Timber retaining walls are a popular choice in Australian landscaping due to their cost-effectiveness, natural appearance, and ease of installation. When properly selected and maintained, the right type of timber can offer both functional support and long-term durability. However, not all timber is suitable for retaining walls—particularly when accounting for varying climate conditions, soil moisture, and load requirements. This article outlines the types of timber commonly used, their classifications, treatments, and considerations when selecting the best option for structural and aesthetic purposes.
Timber Selection
Before choosing a timber type, several practical factors must be assessed:
- Load-bearing requirements: Retaining walls must support soil pressure and additional loads like nearby structures or water runoff.
- Soil and moisture levels: Wet soils accelerate timber decay, requiring higher durability or treatment classes.
- Termite risk: Certain regions in Australia, particularly northern and coastal areas, are high-risk zones for termite activity.
- Wall height and design: Taller walls require stronger timber sections, potentially using engineered wood or composite materials.
Timber used in structural applications, including retaining walls, must meet AS 1684 (Residential timber-framed construction) and AS 5604 (Timber—Natural durability ratings).
Types of Timber
Treated Pine
Treated pine is widely used across residential projects due to its affordability and ready availability. It is typically treated to H4 or H5 hazard levels using preservatives such as copper chrome arsenate (CCA), ACQ (alkaline copper quaternary), or LOSP (light organic solvent preservatives).
Durability: Low in its natural state, treated pine must be pressure-treated to achieve structural use.
Treatment Level:
- H4: For ground contact where the timber is exposed to moisture and termites. Suitable for posts and sleepers.
- H5: Required for higher-risk situations with continuous water exposure, such as retaining walls over 1m or in wet areas.
Pros:
- Economical
- Readily available
- Easy to cut and install
Cons:
- Susceptible to warping and twisting if not properly sealed
- Limited aesthetic appeal unless painted or stained
Hardwood Sleepers
Hardwood sleepers are naturally more durable than softwoods and are often used without chemical treatment. Popular hardwoods include Ironbark, Spotted Gum, and Red Gum, which are rated highly for in-ground durability (Class 1 or 2).
Durability Class:
- Class 1: Very durable—lifespan of 25+ years in ground contact
- Class 2: Durable—15 to 25 years in ground contact
Pros:
- High strength and load resistance
- Rich, natural aesthetics
- Lower susceptibility to termites (especially Ironbark and Tallowwood)
Cons:
- More expensive than pine
- Heavier and harder to work with
- Supply can be inconsistent depending on regional availability
Recycled Timber
Recycled hardwood railway sleepers and bridge timbers are increasingly used in landscaping. These timbers are often extremely dense and already proven in high-stress applications.
Durability: Often Class 1 due to the origin of the timber
Best Use: Ideal for rustic finishes, structural applications, and where sustainability is a concern
Pros:
- Environmentally responsible choice
- Unique aesthetic appeal
- High structural integrity
Cons:
- Difficult to source in consistent sizes or grades
- May contain old nails, creosote, or other residues
Treatment Classes
Australia’s hazard rating system defines the level of timber treatment required depending on exposure:
Hazard Class | Use Environment | Example Application |
|---|---|---|
H1 | Indoors, dry, no insects | Internal framing |
H2 | Indoors, dry, termite risk | Internal structural use |
H3 | Outdoors, above ground | Decking, cladding |
H4 | Outdoors, in-ground | Retaining wall sleepers |
H5 | In-ground, wet area exposure | Posts in wet or flood-prone soil |
H6 | Marine or high-salinity zones | Timber in sea water |
For retaining walls, H4 and H5 are the most relevant ratings. Using timber below the appropriate hazard level will significantly reduce the wall’s lifespan.
Design and Installation
Timber choice alone is not sufficient. Correct design, drainage, and construction methods are critical in extending the life of a timber retaining wall:
- Drainage: Install proper behind-wall drainage (ag-pipe, scoria, and geotextile wrap) to prevent water buildup and timber rot.
- Backfill material: Use granular backfill material rather than clay or compacted soils.
- Protective sealing: Applying bitumen coatings or sealing end-grain cuts can further improve moisture resistance.
Retaining walls over 1 metre in height should be engineered and comply with local council requirements and the National Construction Code (NCC).
Timber Alternatives
In high-moisture zones or for long-term low-maintenance applications, consider timber alternatives:
- Composite sleepers: Made from recycled plastic and wood fibres; termite and rot-proof
- Concrete sleepers: Durable and structurally superior, though more expensive and difficult to install
- Steel-reinforced timber: Hybrid systems use steel channels or galvanised posts to support timber panels
These alternatives increase upfront cost but reduce long-term maintenance and replacement cycles.
For most Australian residential retaining wall applications, treated pine (H4 or H5) offers an economical and practical solution, particularly in suburban landscaping. However, where higher structural integrity, visual appeal, or longevity are critical—particularly in coastal or high-moisture areas—Class 1 hardwoods or recycled timber are better suited. Always factor in local soil conditions, termite prevalence, and compliance standards. When in doubt, consult a structural engineer or qualified landscaper to ensure the timber choice aligns with safety and durability expectations.