Fire resistance

The main factor that influences the performance of a timber species when exposed to bushfire is its density. In general, the higher the density, the slower it will burn. Mineral content in the wood cells is also a factor.

When timber burns, it develops a layer of charcoal on the outside, which helps to insulate the inner core. Timber species with a higher natural resistance to fire are better at slowing down the combustion rate and retaining their strength in the inner core for longer.

Australian Standard AS 3959 Construction of buildings in bushfire-prone areas sets out the recommended building practices for improving the performance of a building during a bushfire attack. It also includes lists of timber species that are most suitable for different parts of the building (cladding, decking, window joinery, bearers, joists, etc) under various Bushfire Attack Levels (BAL).

BAL zones take into account the type of vegetation in the surrounding area, the slope of the land and the distance between the vegetation and the building. The BAL numbers represent the expected ‘heat flux’ of the fire, expressed in kilowatts per square metre (kWm²). Heat flux is also called ‘heat flow rate intensity’, or the rate of heat energy that passes through a surface.

The table below shows a brief summary of the BAL zones described in AS 3959.

Bushfire Attack Level	Description	Building requirements
BAL-LOW	Low risk	No special building requirements
BAL-12.5	Risk of ember attack – heat flux up to 12.5 kWm2	Approved timber species (or timber treated with fire retardants) plus specified construction methods – varying for different parts of the building and different BAL zones
BAL-19	Ember attack and burning debris – heat flux up to 19 kWm2
BAL-29	Ember attack and burning debris – heat flux up to 29 kWm2
BAL-40	As above, possible exposure to fire front – heat flux up to 40 kWm2
BAL-FZ	Extreme risk of ember attack, direct exposure to flames

At the higher BALs, timber components used in exposed situations need to have a higher natural resistance to fire or be treated with a fire retardant. The construction methods also need to comply with stricter Building Code of Australia requirements.

Research conducted by the CSIRO has shown that when timber houses are lost during a bushfire, they tend to burn from the inside out after the embers have found their way into the building. This can occur when embers enter through broken windows, gutters, cracks in roof tiles and other openings, and then start burning furniture and internal materials.

Buildings are much less likely to burn from the outside in, which means that as long as the right construction methods and materials are used, timber can still be incorporated into the overall design even at BAL-FZ.

The timber species that are suitable for use in bushfire prone areas are listed in AS 3959 under three categories:

• ‘Bushfire resisting timbers’ – species that have an inherent resistance to fire (or are treated with a fire retardant)
• Timber species with a density of 750 kg/m³
• Timber species with a density of 650 kg/m³ or greater.

Below are examples of timber species from each of these categories.

Bushfire resisting timbers	Density of 750 kg/m3 plus	Density of 650 kg/m3 plus
blackbutt ironbark (red and grey) spotted gum merbau (kwila) river red gum turpentine silvertop ash	all ‘bushfire resisting timbers’ grey box jarrah Sydney blue gum messmate stringybark	all ‘bushfire resisting timbers’ and 750 kg/m3 + alpine ash mountain ash shining gum white cypress slash pine