Retaining Walls in Bushfire Zones, Blue Mountains BAL Material and Design Guide

The Blue Mountains has some of the highest bushfire risk of any urban area in Australia. Virtually every town in the LGA has properties mapped as bushfire prone land, and large portions of residential areas, particularly at the urban-bush interface in Springwood, Blaxland, Lapstone, Warrimoo, Faulconbridge, and parts of Katoomba, Leura, and Blackheath, carry significant BAL ratings.

Bushfire attack level (BAL) ratings affect what materials can be used for construction within a set distance of bushland. Most people know BAL requirements apply to houses and sheds, many don’t realise they also apply to retaining walls close to the dwelling or within the asset protection zone.

Bottom line: if your retaining wall is within 10-50 metres of a bushland boundary and your property is BAL-rated, the wall’s material and design may be subject to the Australian Standard AS 3959 (Construction of Buildings in Bushfire Prone Areas) requirements. This guide explains what that means in practice.

How BAL Ratings Work

A Bushfire Attack Level rating is assigned based on:

• The Fire Danger Index for the region
• The vegetation type adjacent to the property
• The slope of the ground between the vegetation and the structure
• The distance between the structure and the vegetation

In the Blue Mountains, most residential areas are classified as a flame zone or high BAL zone due to the eucalypt forest and scrubland that surrounds them. The NSW Rural Fire Service maps bushfire prone land, and individual properties get a BAL rating from a Bush Fire Assessment.

BAL ratings in order of increasing severity:

Most Blue Mountains residential areas at the bush interface fall in the BAL-12.5 to BAL-40 range. Some areas near National Park boundaries have BAL-FZ sites.

When Do BAL Requirements Apply to Retaining Walls?

AS 3959 technically applies to “buildings” in bushfire prone areas, and retaining walls alone are not classified as buildings. However, the situation is nuanced:

Retaining walls within the asset protection zone (APZ): The APZ is the managed zone around a dwelling where vegetation is maintained to reduce fire intensity. Retaining walls within the APZ that are part of a DA or building approval package are generally assessed for material suitability and may need to comply with BAL-appropriate materials.

Retaining walls as part of dwelling construction: When a retaining wall is being built as part of a new dwelling or major renovation requiring a DA or CDC, the entire project falls under BAL requirements, and structural elements, including retaining walls, may need to use BAL-appropriate materials.

Retaining walls near the dwelling: Combustible retaining walls (timber sleeper walls) adjacent to the dwelling itself present a fire-spread pathway. In BAL-29 and above zones, BMCC and certifiers are increasingly likely to raise this as a concern.

Council interpretation: BMCC has discretion in how it applies bushfire provisions to ancillary structures. In practice, for DA applications in BAL-29 and BAL-40 zones, you should expect questions about retaining wall materials and may be required to use non-combustible materials for walls within the inner APZ.

See [/guides/bush-fire-attack-level-retaining-walls/] for the detailed BAL guide for retaining wall materials.

Material Choices by BAL Zone

BAL-LOW and BAL-12.5

At low to moderate BAL ratings, retaining wall material choice is essentially unrestricted by bushfire requirements. All common materials, concrete sleeper, sandstone, block, timber, are used in these zones. Standard building and BMCC planning controls apply.

Note: Timber sleeper retaining walls are still combustible, but at BAL-12.5 the risk is generally considered acceptable for walls not immediately adjacent to the dwelling.

BAL-19

At BAL-19, some certifiers and BMCC officers begin to raise questions about combustible materials near the dwelling. Timber sleeper walls immediately adjacent to a house (within 3-5 metres of a wall or opening) may be flagged. Non-combustible alternatives are recommended:

• Concrete sleeper walls (non-combustible)
• Sandstone (non-combustible)
• Block/Besser block (non-combustible)

BAL-29

At BAL-29, the use of combustible retaining wall materials close to the dwelling or within the inner APZ is generally not recommended and may be challenged in a DA. Non-combustible materials should be specified for all walls within 20 metres of the dwelling.

Recommended materials for BAL-29:

• Concrete sleeper walls, preferred for most applications
• Sandstone, excellent choice, particularly for heritage-compatible sites
• Concrete block / Besser block, suitable for all applications
• Gabion walls with steel mesh, non-combustible and appropriate

Not recommended for walls near the dwelling in BAL-29:

• Timber sleeper walls (hardwood, treated pine)
• Timber post and rail walls

BAL-40

At BAL-40, non-combustible materials are essential for retaining walls near the dwelling and within the inner APZ. BMCC and certifiers will require documentation of material compliance.

Steel H-posts in concrete sleeper systems are non-combustible, but the question of whether the polymer or neoprene joint strips used in some sleeper systems are BAL-compliant is worth verifying with your supplier. Sandstone and concrete block walls are straightforwardly non-combustible and unambiguous.

BAL-FZ (Flame Zone)

In flame zone areas, all construction must be assessed by a qualified fire engineer or bushfire consultant. Retaining walls in FZ areas that form part of any development should be treated as non-combustible structural elements. Contact BMCC and the NSW RFS before proceeding with any development in a BAL-FZ area.

Ember Guards and Gaps

One aspect of bushfire design that applies to retaining walls in high BAL zones is the elimination of gaps and cavities that can trap embers. Timber sleeper retaining walls have gaps between sleepers that can allow ember accumulation and ignition. Concrete sleeper systems and solid masonry walls don’t have this problem.

In BAL-29 and above, any retaining structure near the dwelling with gaps (such as timber sleepers or open timber crib walls) should be assessed and potentially enclosed or replaced with solid-face alternatives.

Fire Access and Retaining Walls

In the Blue Mountains, fire access tracks and asset protection zones must remain clear for fire truck access. Retaining walls near fire access tracks or at property boundaries that border APZs need to be positioned and designed so they don’t obstruct access.

For properties where retaining walls are at or near the fire access boundary, coordinate with your design with reference to [/guides/retaining-wall-for-fire-access-track/].

Does BAL Affect the Approval Process?

Yes. For any DA or CDC application in a bushfire prone area:

• The application must include a “10/50 vegetation clearing entitlement” assessment or equivalent
• A Bushfire Risk Assessment (from an accredited bushfire consultant) may be required
• BMCC will refer significant applications to the NSW Rural Fire Service for comment
• The RFS may impose conditions on the DA regarding construction standards

This adds time and cost to the approval process, but it’s a non-negotiable part of building in the Blue Mountains. See [/guides/bmcc-council-approval-retaining-walls/] for the full DA process.

Costs of Bushfire-Compliant Retaining Walls

Specifying non-combustible retaining wall materials adds cost compared to timber, but the premium over standard concrete sleeper or block construction is modest:

When you factor in lifespan, the concrete sleeper wall is also cheaper over time. See [/guides/retaining-wall-cost-guide-blue-mountains-2026/] for a full cost comparison.

FAQ

My property is BAL-29, do I need to replace my existing timber sleeper retaining walls? Existing structures are generally not required to be upgraded unless you’re undertaking development that triggers a DA. If you’re building a new structure or doing significant alterations, the DA may require that non-compliant existing walls near the dwelling be assessed and potentially upgraded.

Can I use hardwood sleepers for a garden retaining wall in a BAL zone? For low walls (under 600mm) as garden edging, hardwood sleeper use in BAL zones is generally accepted. For structural retaining walls over 600mm near a dwelling in BAL-29 or BAL-40, non-combustible materials are recommended and may be required.

Do gabion walls comply with BAL requirements? Steel mesh gabion walls filled with stone are non-combustible and generally comply with BAL material requirements. They are a good option for bushfire-prone areas where a natural, permeable wall is wanted. See [/guides/gabion-walls-blue-mountains/].

How do I find out my property’s BAL rating? Your BAL rating is determined by a Bush Fire Assessment, which can be done by a qualified bushfire consultant. The NSW RFS also publishes indicative maps online. For a formal rating, engage an accredited bushfire consultant.

Does Blue Mountains Retaining Walls work in high BAL areas? Yes. BAL-19 to BAL-40 areas are common across the Blue Mountains, and non-combustible materials are the standard specification for walls near dwellings in these zones. Get a site assessment at [/contact/#quote].

Get a bushfire-compliant retaining wall quote, /contact/#quote