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By Willow Aliento
July 30, 2018
There is more to constructing buildings able to withstand the impacts of bushfire than just adding extra features at the finishing stage. Thinking about fire risk needs to happen at the earliest planning stages, according to Chris Wyborn, the general manager of Education and Bushfire Services at Fire Protection Association of Australia.
Wyborn tells Jobsite that planning and design is influenced by both the state bushfire risk mapping, and planning approval requirements which apply to every site.
Mapping is based on aerial images that show vegetation areas and other factors. That is why it is possible to have a suburban site classified as having a high Bushfire Attack Level (BAL) due to proximity to a nature reserve, and equally possible to have a rural site with a low BAL.
Bring in a Safety Expert Early
Not considering the details of a site’s specific BAL and the resulting requirements can mean a lot of wasted time and money when the project proposal is lodged for planning approval.
Involving a bushfire safety expert at the outset is the smart way to go.
“It is critical builders understand the need for planning approval. The planning system, when it comes to bushfires, is all about risk minimisation,” Wyborn says. “If you don’t involve a bushfire expert at the beginning, you could find out at planning approval stage that the architecture and the materials specifications are not right, or even that the building envelope needs to be relocated to elsewhere on the site.”
While clearing vegetation around a site will lower the BAL, Wyborn points out the building codes applied to the actual building “can’t condition the land.” Therefore, a project needs to meet the applicable standards without consideration of landscape plans.
According to the New South Wales Rural Fire Service, the BAL categories are determined by the type of vegetation around the building. That includes neighbouring areas not on the actual site; how close the building is to the vegetation; the effective slope, as fire runs more readily and with greater intensity uphill; and the fire danger index applicable to the region.
They are explicitly defined in Australian Standard AS 3959, which divides bushfire-prone areas into six BAL categories. They range from BAL-LOW, which is very low risk through to BAL-40, very high risk, and BAL-FZ, the flame zone and the most extreme level of risk. The division is based on the building’s potential exposure to ember attack, radiant heat and direct flame contact.
AS 3959 also sets out the materials performance requirements, design requirements and other building envelope factors required to ensure the building is designed for its defined risk level.
For example, if a building is situated in an area where ember attack is a risk, window and door screens must have gaps smaller than a 3mm, so that a 3mm probe cannot pass through. 3mm is considered the minimum size of ember that can enter a home with sufficient thermal energy to spark a fire inside.
Where radiant heat is a factor in the BAL level, the standard will specify the type of glass that must be used for windows and doors. Standard single glazing is unsuitable for buildings exposed to high radiant heat loads from fire—it can crack, allowing embers or flames to enter the building.
Myths & Misconceptions of Bushfire Disasters
Wyborn says there are “a lot of myths and misconceptions” about how buildings burn down in bushfire scenarios.
Around 80 per cent of buildings are actually lost due to ember attack—not to the fire front.
Embers with sufficient heat to spark a fire may travel up to 200 metres or 300 metres ahead of the actual fire front, he says.
If they land on flammable materials near a home, then any weakness the home has can result in it being vulnerable. House-to-house fire transmission also becomes more likely.
Such was the case of one of Australia’s worst bushfire disasters, Victoria’s Black Saturday fires. The homes lost in one community were all destroyed due to fires started by ember attack and not by the flame front.
One of the crucial factors builders need to be aware of when constructing a building for bushfire risks is ensuring there are no gaps where embers can enter a home—particularly, in the facade, eaves, roofline, around doors and windows, as well as under the home.
This is a matter of both design and workmanship.
The Right Fire Safety Features
For higher BAL-zoned properties, fire resistant sarking is required for the building’s protection. Wyborn says correct installation and proper sealing are vital.
To deliver a best practice building, Wyborn suggests builders should not only upsell customers on cosmetic inclusions, such as stone benchtops, but also fire safety features.
Fire sprinklers inside a building are usually not required under the building code for low-rise general residential or detached dwellings. However, Wyborn says adding them to a home, particularly in the roof space, makes that building safer both as a fire refuge for the owners or occupants and in terms of its likelihood of withstanding fire impacts should they evacuate the property ahead of an approaching fire.
Where power loss or mains water loss is possible during a fire, adding an independent water source that can supply the sprinkler system, such as a rainwater tank,and a stand-alone pump can also be a worthy inclusion.
Wyborn says engaging a bushfire consultant can help a builder achieve better outcomes for their customers also by assisting with matters that are not covered by the standard like developing a site and building-specific bushfire survival plan for the building owners.
“A home owner often doesn’t understand their own risk,” he says. “Builders have a duty to educate the person buying the house. There is a need for the building fraternity to provide some relevant material to owners, because we need them to understand the risks, especially those specific to the area they are living in.”
Climate Change = More Intense Bushfires
Due to climate change, the risks are also becoming relevant to more property owners than ever before.
The CSIRO has predicted that with climate change, we will start to see more intense bushfires and see them more frequently, Wyborn explains. More intensity is a significant issue in terms of planning, design and construction, as the BAL levels applying to a property are based on current modelling of the current site conditions.
However, these are likely to change and not for the better. Hotter drier days will increase risk, and with the higher level of carbon dioxide in the atmosphere, Wyborn says vegetation grows faster and sheds more dead material in the hot weather. This leads to more fuel at ground level and therefore more intense fires.
“Every day post sign-off a building becomes more vulnerable,” he says. “The houses being built today are not being built for tomorrow’s fire.”
There are a number of ways builders can upskill themselves in planning, design, and construction to reduce bushfire impacts.
On September 6-8 2018, the 4th annual Australian Bushfire Building Conference is being held in the Blue Mountains. The agenda includes an expert analysis of the 2017 California bushfires and the 2018 Tathra NSW bushfires; explanations of AS 3959 and other relevant regulations; an expert panel; and an expo of products and service providers.
The FPAA also offers training in planning and building for bushfire. In Perth, there is a short course in October, which will help practitioners understand and apply the new bushfire and planning requirements introduced in WA.
General information can also be obtained from sources including the Housing Industry Association, the Australian Windows Association, and state bushfire agencies, such as the Victorian Country Fire Authority, NSW Rural Fire Service and the Queensland Rural Fire Service.
The Bushfire Building Council of Australia offers a number of resources, including links to state and territory-specific risk mapping, best-practice Innovation Award winner case studies, retrofit guides, and explanations of ember attack, and radiant heat impacts.
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