BAL Rated Windows and Doors
Reviewed by M. Zeng, M. Eng. September 2026.
Windows and doors are among the most vulnerable parts of a building in a bushfire. Glass can crack under radiant heat, timber frames can ignite, and gaps around openings let embers into wall cavities and roof spaces. Australian Standard AS 3959 sets out what each window and door assembly must withstand at every bushfire attack level, from BAL-LOW through to BAL-FZ. The requirements cover glazing type and thickness, frame materials, protective screens, seals and – at the highest ratings – fire resistance levels for the entire assembly.
How Bushfire Attacks Windows and Doors
A bushfire threatens openings through three mechanisms, and AS 3959 addresses all three with escalating requirements as the BAL rating rises.
Ember attack is the most common cause of house loss in Australian bushfires. Embers travel ahead of the fire front – sometimes kilometres ahead – and enter buildings through gaps around window frames, under doors and through unscreened openings. Once inside a wall cavity or roof space, a single ember can ignite timber framing. Screens with a maximum 2 mm aperture are the primary defence, and they become mandatory at every BAL level above BAL-LOW.
Radiant heat is the invisible thermal radiation from flames and burning material. Standard annealed glass begins to crack at heat flux levels around 10-15 kW/m2, which is why AS 3959 moves from safety glass at BAL-12.5 to toughened glass at BAL-19 and above. Toughened glass withstands roughly four times the thermal stress of annealed glass before failure.
Direct flame contact applies only at BAL-40 and BAL-FZ, where the building may be exposed to the fire front itself. At these levels, the entire window or door assembly – not just the glass – must resist sustained flame and heat. This is where fire resistance levels (FRLs) and full-system testing under AS 1530.8.1 and AS 1530.8.2 become mandatory.
Glazing Requirements at Each BAL Level
The glazing specification changes at every step up the BAL scale – thicker, stronger glass is required at each level, and the type of glass changes fundamentally between BAL-12.5 and BAL-19.
BAL-LOW. No specific glazing requirements. Standard window glass is acceptable.
BAL-12.5. Minimum 4 mm Grade A safety glass to AS 1288, or glass blocks. For insulating glass units (IGUs or double glazing), the outer pane must be minimum 4 mm Grade A safety glass. The inner pane has no additional requirement beyond standard compliance. Grade A safety glass includes both toughened and laminated glass.
BAL-19. Minimum 5 mm toughened glass. This is the first level where toughened glass specifically – not just any safety glass – is required. Annealed glass is still permitted if the entire window is protected by a compliant external screen, but in practice most builders specify toughened glass rather than relying on screening alone. For double-glazed units, the requirement applies to the outer pane only.
BAL-29. Minimum 5 mm toughened glass for windows. Doors require a minimum of 6 mm toughened glass. The distinction matters because doors are assumed to be closer to potential ignition sources at ground level. For double-glazed units, the outer pane carries the requirement.
BAL-40. Minimum 6 mm toughened glass for both windows and doors. This is a step up from BAL-29 for windows, which only required 5 mm. All glazed portions – fixed and openable – must be screened, and the screening material must be corrosion-resistant steel or bronze. Aluminium mesh is no longer permitted at this level.
BAL-FZ. Glazing alone is not sufficient. The entire window or door assembly must either achieve a fire resistance level (FRL) of at least -/30/- (30 minutes integrity), be tested to AS 1530.8.2, or be protected by a compliant bushfire shutter. In practice, BAL-FZ windows are specialist manufactured systems tested and certified as complete assemblies.
Frame Materials
The frame is the structural element that holds the glazing in place. If the frame fails, the glass falls out of the opening regardless of its thickness or type. AS 3959 progressively restricts which frame materials are permitted as the BAL rating increases.
BAL-LOW and BAL-12.5. No restrictions on frame materials. Standard timber, aluminium, PVC-U and composite frames are all acceptable.
BAL-19 and BAL-29. Frames must be one of the following – bushfire-resisting timber (species listed in AS 3959 Appendix F, including blackbutt, spotted gum, ironbark, merbau and turpentine), metal (including aluminium), or metal-reinforced PVC-U where the reinforcing members are aluminium, stainless steel or corrosion-resistant steel. Standard softwood timber frames and unreinforced PVC-U are no longer compliant at these levels.
BAL-40. The standard requires that window and door frames be metal, or tested as part of a complete system to AS 1530.8.1. Bushfire-resisting timber is still technically permitted for some door applications, but in practice most BAL-40 window systems use aluminium or steel frames. The critical change from BAL-29 is that the complete assembly – frame, glass, seals, hardware – must perform as an integrated system. A compliant frame material alone does not make the opening compliant.
BAL-FZ. Non-combustible frames are effectively mandatory. Tested systems from specialist manufacturers are available in steel and aluminium, but these are purpose-built BAL-FZ assemblies, not standard window frames rated upward. Some manufacturers offer tested timber window systems for BAL-FZ, but they are uncommon and typically custom-made.
Screen Requirements
Screens serve two functions in bushfire construction – they block embers from entering through or around openings, and at higher BAL levels they reduce the radiant heat reaching the glass. The material, aperture and coverage requirements all change with the BAL rating.
BAL-LOW. No screening required.
BAL-12.5. Openable portions of windows must be screened with mesh or perforated sheet having a maximum aperture of 2 mm, made from corrosion-resistant steel, bronze or aluminium. Fixed portions of windows do not require screening at this level. This is about ember exclusion – keeping burning material out when windows are left open.
BAL-19. Same screening requirements as BAL-12.5. Openable portions screened, fixed portions not required to be screened. Aluminium mesh remains acceptable.
BAL-29. Openable portions must be screened, plus any glazing within 400 mm of the ground, or within 400 mm above decks, carport roofs, awnings and similar horizontal surfaces. This low-level screening requirement reflects the higher ember density and radiant heat at ground level. The screen frame must be metal or bushfire-resisting timber.
BAL-40. All glazed portions – openable and fixed – must be screened externally. The mesh must be corrosion-resistant steel or bronze with a maximum 2 mm aperture. Aluminium mesh is no longer compliant at this level because aluminium’s melting point (approximately 660 degrees Celsius) is too low to withstand direct flame contact. The screen is now performing a radiant heat reduction function as well as ember exclusion.
BAL-FZ. Where screens are used (as part of a tested system or in combination with bushfire shutters), the mesh must be corrosion-resistant steel or bronze only. Aluminium is explicitly excluded. At this level, however, screens alone rarely satisfy the requirement – most BAL-FZ solutions rely on tested assemblies or bushfire shutters.
Door-Specific Requirements
Doors have distinct requirements from windows because they are functional openings that must seal tightly when closed while still allowing passage. AS 3959 treats side-hung doors, sliding doors and garage doors separately.
Side-hung doors (including French doors, panel-fold and bi-fold doors). From BAL-19 upward, side-hung doors require toughened glass at 6 mm minimum when glazed. At BAL-29, weather strips, draught excluders or draught seals must be installed at the base. Door frames follow the same material restrictions as window frames at each BAL level. At BAL-29, a solid timber, laminated timber or reconstituted timber door with a minimum thickness of 35 mm for the first 400 mm above the threshold is an alternative to glazed construction. Hardware supporting the panel in opening and closing must be metal.
Sliding doors. These have a distinct treatment in AS 3959 because they are assumed to be closed during a bushfire event – occupants are not expected to leave a sliding door open and evacuate through it. At BAL-12.5, sliding doors must comply with AS 1288. From BAL-19 to BAL-29, glazing must be toughened glass (5 mm at BAL-19, 6 mm at BAL-29), and frames must be bushfire-resisting timber, metal or metal-reinforced PVC-U. There is no requirement to screen the openable portion at BAL-29, provided the door is manufactured to prevent ember entry when closed. At BAL-40, sliding doors must either achieve an FRL of at least -/30/- (30 minutes integrity) or be completely screened with steel or bronze mesh. At BAL-FZ, sliding doors must achieve -/30/- or be tested to AS 1530.8.2.
Garage doors. Vehicle access doors have specific requirements from BAL-19 upward. The lower 400 mm of the door (when closed) must be non-combustible, bushfire-resisting timber, or minimum 6 mm fibre-cement sheet. Panel-lift, tilt and side-hung garage doors need weather strips, draught excluders, seals or guide tracks with a maximum gap of 3 mm. Roller doors need guide tracks with a maximum 3 mm gap and a nylon brush in contact with the door. Ventilation slots are not permitted in garage doors from BAL-19 upward.
Bushfire Shutters
A bushfire shutter is a fixed, non-removable external covering that protects the entire window or door assembly. Shutters are a compliance alternative at every BAL level from BAL-12.5, and at BAL-FZ they are often the most practical solution.
To comply with AS 3959, a bushfire shutter must be made from non-combustible material or bushfire-resisting timber, be fixed to the building and non-removable, have no gap greater than 3 mm between the shutter and the wall, sill or head when closed, be manually operable from both inside and outside, and protect the entire window or door assembly. Where perforated, the apertures must be a maximum of 3 mm for radiant heat protection or 2 mm where ember protection is also required.
If bushfire shutters are fitted to all external doors, at least one must be operable from the inside to allow safe egress.
Bushfire-rated roller shutters are the most common shutter type. Costs vary significantly by BAL level, shutter size and supplier – obtain quotes from specialist bushfire shutter manufacturers for your specific openings and BAL rating.
The BAL-29 to BAL-40 Threshold
The transition from BAL-29 to BAL-40 deserves specific attention because it represents the most significant single step change in window and door requirements across the entire BAL scale.
At BAL-29, standard aluminium-framed windows with 5 mm toughened glass and corrosion-resistant screens on the openable portions will generally comply. These are available from most major window manufacturers as part of their standard product range. The glazing and screening can often be specified at the quoting stage and supplied with the window order.
At BAL-40, the requirements change in four ways simultaneously. The glazing thickness increases from 5 mm to 6 mm for windows. Screening becomes mandatory on all portions – fixed and openable – rather than just the openable portions. The screen mesh must be steel or bronze, not aluminium. And the entire assembly must be tested and certified as a complete system, not just assessed component by component.
This means a homeowner at BAL-29 can often work with their regular window supplier. A homeowner at BAL-40 typically needs to source purpose-built, tested window assemblies from specialist manufacturers – and those manufacturers may have longer lead times, fewer design options and significantly higher prices.
For kit home builds, this threshold has a practical consequence. Kit home suppliers typically supply window and door packages as part of the kit, but BAL-40 rated assemblies may need to be sourced separately and installed on site. This can affect the overall build programme and cost.
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Common Mistakes with BAL Rated Windows and Doors
Assuming the frame material alone determines compliance. A window built with a compliant frame material, compliant glass and compliant screens can still fail to meet the standard if the seals, hardware, installation details or overall assembly have not been tested or assessed as a system. This is especially true at BAL-40 and above, where compliance depends on the complete assembly, not individual components. A manufacturer’s BAL rating applies to specific products in specific configurations – not to every product in their range.
Using aluminium mesh at BAL-40 or BAL-FZ. Aluminium mesh is compliant up to and including BAL-29, but aluminium’s relatively low melting point means it is not permitted at BAL-40 or BAL-FZ. Corrosion-resistant steel or bronze mesh is required. This is one of the most commonly missed details when owners or builders source their own screens.
Ignoring the 400 mm ground-level rule. At BAL-29 and above, any glazing within 400 mm of the ground or within 400 mm above adjacent horizontal surfaces must be screened. This catches low-set windows, sidelights beside front doors, and glazing adjacent to decks and verandahs. It is often missed in the initial design because the architect or designer draws the elevation without accounting for the finished ground level or deck height.
Specifying sliding doors without checking the BAL-40 FRL requirement. Sliding doors have no screening requirement for the openable portion at BAL-29 because they are assumed to be closed during a bushfire. At BAL-40, however, sliding doors must either achieve an FRL of -/30/- or be completely screened. This is a different compliance pathway from side-hung doors and catches owners who assume sliding and hinged doors share the same requirements.
Overlooking garage doors. Garage door requirements are often treated as an afterthought, but from BAL-19 upward the standard requires non-combustible or bushfire-resistant material in the lower 400 mm, specific sealing, and no ventilation slots. A standard sectional garage door may not comply without modification.
Treating double glazing as double the protection. In a double-glazed unit, AS 3959 only requires the outer pane to meet the bushfire glazing specification. The inner pane follows standard glazing requirements. Double glazing provides thermal and acoustic benefits, but it does not reduce the bushfire glazing requirement – the outer pane must still be the specified thickness and type.
Worked Example
A homeowner is building a three-bedroom kit home on a site assessed at BAL-29 in regional Victoria. The house has a standard layout with fourteen glazed openings – ten windows and four doors (two side-hung entry doors, one set of sliding doors and one garage door).
Windows (ten openings). Each window requires minimum 5 mm toughened glass. Openable portions must be screened with corrosion-resistant mesh (steel, bronze or aluminium, maximum 2 mm aperture). Two windows have sills within 400 mm of the finished ground level, so those also need external screening on the lower fixed portion. Frames must be bushfire-resisting timber, metal or metal-reinforced PVC-U. Standard pine frames are not compliant.
Side-hung entry doors (two openings). Glazed panels require minimum 6 mm toughened glass. Door frames must be bushfire-resisting timber or metal. Weather strips and draught seals are mandatory at the base. Any glazing within 400 mm of the finished floor level must be screened.
Sliding doors (one set). Glazing must be minimum 6 mm toughened glass. Frames must be bushfire-resisting timber, metal or metal-reinforced PVC-U. No screening is required on the openable portion at BAL-29, provided the door is tight-fitting in the frame and manufactured to prevent ember entry when closed.
Garage door (one opening). The lower 400 mm must be non-combustible, bushfire-resisting timber or minimum 6 mm fibre-cement sheet. Guide tracks or seals must limit gaps to 3 mm maximum. No ventilation slots are permitted.
Cost implications. The cost of bushfire-rated windows and doors varies by BAL level, product, supplier and region. At BAL-29, the glazing, screening and frame upgrades across fourteen openings represent a measurable uplift over standard construction. If the same house were assessed at BAL-40, the requirement for tested assemblies, steel or bronze screening on all portions, and potentially FRL-rated sliding doors would increase that uplift significantly. Obtain quotes from window and door suppliers experienced in your BAL level before finalising the budget.
Frequently Asked Questions
Can I use standard window frames at BAL-29?
Standard softwood timber frames (pine, oregon) are not compliant from BAL-19 upward. You can use aluminium, steel, metal-reinforced PVC-U, or specific hardwood species listed in AS 3959 Appendix F – including blackbutt, spotted gum, ironbark, merbau, silvertop ash and turpentine. Check with your frame supplier that the specific product is rated for your BAL level.
Do I need to screen every window?
It depends on the BAL level. At BAL-12.5 to BAL-29, only the openable portions need screening (plus any glazing within 400 mm of ground level at BAL-29). At BAL-40, all glazed portions – fixed and openable – must be screened. At BAL-FZ, screening alone is usually insufficient and the assembly needs fire resistance testing or bushfire shutters.
Are bushfire shutters required?
Bushfire shutters are never mandatory at any BAL level. They are one of several compliance pathways. However, at BAL-FZ they are often the most practical option because the alternative – a window assembly tested to AS 1530.8.2 – involves specialist products with limited availability and higher cost.
Does double glazing help with bushfire protection?
Double glazing does not reduce the bushfire glazing requirements. The outer pane must still meet the specified minimum thickness and type for the BAL level. However, the inner pane can be standard glass, and the insulating air gap may provide some additional thermal resistance in practice. Some manufacturers offer double-glazed BAL-rated units that combine bushfire compliance on the outer pane with energy efficiency through the inner pane and air gap.
Can I upgrade individual windows without replacing the whole house?
If you are adding or replacing windows on an existing house in a bushfire-prone area, the new or altered work must comply with the current BAL rating for the site. Existing windows that are not being altered are generally not required to be upgraded, though this varies by state and local authority. Check with your local council or building certifier.
What is the difference between Grade A safety glass and toughened glass?
Grade A safety glass is a classification under AS 1288 that includes both toughened glass and laminated glass. Toughened glass is a specific type of safety glass that has been heat-treated to be approximately four times stronger than annealed glass of the same thickness, and it breaks into small granular pieces rather than sharp shards. From BAL-19 upward, AS 3959 specifically requires toughened glass – laminated glass alone (even though it is Grade A) does not satisfy the requirement unless the window is protected by a compliant bushfire shutter or screen.
Do the same requirements apply to skylights and roof windows?
Glazed roof penetrations have their own requirements under AS 3959. At BAL-12.5, roof glazing must be Grade A laminated safety glass or polymer with a Grade A safety glass diffuser installed underneath. At BAL-29, glazed roof penetrations in roofs with a pitch of less than 18 degrees must also be protected by ember guards. At BAL-40 and BAL-FZ, roof glazing must achieve specific fire resistance levels. The requirements vary from wall-mounted windows, so check the relevant section of the standard.
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Glossary
Annealed glass. Standard float glass that has not been heat-treated. It breaks into large sharp shards and has the lowest resistance to thermal stress.
AS 1288. Australian Standard for glass in buildings, covering selection and installation including safety glass classifications.
AS 1530.8.1. Test method for determining the resistance of building elements to bushfire attack – for BAL levels up to and including BAL-40.
AS 1530.8.2. Test method for determining the resistance of building elements to bushfire attack in the flame zone – for BAL-FZ.
AS 3959. Australian Standard for construction of buildings in bushfire-prone areas. The current edition is AS 3959:2018.
BAL. Bushfire attack level. The measure of a building’s potential exposure to ember attack, radiant heat and direct flame contact, as determined by AS 3959.
Bushfire-resisting timber. Timber species tested and listed in AS 3959 Appendix F as suitable for use in bushfire construction. Includes blackbutt, spotted gum, red ironbark, merbau (kwila), turpentine, silvertop ash and red river gum.
Bushfire shutter. A fixed, non-removable external protective covering for a window or door assembly, made from non-combustible material or bushfire-resisting timber, operable from both inside and outside.
FRL (fire resistance level). A grading period in minutes for structural adequacy, integrity and insulation, expressed in the format -/30/- (no structural requirement, 30 minutes integrity, no insulation requirement). Relevant to window and door assemblies at BAL-40 and BAL-FZ.
Grade A safety glass. A classification under AS 1288 that includes both toughened and laminated glass meeting specific impact requirements.
IGU (insulating glass unit). A double- or triple-glazed unit consisting of two or more panes of glass separated by a sealed air or gas-filled gap. In bushfire construction, the outer pane must meet the BAL glazing requirement.
Toughened glass. Glass that has been heated to approximately 620 degrees Celsius and then rapidly cooled, creating internal stresses that make it approximately four times stronger than annealed glass. When it breaks, it fractures into small blunt granules rather than sharp shards.
Sources
- Standards Australia, AS 3959:2018 Construction of buildings in bushfire-prone areas
- Standards Australia, AS 1288 Glass in buildings – Selection and installation
- Standards Australia, AS 1530.8.1 and AS 1530.8.2 Methods of test for fire properties of building materials and structures
- Hume Doors, Bushfire Attack Level BAL Specifications, Version 1.0
- James Hardie, Construction of buildings in bushfire prone areas to AS 3959:2018, Technical Supplement May 2023
- My Building Certifier, BAL-29 Summary of Construction Standards, August 2014
Reviewed by M. Zeng, M. Eng. September 2026.
This article is general guidance only. Requirements under AS 3959 can vary by state and territory, and are subject to local planning overlays and conditions of development consent. Always confirm the specific requirements for your site with your building certifier or relevant fire authority before specifying windows and doors.