Much of the Blue Mountains is classified as bushfire prone land. The 2019-20 fire season demonstrated that threat in the starkest possible terms, with fires running to the edge of the urban areas in Blackheath, Katoomba and across the escarpment. For property owners planning retaining walls, the Bush Fire Attack Level (BAL) rating of their property is a planning consideration that affects material choices, drainage design and what BMCC will require as part of a development consent.
This guide explains how BAL ratings work, what they mean specifically for retaining walls, and how to navigate the requirements without over-engineering a simple garden wall.
Quick answer (BLUF)
BAL ratings primarily affect buildings, but retaining walls used as part of an ember-blocking or fire-access design, or walls that form part of a habitable building’s structural system, must comply with AS 3959 Construction of Buildings in Bushfire-Prone Areas. For standalone garden retaining walls, BAL restrictions are less prescriptive, but material choices still matter, particularly around combustible elements like timber sleepers and plastics near the bush interface.
How BAL ratings work
The Bush Fire Attack Level is a measure of the fire threat at a specific location based on:
- Distance from the nearest classified vegetation (bush, scrub, grassland, woodland)
- The slope the fire would travel up to reach the site
- The type of vegetation
Ratings range from BAL-LOW (no specific requirements), through BAL-12.5, BAL-19, BAL-29, BAL-40, to BAL-FZ (Flame Zone, the highest risk, applicable to properties in the direct path of a flaming fire front). The numbers in BAL-12.5 to BAL-40 refer to the expected radiant heat exposure in kilowatts per square metre; BAL-FZ means exposure above 40 kW/m² with direct flame contact likely.
In the Blue Mountains, properties on bush-edge lots across Blackheath, Katoomba, Mount Victoria, Wentworth Falls and Leura frequently carry BAL-12.5 to BAL-40 ratings. Some escarpment-adjacent properties carry BAL-FZ.
What BAL means for retaining walls specifically
The standard that governs construction in bushfire-prone areas is AS 3959:2018 Construction of Buildings in Bushfire-Prone Areas. This standard is primarily focused on buildings, walls, roofs, windows, decks and openings. However:
Where retaining walls adjoin or are structurally integrated with a building, they take on the BAL requirements of the building. A retaining wall that forms part of the boundary of an enclosed sub-floor space, for example, must meet the construction standards for that BAL.
Standalone garden retaining walls are not directly subject to AS 3959 requirements in most cases. They are not buildings and are not regulated under the NCC/BCA. However, BMCC may impose conditions on materials and design for retaining walls that form part of a development application in bushfire prone areas.
Timber sleeper walls in high-BAL zones: Timber sleepers are combustible. In BAL-40 and BAL-FZ zones, BMCC and the NSW Rural Fire Service (RFS) will typically require non-combustible materials for structures near the building. A timber sleeper wall at the edge of a BAL-40 property, immediately upslope of the house, creates a fire risk pathway. Concrete sleepers, masonry or sandstone are preferred alternatives in these situations.
NSW RFS Asset Protection Zones and retaining walls
As part of the development consent process for properties in bushfire prone areas, BMCC refers DAs to the NSW Rural Fire Service for assessment. The RFS applies Planning for Bushfire Protection (PBP 2019) guidelines, which specify Asset Protection Zones (APZs), managed vegetation buffer zones around structures.
Retaining walls within the APZ, or walls used to create a fire access track, cleared area or asset protection zone boundary, are assessed for their contribution to fire safety. In this context, non-combustible materials are strongly preferred, and walls that can deflect ember flow or provide a physical barrier to fire spread are positively regarded.
Practical material implications by BAL rating
| BAL Rating | Timber sleepers | Concrete sleepers | Sandstone block | Besser block |
|---|---|---|---|---|
| BAL-LOW | Acceptable | Acceptable | Acceptable | Acceptable |
| BAL-12.5 | Generally acceptable | Preferred | Preferred | Acceptable |
| BAL-19 | Use caution near house | Preferred | Preferred | Preferred |
| BAL-29 | Avoid near buildings | Strongly preferred | Strongly preferred | Preferred |
| BAL-40 / FZ | Not recommended | Required near buildings | Strongly preferred | Required near buildings |
This table is a general guide, site-specific RFS advice through the DA process may differ.
Gaps, joints and gabions
Ember accumulation is a design issue as well as a material one. Timber sleeper walls and open timber crib walls have gaps where embers can lodge and ignite, so in BAL-29 and above, any gapped structure near the dwelling is worth assessing and, where needed, replacing with a solid-face wall. Concrete sleeper and solid masonry walls do not have this problem. Steel H-posts and concrete panels are non-combustible, but some sleeper systems use polymer or rubber joint strips, so check their fire performance with the supplier in high-BAL zones. Steel mesh gabion walls filled with stone are non-combustible and suit bushfire-prone sites where a natural, permeable wall is wanted; see Gabion Walls in the Blue Mountains: Pros and Cons. For low garden edging well away from the house, hardwood sleepers are often accepted, but confirm through the DA if the property is on bushfire prone land.
Drainage and fire: an additional consideration
In bushfire prone areas, retaining wall drainage outlets must be protected from ember entry. Open ag pipe outlets at the wall face can allow embers to enter the drainage layer and smoulder against combustible backfill or adjacent materials. In BAL-29, 40 and FZ zones, drainage outlets should be fitted with ember guards, metal mesh covers that prevent ember entry while allowing water flow.
Getting BAL clarity for your property
Your property’s BAL can be formally assessed by a bush fire consultant or determined through BMCC’s BMaps overlay tool (approximate indication only). A formal BAL assessment is required for any DA that involves construction in bushfire prone areas.
If your property is in a high-BAL zone and you’re planning significant retaining work, engage a contractor who understands the material requirements and can co-ordinate with the RFS through the DA process. Most experienced Blue Mountains retaining wall contractors work in bushfire prone areas routinely and understand what is required.
FAQs
Does my entire block have a single BAL rating?
No. BAL varies across a lot depending on the distance from vegetation at each point. The front of your property (near the road) may be BAL-LOW while the rear (adjoining bushland) may be BAL-40. A formal BAL assessment maps the rating across the site.
My existing timber sleeper wall is in a BAL-40 zone. Does it need to be replaced?
Existing lawful development is generally not required to be upgraded to comply with new BAL requirements, the requirement applies to new construction. However, if you’re replacing the wall anyway (due to failure or renovation), the replacement must comply with current requirements, which effectively means non-combustible materials in BAL-40 zones near buildings.
Can I use reclaimed railway sleepers for a garden wall near the bush?
In BAL-19 and above zones, reclaimed hardwood sleepers are not recommended for walls adjacent to buildings. For a low garden terracing wall well away from the house and outbuildings, for example, in the front garden, there may be more flexibility, but you should confirm this through your DA if the property is in a bushfire prone land zone.
Who assesses the BAL requirement, BMCC or the RFS?
BMCC refers DA applications involving bushfire prone land to the NSW RFS, which assesses them under the PBP 2019 framework and issues a bushfire safety authority or conditions. Your contractor or certifier can advise whether your specific project triggers an RFS referral.