In Australia,
building with timber in bushfire-prone areas is governed by the Australian Standard AS 3959. This standard defines Bushfire Attack Levels (BAL), which determine what types of timber can be used based on the site’s potential exposure to embers, radiant heat, and direct flame.
Understanding these ratings is the first step in ensuring a home is both beautiful and resilient against the unique Australian climate.
What is a BAL Rating?
A BAL rating measures the severity of a building’s potential exposure to a bushfire. It is calculated based on the region’s Fire Danger Index (FDI), the slope of the land, and the proximity and type of surrounding vegetation.
| BAL Rating | Risk Level | Radiant Heat Flux (kW/m2) | Timber Suitability |
| BAL–LOW | Very Low | $\leq 12.5$ | No special requirements; any timber is allowed. |
| BAL–12.5 | Low | $12.5$ | Ember protection required. High-density hardwoods are common. |
| BAL–19 | Moderate | $12.5 – 19$ | Increasing ember risk. Specific density requirements for timber. |
| BAL–29 | High | $19 – 29$ | Significant radiant heat. Only “Bushfire Resisting Timbers” allowed. |
| BAL–40 | Very High | $29 – 40$ | High risk of flame contact. Timber use is strictly limited. |
| BAL–FZ | Extreme | $> 40$ | Direct flame zone. Requires specialized fire-rated systems. |
1. The “Big Seven”: Bushfire Resisting Timbers (BRT)
These species are the “gold standard.” They have been tested and are deemed compliant for use in applications up to BAL–29 without any additional fire-retardant treatment.
| Species Name | Scientific Name | Common Uses |
| Blackbutt | Eucalyptus pilularis | Decking, cladding, flooring |
| Spotted Gum | Corymbia maculata | Heavy engineering, decking, cladding |
| Red Ironbark | Eucalyptus sideroxylon | Posts, poles, decking |
| Silvertop Ash | Eucalyptus sieberi | Cladding, flooring, joinery |
| River Red Gum | Eucalyptus camaldulensis | Fencing, landscaping, furniture |
| Turpentine | Syncarpia glomulifera | Cladding, decking, marine piles |
| Merbau (Kwila) | Intsia bijuga | Decking, screening, outdoor furniture |
Timber Selection and Compliance
For most timber applications in bushfire zones, the standard categorizes wood into three main groups based on their natural fire resistance.
1. Bushfire Resisting Timbers (BRT)
These are specific species that have been tested and proven to withstand higher heat flux (up to BAL–29) without additional treatment.
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Common Species: Spotted Gum, Blackbutt, Red Ironbark, Silvertop Ash, Merbau (Kwila), River Red Gum, and Turpentine.
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Best For: External cladding, decking, and window frames in high-risk zones.
2. High-Density Hardwoods
Timbers with a natural density of 10$750\text{ kg/m}^3$ or greater are generally permitted for use in BAL–12.5 and BAL–19 zones for general construction.11 For window and door joinery, the density requirement is slightly lower at $650\text{ kg/m}^3$.
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Species: Jarrah, Karri, Sydney Blue Gum, and Brush Box.
3. Treated or Protected Timber
If you wish to use a timber that isn’t naturally fire-resistant (like Radiata Pine or Western Red Cedar) in a higher BAL zone, it must be:
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Fire-Retardant Treated (FRT): Impregnated with chemicals to reduce flammability.
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Protected by a System: Used as part of a fire-rated wall system (e.g., placing fire-grade plasterboard behind the timber cladding).
Critical Construction Rules
Building with timber isn’t just about the species; it’s about where and how it’s installed.
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The 400mm Rule: In BAL–12.5 to BAL–29 zones, any timber used within 400mm of the ground or an external horizontal surface (like a deck) must be a Bushfire Resisting Timber or non-combustible.15 This prevents ground-level embers from igniting the walls.
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Subfloor Protection: For elevated houses, if the subfloor is “unenclosed,” the bearers and joists must be fire-rated or non-combustible if they are within 400mm of the ground.
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Decking Gaps: Standard AS 3959 requires gaps between decking boards to be managed to prevent ember entrapment, often necessitating a maximum gap of 3mm or the use of non-combustible materials beneath the deck.
Summary of Use Cases
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Decking: Use Spotted Gum or Merbau for a natural look that meets BAL–29 requirements.
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Cladding: If you are in BAL–40, you will likely need to move away from standard timber cladding unless you use a certified “Flame Zone” wall system.
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Windows/Doors: Metal frames are common, but “Bushfire Resisting” timber frames are permitted up to BAL–29.
Note: Always consult a BAL Assessor or a building certifier before finalizing your material list. Requirements can vary slightly between states (e.g., NSW and Victoria have specific variations to the national code).
Pro Tip: Timber density is measured at 12% moisture content. If you are buying “Green” (unseasoned) timber, its density will be higher, but you must ensure it meets the requirements once it has dried to its service equilibrium.