Floors and subfloors
This section describes a range of design and construction principles to maximise the bushfire resilience of your new floor system. Consider using a cost effective design that reduces the risk of ignition and which prevents floors and underfloors from burning and affecting other parts of the building. Your main objective should be to prevent flames and embers from entering the home via the flooring system, and to safeguard supports and structures from collapse, displacement, breach, and from burning and producing additional heat and toxic gases.
Designing out the risks
As with all buildings systems, consider what risks can be ‘designed out’ early in your design process. Many of the risks associated with a flooring system can be eliminated by building on the ground using a concrete slab. On the other hand, a raised floor can be built to be bushfire resilient, however it will require the right combination of substrate, support structures and enclosure materials.
Consider what works best for your build – and don’t forget to factor in the desired look, feel, and function of your new home in addition to its bushfire resilience.
Building on a slab removes the risk of flames and embers attacking from under the house
Do’s
- Lay the slab on ground.
- Fully enclose the floor system using non-combustible materials and support frames with the necessary fire ratings. Refer to AS 3959:2018, and check what measures are required for your BAL rating. Note that in some cases, a fire rated product is combustible, meaning that it may burn if the fire exceeds the rated threshold. Obviously this is not a desirable outcome, so we suggest using materials that are both fire-rated and non-combustible.
- Use non-combustible flooring and support structures that have the necessary fire-rating for your BAL.
- Protect (cover) all combustible flooring elements with non-combustible coverings.
- Any unprotected support structures should be non-combustible and have the necessary fire-rating for your BAL.
Don’ts
- Do not store combustible objects in an open underfloor space.
- Do not install a combustible access door to the enclosed underfloor space.
- Do not use a combustible support structure immediately behind non-combustible cladding.
- Do not rely on a minimum ground separation distance to justify the use of combustible flooring.
- Do not rely on conventional sarking to protect against flame and ember attack. Conventional sarking products are not suitable as an ember or flame barrier, even if they are rated with a flammability index. The flammability index specifies the threshold at which the material will freely burn, rather than its ability to act as an effective barrier. A small number of specialised sarking products are available which can resist flame and embers attack; pay attention to what your product can do.
Construction and materials
The following table contains information on different floor systems and the level of bushfire protection they provide. Consider the Bushfire Attack Level (BAL) at the site of your build and design a system that will protect against the predicted level of exposure.
| Type of subfloor | Flooring substrate | Support posts and frames | Enclosure | Exposure protection – BAL |
| Slab on the ground | NA | NA | NA | BAL-FZ and consequential fire |
| Raised and fully enclosed subfloor | Any | Non-combustible frame behind fully enclosed cladding | Non-combustible enclosure (e.g., brick, cement sheet, fibre cement), with no gaps larger than 2mm | BAL-29 and surface fire |
| Non-combustible enclosure, with 30min fire-rated enclosure material | BAL-FZ | |||
| Non-combustible enclosure, with 60min fire-rated enclosure material | BAL-FZ and consequential fire | |||
| Raised floor | Non-combustible substrate (e.g., concrete, aerated concrete, cement sheet, or fibre cement) | None | Concrete or steel posts | BAL-29 and surface fire |
| Fire-protected or 30min fire-rated concrete or steel posts | BAL-FZ |