We had a client call last year, property in the lower Blue Mountains, BAL-40 rated. He’d already picked out his composite boards. Done the research, knew they were fire-resistant, felt sorted. He wanted a quote to put them over his existing pine frame.
We had to explain that the boards were the least of his problems.
It’s a common misread. People focus on what they can see and touch (the decking surface) and assume that’s where the fire safety lives. It isn’t. Under the National Construction Code and AS 3959, from BAL-29 onwards, the structural frame of a deck must be non-combustible. Treated timber, regardless of how it’s been treated, doesn’t make that cut.
Which Parts of Sydney This Actually Affects
More of it than most people realise. The Blue Mountains corridor is the obvious one: Springwood, Blaxland, Glenbrook, Wentworth Falls, Leura, Katoomba. Properties there, especially anything on an escarpment edge or backing onto reserve, are routinely coming in at BAL-40 or BAL-FZ. We’ve had BAL-FZ jobs in Katoomba where we’re talking direct flame contact in a worst-case scenario. The requirements there are serious.
But it’s not just the Mountains. Hornsby Shire, Ku-ring-gai, the Hills District. Dural, Galston, Berowra, Terrey Hills, Ingleside. BAL-29 is common in those suburbs, and BAL-40 isn’t unusual. Large chunks of the Northern Beaches too. Sydney’s bushfire prone footprint is genuinely big, and a lot of homeowners only find out where they sit when they go to build something and council comes back with requirements they weren’t expecting.
What the Ratings Actually Mean for Your Frame
BAL-29: non-combustible framing required for exposed structural elements: joists, bearers, posts. BAL-40: non-combustible across the board, no exceptions. BAL-FZ: all of that, plus the structure needs to achieve a minimum Fire Resistance Level of 30/30/30.
Steel is non-combustible by definition. Galvanised, powder-coated steel framing doesn’t contribute to fire spread, won’t ignite from ember attack, won’t fail the way a burning timber frame does. It satisfies those requirements without needing workarounds or special treatment.
Why the Frame Is the Critical Part
Bushfire doesn’t just arrive as a wall of flame. Ember attack is responsible for a large share of residential losses in major fire events, with embers travelling kilometres ahead of the fire front, landing under decks, accumulating in gaps, finding dry organic material to work with.
A timber frame under a deck is exactly what they’re looking for. Enclosed. Sheltered. Full of material that will catch and hold a flame. When that frame ignites, you get sustained fire underneath the structure, which then starts working on the house itself: fascias, eave linings, windows. That’s the sequence AS 3959 is designed to stop.
A steel frame gives embers nothing to work with. They land, they don’t find a fuel source, the sequence doesn’t start.
I’m not overstating this. It’s why the NCC requirements exist, and it’s why we won’t frame a deck in timber in a BAL-29 or higher zone. Not because the rules say not to. Because the rules are right.
What Treated Timber Actually Does in a Fire
Timber char-burns. The charring gives some short-term protection to the core, slows the burn a bit. But sustained heat (the kind you get in an actual bushfire scenario) will get through. The chemical treatment that protects against rot and termites does essentially nothing for fire resistance. Those are separate problems with separate solutions, and H3 or H4 timber has only addressed one of them.
Steel under sustained heat will eventually lose structural integrity too. Nothing’s invincible. But it lasts significantly longer than a burning timber frame, and crucially, it doesn’t contribute fuel to the fire in the process.
What a Properly Built Bushfire Zone Deck Involves
Galvanised structural steel frame, engineered for the span and load. Boards that carry the relevant BAL certification: composite products for most BAL-29 and BAL-40 applications, fibre cement at BAL-FZ. Fasteners, board gaps, balustrades, any enclosed sections under the deck. All need to meet the spec for the specific rating.
Council approval requires all of it to stack up. An inspector who finds a timber frame in a BAL-40 zone will not let it through. And in the event of a real fire, a deck that doesn’t meet its rated specification isn’t just a compliance problem.
If you’re building or replacing a deck in a bushfire prone area around Sydney, take a look at our bushfire zone decking solutions in Sydney. Happy to talk through what your block actually needs.
