Building in bushfire-prone areas: BAL ratings explained, and the details that keep embers out

With spring days away and the bush fire danger period approaching across much of NSW, it is a good time for builders working on the urban fringe, regional areas and the bush to revisit how bushfire construction works. A large share of new housing in NSW sits on or near bushfire-prone land, from the edges of Sydney’s growth areas to coastal and regional towns.

The key concept is the bushfire attack level, or BAL. It sets how much a house must resist, and it drives the specification of almost everything on the outside of the building. This guide explains the levels, how a BAL is assessed, what changes in construction, and the ember-protection details that often decide whether a house survives.

How houses are lost in bushfires

Research into house losses has consistently found that most houses are ignited by embers rather than by direct flame contact from the fire front. Embers can be carried long distances ahead of a fire, and they find their way into small gaps, vents, gutters full of leaves, and under decks. A house can survive the passing fire front and then burn down hours later from an ember that lodged in a gap and smouldered.

That is why bushfire construction starts with keeping embers out, at every level, before it deals with radiant heat and flame.

The bushfire attack levels

The Australian Standard for construction of buildings in bushfire-prone areas, AS 3959, defines six bushfire attack levels, based on the expected exposure of the building to embers, radiant heat and flame.

The numbers in BAL-12.5, BAL-19, BAL-29 and BAL-40 refer to the radiant heat exposure the level is based on. Each step up adds requirements to the previous level. BAL-FZ, the flame zone, means the building may be in direct contact with flames and requires the most stringent construction.

How a BAL is assessed

Councils map bush fire prone land. If a site is on the map, a bushfire assessment is needed to determine the BAL. The assessment considers the type of vegetation near the site, its distance from the proposed building, and the slope of the land under the vegetation, along with the fire danger for the region. The BAL can vary around a building; the building is generally designed to the highest level that applies, although some elements facing away from the hazard may be treated differently where the standard allows.

In NSW, development on bush fire prone land must also comply with Planning for Bush Fire Protection, published by the NSW Rural Fire Service, which covers matters beyond the building itself, such as asset protection zones, access for fire fighters and water supply. Some developments require a bush fire safety authority or referral to the RFS.

What changes in construction

As the BAL increases, requirements tighten for:

• Windows and glazed doors: glass type and thickness, frame materials, screens and shutters.
• External doors: door construction, frames, seals and screens.
• External walls: cladding materials and how junctions and gaps are sealed.
• Roofs: roofing materials, sarking, sealing of gaps at ridges, valleys and edges, and gutters.
• Vents and weepholes: ember-proof screening.
• Subfloors: enclosure or protection of the underfloor space on elevated homes.
• Decks, verandahs and steps: materials and their separation from the house.
• Service penetrations: sealing and protection of pipes, evaporative coolers and other roof equipment.

At the higher levels, particularly BAL-40 and BAL-FZ, requirements become demanding enough that tested systems and specialist design are common. Windows are often the most expensive single element of building to a high BAL.

Ember protection: the details that matter

These are small details, often left to trades who may not know the BAL. A weephole without a screen, a gap under a roller door, a vent without mesh, a timber deck butting against the wall: each can let embers in or give them a place to ignite. A good bushfire build is one where every trade knows the BAL and what it means for their work.

Materials at higher levels

As the BAL rises, material choices narrow. Non-combustible cladding such as masonry, fibre cement and metal is commonly used on exposed elevations at higher levels, while timber cladding and decking can be used at lower levels subject to the standard’s requirements for the species and treatment. Roofing, sarking and the sealing of roof junctions become more tightly specified. Glazing moves from standard glass to toughened glass of specified thickness, and at higher levels to tested window systems or bushfire shutters. Decks close to the house need non-combustible or suitably rated materials, and the space under decks must be managed.

The practical point for builders is that the BAL should be on every product order and every trade’s scope. A cladding installer, window supplier or roofer who does not know the BAL cannot be expected to supply the right product.

Bushfire and energy efficiency together

Bushfire requirements and energy requirements pull on the same elements: windows, cladding, roofs and vents. A window that meets the BAL must also meet the energy assessment’s performance values; a vent that is screened for embers must still provide the ventilation the design needs. Coordinate the bushfire assessment, the energy assessment and the design early so the chosen products satisfy both, rather than discovering a conflict when ordering.

Photographs and records

Many bushfire details are hidden once the house is finished: sarking, sealed junctions, ember screens behind cladding, subfloor enclosures. Photograph them before they are covered, and keep product documentation showing BAL ratings. If the house is ever damaged in a fire, or the certifier or an insurer asks how it was built, those records matter.

Renovations and additions

Alterations and additions to existing homes on bush fire prone land generally need to meet bushfire construction requirements for the new work, and the interface between old and new needs attention. An extension built to BAL-29 attached to an older house with open eaves and unscreened vents does not make the whole house resilient. Renovation builders can add value by recommending upgrades to the existing house, such as screening vents, sealing gaps and fitting gutter guards where suitable.

Water, access and services

Planning for Bush Fire Protection also deals with the things around the house that help it survive and help fire fighters defend it: access for fire trucks, a static water supply in some areas, and protection of gas and electricity services. These are often development consent conditions, and they need to be built and in place before the occupation certificate.

Landscaping and maintenance

The house is only part of the picture. Vegetation near the house, stored firewood, timber fences and garden mulch against the walls all add fuel. Asset protection zones, where required, need to be established and maintained. Builders should make sure the handover pack explains the BAL, the features that protect the house, and the owner’s role in maintaining gutters, screens and the surroundings.

A worked example: the BAL-29 home on the fringe

A builder is building a house on the edge of a regional town, backing onto bushland. The bushfire assessment determines BAL-29. The designer specifies compliant windows and doors, non-combustible cladding on the elevation facing the bush, sarking, sealed roof junctions, ember-proof screens on vents and weepholes, and a steel-framed deck with non-combustible decking near the house.

During construction, the window supplier offers a faster alternative that is not rated for BAL-29. The supervisor checks the rating and declines. The roofer is briefed on sealing the ridge and valley junctions, and the bricklayer on screening the weepholes. The builder photographs the ember-protection details before they are covered and includes them in the handover pack with a maintenance guide. Two summers later, a grassfire comes close; the house is untouched. Good bushfire construction is invisible until the day it matters.

Costs and client conversations

Building to a higher BAL costs more, mainly in windows, doors, cladding and roofing details. Clients sometimes react by asking whether the BAL can be reduced. The BAL is a matter of assessment, not negotiation, although design choices, such as moving the house further from vegetation or establishing an asset protection zone where permitted, can sometimes lower it. Raise the BAL and its cost early, before the client falls in love with a design that is expensive to build at their level.

The bottom line

On bushfire-prone land, the BAL decides what the outside of the house must resist. Get the assessment early, design to the level, keep embers out with careful attention to gaps, vents, gutters and decks, check every substitution against the BAL, and brief every trade. The details are small; the consequences are not.

Questions builders ask

• Bushfire
• Climate resilience
• NSW
• Windows and glazing

Published 26 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.