at the bottom of the page, this includes the sites BAL and estimates of the fires speed and flame height
- After entering the information in Step 1, read the distances for each BAL category from the image at the top of the BAL calculator – or in the table displayed at the bottom.
- Note the BAL for each side of the house and at the locations of other structures or features on your property (such as sheds, fences, driveways and gardens). Fixed structures (like homes, garages and sheds) in an area with a BAL rating of BAL-12.5 or above, may require specific measures to mitigate their risks. Read the rest of the Bushfire Best Practice Guide for advice on retrofitting your home and garden.