Decking and Subfloor Materials for Bushfire Prone Zones

What This Requirement Covers

This requirement covers the selection, design and installation of decking and subfloor materials where buildings (or attached decks) are located in designated bushfire prone areas in Australia. Its purpose is to reduce the risk of ignition and structural failure during a design bushfire, limit ember penetration and flaming contact, and maintain occupant tenability and safe egress. The rules apply to the construction and detailing of decks, subfloor ventilation and clearances, material durability and protection, and where relevant, fire-resisting construction of elements adjacent to a building envelope exposed to bushfire attack.

Who this applies to: builders, certifiers, designers, and homeowners involved with Class 1 and 10a buildings (housing and associated decks) and the Class 2-9 buildings that are captured by NCC Volume One bushfire provisions. Where State or Territory schedules modify the NCC, the applicable state schedule also applies.

Key Requirements

1. Design bushfire intensity and application - Buildings in designated bushfire prone areas must be designed to resist ignition from a design bushfire with the specified annual exceedance probability for that building class, e.g., Class 1 and Class 10a decks: 1:50 years in many NCC provisions for Volume Two, while certain Class 9 buildings use more stringent probabilities; see NCC Volume Two, Section H7P5 and NCC Volume One Part G5 for higher importance buildings. (See state schedules for variations.)
2. Material selection for decking
3. Use non-combustible materials or appropriately treated/limited-combustibility materials where required by the relevant bushfire attack level and NCC provisions. Specify materials in accordance with NCC and AS references where applicable (for example, use of durable timber in accordance with AS 1684 for framing and treated timbers to the specified preservative levels).
4. Where combustible decking is used adjacent to a building that must resist bushfire attack, designers should reference AS 3959 for construction requirements relating to external walls and ember protection and NCC G5/H7 provisions for the applicable design bushfire intensity.
5. Subfloor framing and durability
6. For subfloor framing above ground use above-ground durability Class 1 or 2 timbers or H3 preservative treated timbers in accordance with AS 1684.2/AS 1684.3/AS 1684.4, unless alternative measures are provided (NCC Volume One, F1D8 and related notes).
7. Where framing is in ground, use in-ground durability Class 1 or 2 timbers or H5 preservative treated timbers, in accordance with AS 1684.2/AS 1684.3/AS 1684.4 (NCC Volume One, F1D8).
8. Steel framing must comply with NASH Standard ‘Residential and Low-Rise Steel Framing’ Part 2 (as referenced in NCC Volume One, F1D8).
9. Subfloor ventilation, ground sealing and clearance
10. Minimum aggregate subfloor ventilation openings (mm2 per m of wall) depend on climatic zone as set out in Table F1D8 of NCC Volume One. If the ground within the subfloor is sealed with an impervious membrane, required ventilation area is reduced per the same table (see Table F1D8 - Subfloor openings and ground clearance; NCC Volume One, F1D8).
11. Minimum ground clearance where termite inspection or management is not required: 400 mm (note exceptions for sloping sites where this may be reduced to 150 mm within 2 m of external walls) - see Table F1D8 notes (NCC Volume One, F1D8).
12. Where openings cannot be provided, additional measures must be adopted to maintain ventilation or durability such as sealed membrane or specified durability timbers (NCC Volume One, F1D8(3)-(4)).
13. Ember protection and gaps
14. Close or protect gaps and junctions where embers could enter the subfloor or under-deck spaces. Use non-combustible or ember-resistant closures and screens consistent with NCC bushfire provisions and AS 3959 recommendations for ember protection of external openings.
15. Linkages to Australian Standards and clauses
16. NCC Volume Two, Section H7P5 - Buildings in bushfire prone areas (applies to Class 1 and Class 10a decks associated with Class 1 buildings).
17. NCC Volume One, Part G5 - Construction in bushfire prone areas (applies to certain Class 2-9 buildings and notes on higher importance buildings).
18. NCC Volume One, F1D8 - Subfloor openings and ground clearance requirements (ventilation, clearance, durability options).
19. AS 1684 series - Timber framing and durability classes for subfloor framing and deck framing.
20. AS 3959 - Construction of buildings in bushfire-prone areas - for detailing external walls, ember protection and construction to required BAL (bushfire attack level).
21. NASH Standard - Residential and Low-Rise Steel Framing Part 2 - where steel framing is used for subfloor members.

Residential vs Commercial

1. Residential (Class 1 and Class 10a decks):
2. Subject to NCC Volume Two bushfire provisions (H7P5) which require design for the design bushfire with an annual exceedance probability commonly 1:50 years for Class 1 and Class 10a associated decks (check the relevant state schedule for variations).
3. Decks attached to Class 1 buildings that are within designated bushfire prone areas must be detailed to reduce ember entry under deck and to use appropriate materials or protective measures (NCC Volume Two, H7P5; AS 3959 for BAL-derived detailing).
4. Commercial / higher importance (Class 2-9):
5. NCC Volume One Part G5 applies. Several Class 9 and other higher importance buildings have more stringent design bushfire annual exceedance probabilities (for example, some importance levels specify 1:200 or 1:500 design AEPs depending on importance level and analysis method) and require more rigorous prevention of internal ignition and maintenance of structural integrity for longer durations (NCC Volume One, G5P1 and G5P2).
6. Decks or external elevated platforms associated with Class 2-9 buildings that must meet Part G5 will often require non-combustible construction, ember barriers and more robust detailing in line with AS 3959 and state-specific schedules.

Exceptions and Exemptions

1. Smaller, detached Class 10b structures or minor elements not captured by the definition of buildings in a state schedule may be exempt, but confirm with the local jurisdiction. State or territory schedules to the NCC may list additional exemptions or modified application rules - always check the relevant schedule.
2. Where subfloor ventilation cannot be provided, the NCC permits alternative compliance by increasing ventilation by 50%, sealing the ground in the subfloor space with an impervious membrane, or specifying higher durability timbers/steel (NCC Volume One, F1D8 clauses and notes). These are considered alternative solutions rather than blanket exemptions.
3. Alternative solutions such as use of non-combustible decking materials, ember-proof under-deck enclosures, or fuel-reduced landscaping may be accepted by a certifier where they provide an equivalent level of safety to the deemed-to-satisfy provisions.

State and Territory Variations

1. The NCC is national, but each state and territory may have a schedule that modifies Part G5 or H7 provisions. Examples:
2. Tasmania: specific bushfire provisions replace G5P1 and G5P2 and explicitly apply to certain building classes and Class 10a decks immediately adjacent to listed higher-risk buildings (see TAS G5P1, TAS G5P2 in NCC Volume One and NCC Volume Two schedules).
3. Queensland, NSW, Victoria, WA, SA, ACT and NT each maintain state schedules or additional planning overlay requirements that can alter the design bushfire AEP, the application to Class 10a decks, or provide stricter requirements for ember protection and materials. Refer to the relevant state schedule in NCC 2022 Volume One (Schedules 4-12) and local jurisdiction planning/bushfire overlay controls.
4. Note: Always verify the applicable state schedule in NCC 2022 Volume One and any local planning or council bushfire overlay requirements before finalising deck or subfloor construction in a bushfire prone location.

Practical Compliance Tips

1. Use the correct BAL (bushfire attack level) determined by site assessment and AS 3959 as the starting point for specifying decking materials and ember protection details.
2. Where possible, specify non-combustible decking or durable, preservative-treated timbers (H3 for above-ground, H5 for in-ground) in accordance with AS 1684 to reduce ignition risk and meet NCC durability requirements.
3. Seal or protect the under-deck and subfloor space against ember entry using ember-proof screens, closely fitted soffits, or impervious membranes where ventilation requirements permit - this reduces the chance of ember accumulation beneath combustible decking or in subfloor voids.
4. Follow Table F1D8 for subfloor ventilation area requirements and maintain the minimum ground clearance of 400 mm (or site-specific reduced clearance of 150 mm within 2 m on sloping sites) unless alternative measures are applied (NCC Volume One, F1D8).
5. Avoid storing combustible materials under decks or in subfloor spaces in bushfire prone areas. Provide clearances from vegetation and keep combustibles out of ember-exposed cavities.
6. Detail junctions between the deck and the building carefully to prevent ember ingress into the building envelope - use flashing, close gaps and consider non-combustible connections where required by the BAL and NCC provisions (AS 3959 guidance is useful here).
7. Confirm state or territory-specific variations and council bushfire overlay controls early in design, and document any alternative solutions proposed to achieve equivalent safety for approval by the certifier.

References (selected)

1. NCC 2022 Volume Two, Section H7P5 - Buildings in bushfire prone areas
2. NCC 2022 Volume One, Part G5 - Construction in bushfire prone areas
3. NCC 2022 Volume One, F1D8 - Subfloor openings and ground clearance (Table F1D8)
4. AS 3959 - Construction of buildings in bushfire-prone areas
5. AS 1684 series - Residential timber framing
6. NASH Standard - Residential and Low-Rise Steel Framing Part 2

Note: State and territory schedules in NCC 2022 Volume One may modify these provisions. If documentation does not clearly address a specific scenario, confirm with a registered building certifier or the relevant state authority.