Bushfire Attack Levels (BAL) — AS 3959
Bushfire Attack Levels (BAL) — AS 3959
In a bushfire-prone area the BAL sets how the building has to be built. Six levels, set by radiant heat — and the jump to Flame Zone changes everything.
BAL levels (radiant heat exposure)
| BAL-LOW | Insufficient risk — no specific construction requirements |
| BAL-12.5 | Up to 12.5 kW/m² — ember attack |
| BAL-19 | Over 12.5 to 19 kW/m² — embers + radiant heat |
| BAL-29 | Over 19 to 29 kW/m² — increasing radiant heat |
| BAL-40 | Over 29 to 40 kW/m² — radiant heat + possible flame contact |
| BAL-FZ (Flame Zone) | Over 40 kW/m² — direct flame contact |
How the BAL is set
- Method 1 (simplified): vegetation type + distance within 150 m + slope, read off the AS 3959 tables for your Fire Danger Index region
- Method 2 (detailed): site-specific radiant heat flux calculation
- Assessed in every direction — the highest result is the site BAL
AS 3959 construction sections
| BAL-12.5 / 19 / 40 | Sections 5 / 6 / 8 |
| BAL-29 | Section 7 |
| BAL-FZ | Section 9 |
BAL-FZ — the big step up
- External walls: non-combustible, a system tested to AS 1530.8.2, or FRL 30/30/30 from the outside
- Subfloors and decks entirely non-combustible; openings/gaps sealed to 3 mm or less
Reference only — a BAL assessment for a specific site should be done by a qualified assessor. AS 3959:2018 is the NCC Deemed-to-Satisfy bushfire construction standard.
Sources: AS 3959:2018 Construction in bushfire-prone areas · AS 1530.8.1 / AS 1530.8.2 · ABCB / National Construction Code
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