One that got away: the aftermath of a fire onsite

Bushfire Hazard Assessment FAQ

Adam Smeeby author page

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Bushfire terminology explained

Our bushfire hazard assessment expert answers all your frequently asked questions about building a home on a bushfire prone site:

 

When do you need a bushfire assessment?

You need a bushfire hazard assessment if you are building a house on a bushfire prone site. If you think that the site might be bushfire prone (see below), I suggest getting the assessment done as early as possible in the process as it can have a significant impact upon the siting of the house and the cost of the project.

 

What is a bushfire prone site?

A bushfire prone site is defined as land that is within 100m of 1ha or greater of bushfire prone vegetation. A bush block obviously meets this definition although a residential lot on the urban fringe may also be bushfire prone. Aerial imagery such as that available online through Google Earth or www.listmap.tas.gov.au is useful in determining whether a site might be bushfire prone.

 

What is bushfire prone vegetation?

Bushfire prone vegetation is considered to be any unmanaged vegetation that is likely to be susceptible to bushfire. Native forest is the most common example although other vegetation types such as grassland and scrubland may also be considered bushfire prone.

 

What is a BAL?

BAL stands for Bushfire Attack Level. A BAL is the level of risk posed to a building in a bushfire prone area determined by an assessment carried out in accordance with the Australian Standard for the Construction of Buildings in Bushfire Prone Areas (AS3959:2009). BALs range from BAL-LOW, where the bushfire risk is very low, to BAL-FZ (Flame Zone) where the risk is extreme. Most buildings in bushfire prone areas will be at least BAL12.5 (low bushfire risk) and the BAL should be limited to BAL19 (moderate bushfire risk) wherever possible.

 

How is a BAL calculated?

A BAL is based upon the separation available between the building and the bushfire prone vegetation (referred to as the Hazard Management Area, see below for further detail), the gradient of the land beneath the vegetation, and how the vegetation is classified in accordance with the Australian Standard. The experience of the Black Saturday bushfires in Victoria in 2009 confirmed that houses with adequate surrounding cleared areas stand a better chance of withstanding a bushfire so the separation between a building and bushfire prone vegetation is a key factor in assessing bushfire risk. Gradient is also a significant consideration as fire moves faster uphill and a bushfire is therefore likely to be more intense at the top of a slope than on flat land. Vegetation type also plays a part in bushfire intensity, for example, native forest has a greater potential fuel load than grassland and results in more intense bushfires.

 

What effect does the BAL rating have on the building?

The Australian Standard provides construction requirements for each BAL. The construction requirements generally focus upon providing ember protection for weak points such as doors, windows, decks, external cladding, and roofing. At the higher BALs, the construction requirements also attempt to protect the building from flame contact and radiant heat.

 

What effect does the BAL rating have on cost?

Construction requirements become more stringent and the options for compliance reduce as the BAL increases. For example, timber cladding is permitted at BAL29 (and at lower BALs) provided it is of a bushfire resistant species. However, timber cladding is not permitted at BAL40 and above. There are also additional requirements at the higher BALs that do not feature at the lower BALs.

Therefore, the additional costs associated with construction requirements increase exponentially with the BAL. For example, Victorian company BAL Assessments estimates that, for an average house, the additional costs at BAL12.5 vary from $3k to $5k but can exceed $30k at BAL40 (http://www.bal.net.au/cost.htm).

 

What is a HMA?

The Hazard Management Area is the area between a building and unmanaged bushfire prone vegetation that is managed in a low fuel state to reduce bushfire hazard. For example, an area of native forest converted to HMA would have all of the understorey and most of the canopy provided by mature trees removed. Individual trees may be retained within the HMA as they may assist in reducing ember attack upon the building but only where there is adequate separation between the canopy, the building, and other vegetation. Slashing is usually the best way to maintain HMA although landscaping with low flammability plants and stones is also an option. As HMAs also provide an area within which fire fighters can fight a bushfire threatening a building, they are also referred to as “defendable space”.

 

What is a BHMP?

BHMP stands for Bushfire Hazard Management Plan. This plan shows how the bushfire hazard posed to the building will be managed. The plan is similar to a site plan in that it shows the location of the building on the site and vehicular access but also includes HMA as well as fire-fighting water supplies (see below).

 

Are there special water supply requirements when building a house in bushfire prone areas?

Yes*, when building a house in a bushfire prone area a fire-fighting water supply must be provided. In serviced areas, this supply may be provided by a fire hydrant provided it is close enough to the site. However, in unserviced areas, a dedicated supply must be provided on the site. An onsite fire-fighting water supply is usually provided within a heat and flame proof water tank but can be provided by a dam or swimming pool.

 

Are there special requirements for vehicular access when building a house on a bushfire prone site?

Yes*, adequate vehicular access is required to ensure that fire-fighting vehicles can enter the site in order to defend the home.

 

*Please note that in Tasmania at least, there is no requirement to provide a fire-fighting water supply or compliant vehicular access where only a dwelling addition such as a house extension is proposed.

 

Adam Smee is principal planning consultant of Southern Planning, specialising in bushfire hazard assessments

 

The next post will focus on the specific bushfire hazard assessment for 60k House and the design implications.

 

Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.

4 Comments

    1. Andrew Kerr

      Thanks for the comment Sam. This appears to only be around Perth, WA and only ascertains the BAL rating. Knowing the BAL of a site is useful, but a full Bushfire Hazard Management Plan will also likely be needed prior to building. Cheers

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