Bushfire Attack Level, and what it changes in the build

Bushfire Attack Level, and what it changes in the build

A lot of Sydney sits against bushland, and any development on land mapped as bushfire prone has to establish its Bushfire Attack Level before the design proceeds.

The rating describes the site, based on the vegetation around it, the slope beneath that vegetation and the distance from it. The construction requirements follow from the rating.

The levels, and what drives them

The assessment produces a level running from the lowest, where the risk is primarily from embers, through increasing levels of radiant heat exposure, to the highest, where direct flame contact is anticipated.

Three inputs decide it. The classification of the vegetation, because forest and grassland behave very differently. The effective slope under that vegetation, since fire runs faster uphill and the radiant heat at the top is higher. And the separation distance between the vegetation and the building.

That last one is the lever. Increasing the separation reduces the level, which is why the siting of the building on the block frequently decides the cost of the construction.

What changes as the level rises

At the lower levels the requirements are largely about embers. Sealing gaps, screening openings and vents, protecting the subfloor and the roof space, and using materials that will not ignite from an ember landing on them.

That matters because ember attack destroys far more houses than flame front contact does. Embers travel well ahead of a fire, find a gap, and get inside.

As the level rises, radiant heat becomes the design case. Glazing has to be protected or upgraded, shutters may be required, external cladding and framing requirements tighten, and decks and their supporting structure come under the requirements.

At the highest level, the requirements approach those for a structure expected to be in direct contact with flame, and the cost difference against the lowest level is substantial.

The structural consequences

Where a deck, a verandah or a carport falls within the requirements, the framing and the substructure change. Timber species have to meet fire resisting requirements or be replaced with steel or non-combustible alternatives.

Subfloor enclosure changes how the ground floor is built and how it is ventilated, which interacts with termite management and with moisture.

Roof construction changes, with sarking, sealed junctions and protected openings, and on a roof with complex geometry those junctions multiply.

The siting decision comes first

The most effective way to reduce the construction cost is to change where the building sits, or to alter what is between it and the vegetation.

That is a design decision made at the concept stage. Once a floor plan is fixed and a development application is lodged, moving the building means starting the design again.

An asset protection zone, the managed area around the building, is often a condition of consent, and it has to be achievable and maintainable on the actual block.

It interacts with everything else

A bushfire prone site is frequently also a sloping site, a site with reactive soil, or one with vegetation subject to a tree preservation order.

Those constraints pull against each other. Increasing separation from vegetation may require removing trees that are protected, or moving the building to a steeper part of the block that costs more in footings and retaining.

Resolving that is a design exercise, and it is far cheaper at concept than at construction certificate stage.

The order

Get the site assessed and the level established. Site the building against it. Then design, with the construction requirements known.

We work alongside bushfire assessments on constrained sites through structural engineering, because the structural implications of the rating land in the framing, the substructure and the footings.