*The WoodSolutions Technical Design Guide #4, which this blog is based on, is currently under review. The content will be updated when the revision has been published.
Bushfires are a natural part of our Australian landscape. However, the extent and frequency of bushfires appears to have changed, making it more important than ever to utilise building materials that ensure the safety of your family and home.
The Australian Standard AS 3959-2018 Construction of building in bushfire-prone areas and the National Construction Code (NCC) have both recognised this and adapted over the years to enhance the protection of Australian properties and lives.
How it works is building construction and planning controls apply to new housing in defined bushfire prone areas. Bushfire prone areas (BPA), put simply, are areas that are likely to be subject to bushfires. For example, in Victoria, bushfire prone areas make up most of the state. So, if your planned build is within a BPA, you’ll need a Bushfire Attack Level (BAL) assessment, but more on that below.
The good news is that by the end of this blog, you’ll have learnt more about BAL-ratings and how you can better utilise timber products like IronAsh cladding and screening in some of the highest rated areas. This means you can still have an attractive timber home whilst creating a fire-resisting building envelope. It’s a win-win!
As mentioned above, when you are building in a BPA, you’ll need to get a BAL assessment. BAL stands for Bushfire Attack Levels, which measures a building’s potential exposure to a bushfire and takes into consideration factors like Fire Danger Index, slope of the land, vegetation and proximity to any building.
Once your land has been assessed, you will be assigned a BAL-rating. The table below outlines each BAL-rating in accordance with AS3959.
For internal joinery applications and framing, there are no limitations on materials. Externally, the material depends on the assigned BAL rating, but typically used are treated timber products, high-density Australian hardwoods and fire-retardant treated timber products.
IronAsh is ASH’s brand of H3 Victorian ash treated products for cladding, screening, structural and mass timber applications. At 650kg m3, IronAsh (Victorian ash) is naturally a BAL-19 (Bushfire Attack Level 19) rated hardwood. This means that IronAsh can be used externally in areas of BAL-19 or lower (12.5 and LOW).
While this rating is widely accepted for the window and door market, IronAsh cladding and screening can be used in BAL‒LOW, BAL‒12.5 and BAL‒19 provided it is not installed within 400mm from the ground – so consider elevating your building 400mm or using a deemed bushfire resistant product such as fibre-cement or corrugated metal sheeting or stone to compliment your stunning purchase of IronAsh cladding.
Read more about using IronAsh in BAL-19 areas in this blog.
We covered above that IronAsh is naturally BAL-19. However, what you may not know is that IronAsh cladding and screening can also be used in BAL-29, BAL-40 and BAL-FZ rated areas if used in a timber wall system.
BAL-29 is the third-highest Bushfire Attack level which has an increasing risk of ember attack and burning debris ignited by windborne embers together with a heat flux not greater than 29 kWm2.
BAL-40 is the second-highest Bushfire Attack Level which has the possibility of ember attack and burning debris ignited by wind-borne embers and an increased likelihood of exposure to bushfire flames. The ‘40’ indicates that a radiant heat no greater than 40kW/m2 is assumed.
BAL-FZ is the highest Bushfire Attack Level and is very bushfire-prone, having a predicted direct exposure risk to flames from a fire front, ember attack and radiant heat which is greater than 40 kW/m2.
As illustrated in the diagram below (obtained from WoodSolutions Technical Design Guide #04), specifiers can comply with the BAL-FZ, BAL-40 and BAL-29 wall requirement as stated in the bushfire standard AS 3959 for timber wall systems by using a 16mm moisture-resistant fire grade plasterboard fixed to the outside of the wall frame.
The key elements to the wall system are detailed in the WoodSolutions Technical Guide #04 here (you’ll need to create an account to gain access to the Guide).
How the wall system pictured above complies is that it achieves a Fire Resistance Level (FRL) of 60/60/60, whereas AS 3959 only requires a FRL 30/30/30. Although keep in mind that some fire authorities do not support the use of materials that flame in these extreme bushfire classifications (BAL 40 or FZ). Architects and builders should always have a discussion with their local fire service and local government authority (e.g. Council, Shire) before finalising any specifications.
AS 3959 permits the use of construction requirements of a higher BAL to be used in a lower BAL.
Although when designing in extreme BAL-40 or BAL-FZ areas with a timber clad wall system, it is recommended that you do not mix and match building elements (e.g. walls and windows) unless the combination has been fire tested. This means that when using external timber cladding, we suggest building to full ‘BAL-FZ’ construction (e.g. walls, roof, windows, doors) in both BAL-40 and BAL-FZ areas. The logic being that BAL-40 windows are typically not fire tested with a timber cladding surround.
This is why in BAL-40 rated areas, it is not recommended to use a timber clad BAL-FZ wall and a BAL-40 window as the timber cladding may adversely impact on the BAL-40 window. Before specifying this combination check with your window supplier to obtain a fire test report, tested to AS 1530.8.1.
Now you can confidently specify IronAsh applications in BAL-LOW, BAL-12.5 and BAL-19 areas, but also use IronAsh cladding and battens in a timber wall system for BAL-29, BAL-40 and BAL-FZ rated areas as per AS 3959. For additional advice, we recommend that you read the WoodSolutions Technical Guide #04 – Building with Timber in Bushfire-prone Areas as it is a fantastic tool when designing for bushfire.
