Know your Bushfire Attack Levels AS3959

You may remember when AS 3959 – 2009 Construction of buildings in bushfire-prone areas (AS3959) was released.  We received a lot of queries from customers regarding the use of timber under the standard.  There are still a number of our customers who are unaware of, or confused by the standard. We’ve covered some basics.

What are BAL levels?

The AS 3959 has levels of risk of bushfire attack.  These levels are referred to as  Bushfire Attack Levels (BAL).  A BAL is a means of measuring the severity of a building’s potential exposure to a bush fire.  The ratings take into consideration dwelling location including nearby vegetation classifications, the distance from these and topography.   Some councils and shires in alpine or heavily forested areas use geo information systems to help make BAL assessments.  The BAL levels are:

  • BAL 12.5 – risk of ember attack.
  • BAL 19 – increased level of ember attack and burning debris, increasing heat flux exposure.
  • BAL 29 – as per BAL19 but with variation to vegetation classification and/or heat flux exposure.
  • BAL 40 – as per BAL29 and with the increased likelihood of exposure to flames.
  • BAL FZ – direct exposure to flames from fire front in addition to hear flux and ember attack.

As the BAL number increases the severity of bushfire attack from embers, radiant heat and direct flame contact also increases.  Severity of a bushfire fire or BAL is influenced by the Forest Fire Danger Index, vegetation type, slope of land under the vegetation and distance the building is away from the vegetation.

What timber can I use?

AS3959 classifies a number of timber species by density and moisture content.  Categories include E1, E2 and Bushfire Resisting Timber (BRT).  Barwon Timber has species that fit all of these categories.   Our Silvertop Ash, Spotted Gum, Red Ironbark, Merbau and Blackbutt are BRT species.

Red Ironbark cladding can achieve a BAL29 rating when applied as per AS3959.

How do I work with BAL levels and timber classifications?

The standard uses the BAL as the basis for establishing the requirements for construction to improve protection of building elements from attack by bush fire.  Timber construction can still be applied to areas with a BAL rating – right up to a BAL FZ.   It’s important to use a suitable timber specie in the right application.  Barwon Timber has a copy of AS3959 and its amendments, we also have copies of guides for the standard including the Wood Solutions Guide – Building with Timber in Bushfire-prone Areas.  This guide is also available online and is easy to read with a lot of diagrams.  The guide also shows diagrams of timber cladding building systems that are BAL-FZ compliant.

If you have any queries regarding the AS3959, please contact our timber office on 03 5229 0080.  We can point you in the right direction toward technical guides and information and have a range of timber available to suit AS3959 design applications.

All information in this article is a guide only and assumed to be correct at time of publishing.  Barwon Timber strongly advises readers check all information against current Australian Standards and guidelines.

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