Bushfires are a common occurrence in Australia. Large expanses of natural woodland combined with the hot temperatures of summer and arid conditions can lead to uncontrollable bushfires that can destroy anything in their path including buildings.
On Christmas Day, 116 families in the Wye River and Separation Creek area lost their homes in a bushfire. Five of these homes were bushfire rated leading experts to question house design standards.
The Bushfire Building Council of Australia (BBCA) claims the existing building standards are flawed. BBCA's chief executive Kate Cotter explained that the type of fire and fire weather conditions (during the Christmas Day fire) were well within the design limitations of the building standards.
Stephen Kip, board member of the Australian Building Codes Board and fire safety engineer pointed out at least half a dozen design flaws in them.
The increasing incidence of bushfire in Australia is cause for concern. A 2016 ABC report citing research by NASA satellite data showed there were 4,595 bushfires per week in Australia in 2013. There has been an increase of 40% since 2007.
Bushfires typically last for 2-5 minutes in a single area. However, they leave behind embers that can smoulder for days, generating a huge amount of heat, and causing damage to property and lives.
Approximately half of all bushfire incidents occur as a result of lightning strikes, which cannot be predicted with certainty. Consequently, most policies focus on mitigation, rather than complete prevention. Buildings in bushfire-risk zones must meet several regulations to protect their residents.
Bushfire risk factors
Tree and Bush Density: The dense growth of trees and bushes is a major risk factor, especially the foliage on the ground, which fuels the spread of flames during a bushfire. More foliage assists with faster spread of a fire. Drought conditions also exacerbate the situation with dry foliage increasing the risk of bushfires.
Wind Speed: During a bushfire, the wind provides more oxygen to the fire, not only increasing its intensity but also igniting embers into full-on flames. Wind can also speed up the spread of flames.
Slopes: Fires use convection and radiation to heat up their fuel sources. As a result, they actually travel faster when they reach an incline with every 10-degree slope causing the fire to spread twice as fast when compared to fire on flat ground. A 20-degree incline spreads the fire four times faster than flat ground. This continues in 10-degree intervals.
Which areas are at risk?
Depending on the Australian climate patterns, some areas are more prone to bushfires than others at different times during the year. For example, the summer to autumn period is peak bushfire season in South Australia. However, spring into summer presents the biggest risk in Queensland and New South Wales. Surprisingly, winter holds the biggest threat for the Northern Territory.
Beyond that, cities present far less risk than suburban and rural areas thanks to their built-up environment, which provides very little fuel to sustain a bushfire. However, the further you get away from the city, the more the risk increases.
However, when Australians want to live in areas that have natural views, it’s important for architects and engineers to ensure their homes are protected against bushfires. Thankfully, most areas carry a Bushfire Attack Level (BAL) rating indicating how much risk an area faces from bushfires. Furthermore, BAL regulations also dictate building measures.
Understanding BAL ratings
Following the Victoria Black Saturday bushfire in 2009, one of the worst in Australian history, each state agreed to implement a common system that would govern the safety of building materials. Dubbed the Bushfire Attack Level, this rating would determine the level of bushfire risk for any area. The BAL rating also dictates the specific materials to be used by builders, architects and engineers.
Australian Standard (AS) 3959 sets out the specific rules to follow for each BAL rating. As a general rule, the closer you are to bushland, the more likely it is that you’ll have to make adjustments to your structure.
There are six BAL ratings. Each one indicates a higher level of risk than the one that precedes it. They are BAL-LOW, BAL-12.5, BAL-19, BAL-29, BAL-40, and BAL-FZ.
Each zone has specific restrictions with regards to the building materials used in any structure. These restrictions apply to subfloor supports, floors, exterior walls, exterior windows, exterior doors, the roof, and any additional outdoor structures, such as decks and balconies.
Both BAL-40 and BAL-FZ are flame zones, so don’t assume that you need only worry about the latter rating.
You can speak to the local council to find out what your region’s BAL rating is, or consult with an independent bushfire specialist. These professionals have specific expertise that may prove invaluable. For example, they may be able to tell you if your property backs into a region with a higher BAL rating.
BAL ratings and windows
BAL ratings only affect exterior windows, and you can use any material for internal windows.
When building in a BAL-40 zone, you can: use standard windows, provided they have bushfire shutters that offer complete protection and comply with AS3959:2009; or build a window system that complies fully with AS 1530.8.1; or build your own windows that meet the following criteria: Window glass has a minimum 6mm thickness; only metal window frames including aluminium; metal hardware is used in the sash that opens or closes the window; openable portions must have metal screens to protect them; and sills, stiles, and threshold materials must have a flammability index rating of five or less.
Architects and building professionals can specify louvre windows to address many of these requirements instead of unsightly bushfire shutters.
When building in a BAL-FZ zone, building your own windows is not an option, and the window system requires compliance with AS 1530.8.2.
Other considerations when building in a BAL-40 or BAL-FZ zone
- Speak to the local council to learn more about the regulations that apply to your region. Bear in mind that you have to pay $149 to receive a certificate for more specific information.
- Check the area you want to build on against the risk criteria above. The presence of slopes near bushes is a particular concern. Ideally, you’ll build at the bottom of a slope, rather than the top.
- The Asset Protection Zone dictates how closely you can build to vegetation. Check it before starting any design work.
- Consider integrating an alternate power supply for the property.
- Consider water supply options, and how you could use the supply to fend off a fire. For example, you could place hosepipes around the house.
- Understand that the higher the BAL rating, the more you’ll spend on construction.
- Check access roads to ensure building occupants have escape routes open to them.
- Remember that a structure isn’t necessarily safe just because it doesn’t back directly onto bushland. Flying embers can cause problems too.
Building in bushfire-risk areas presents some problems, and depending on the BAL rating, several factors need to be considered before beginning construction.
Louvre windows from Safetyline Jalousie provide the perfect alternative to unsightly bushfire shutters. They can serve the same purpose, and make your buildings look more attractive. We can help you design great windows that meet the various building regulations governing bushfires.