What’s the best retaining wall material for the job

Retaining walls are a crucial landscape structure to stabilise sloped areas, prevent soil erosion and create functional outdoor spaces. Effective – and long-lasting – retaining walls require careful planning, especially in terms of material choice and structural integrity, as well as position and drainage. We asked Sunshine Coast builder Paul Smith of the Queensland Landscape and Drainage Group for his best insights into how to build right when working on a retaining wall project, and which materials are best for the job at hand.

“Know what options and alternatives are out there so when you understand what the client’s wants and needs are, you know how you can get the best outcome for everyone,” he says.

Tip: Always wear the appropriate safety equipment (safety glasses, gloves, ear muffs and a mask, for example) and always follow the instructions for the product or equipment.

Using timber sleepers to build retaining walls

There are many reasons timber sleepers remain popular for retaining walls: their natural aesthetic and functionality. “[Timber sleepers] are an affordable choice, and that makes them an attractive option when a project needs to be kept to a tight budget,” Paul says.

Works well when... “I usually choose timber sleepers when it’s a small retaining job,” Paul says, adding the retaining wall should be no higher than 500mm.

Best areas to use it: A rule Paul follows with timber sleepers is to think small. “This might be a gentle slope, which only requires a low level of retention and where the pressure of the soil behind is moderate,” he says.

The pros: “Timber sleepers are durable, lightweight, easy to use and pretty cost-effective,” Paul says. Often, a timber retaining wall may only require one worker and can be installed in a day, and as a renewable resource, timber is a sustainable choice. “Many clients like the natural look, so timber is aesthetically pleasing.”

The watch-outs: Changes in the material over time needs to be considered when using timber. “It can twist, warp, shrink, split and, unless it’s been treated properly, can attract termites,” Paul says. Longevity could also pose a problem. “You may get 30 good years out of timber sleepers, but you also may only get five years, as being exposed to the elements can shorten the lifespan.”

Key takeaways:

• Aesthetically pleasing
• Easy and cost effective
• Not usually suitable for retaining walls over 500mm
• May require replacing sooner than expected

The pros and cons of concrete sleepers for retaining walls

“Reliable sums up concrete sleepers,” Paul says, claiming they’ve become almost as popular for residential retaining jobs as they have on commercial projects. “Their strength means that once they’re in, they’ll do the job for a long time.”

Works well when... “I’ve used concrete sleepers in low-rise jobs to create terraced gardens and in projects as high as a three-metre wall,” Paul says. While concrete is heavier than timber, they still offer versatility, he adds. “There are many ways concrete can be used when retaining.”

Best areas to use it: If holding back a significant amount of high soil pressure is paramount, like on sloped terrains or a terraced garden, then concrete is ideal. “There is a variety of concrete choices with different textures and colours that also make them appealing,” he says.

The pros: Expect a long life with concrete sleepers. “They will be there for decades, and you’ll get a much better warranty from the suppliers,” Paul says. You can also stack them to height. “Of course, it comes down to the engineering specifications, but what can be achieved is impressive.”

The watch-outs: Concrete sleepers require more manpower and a slower installation if access for construction equipment is an issue. “It also takes more work to get all the joins to line up so it’s neat,” Paul says. The look can be ‘too industrial’ for some clients, and the cost can be double that of timber sleepers.

Key takeaways:

• Concrete stands the test of time
• It can hold back large volumes of soil
• Requires more equipment and time to install
• More expensive due to material and labour

Are composite sleepers suitable for retaining walls?

Composite sleepers are a mix of recycled plastic and wood materials bonded with a resin to create a durable material often referred to as fibre-reinforced polymer (FRP). “There are more versions of this on the market, and the quality is good,” Paul says.

Works well when... you use it “just about anywhere,” Paul says. “I believe we’re about to see composite sleepers being used much more on retaining jobs.” Lighter than concrete and stronger than timber, composites offer a high strength-to-weight ratio, allowing for easy installation.

Best areas to use it: With strength and durability, composite sleepers are suitable in a range of settings. “The lightweight versions – some around 5kg – are still strong but can easily be carried across a site and then stacked to a reasonable height,” Paul says.

The pros: Unlike other materials, one worker can usually stack the lightweight composite sleepers into a strong retaining wall, and they will not rot, weather or warp and can be painted. As they are made from recycled materials, they’re an eco-friendly choice, and composite posts are often available.

The watch-outs: As they are newer products on the market, using composite sleepers may require convincing for clients to understand the benefits. “I find them far easier and more cost effective to use, and when built to specifications, do the job superbly.”

Key takeaways:

• Lightweight, durable and effective
• Made from recycled materials so they are eco-friendly
• Requires less labour and equipment, but are a similar price to concrete
• Not as well-known in the market

Why should you consider masonry blocks for retaining walls?

As a traditional method, block retaining walls remain a popular choice due to their durability and pleasing aesthetic. “Blocks always do a good job,” Paul says.

Works well when... used in almost any configuration. “They fit easily and can create a variety of shapes,” Paul says. “Blocks can be stacked to make a solid retaining wall, and some people like a terraced effect. With masonry blocks, there’s a lot of choice.”

Best areas to use it: “Take your pick – blocks are good for both smaller and higher retaining walls,” Paul says. Being sturdy, blocks can effectively hold back large volumes of soil.

The pros: Your client’s retaining wall can be straight, angled or curved, while still offering a strong structure. “Blocks offer good structural integrity in withholding the soil,” Paul says.

The watch-outs: Like bricklaying, blocks require a tradesperson with specific skills to create the layers of the walls, which can be time consuming. “With a higher wall, that’s when you need to go through the engineering process and be governed by the directions of the engineer,” Paul says.

Key takeaways:

• Design flexibility
• Affordable and aesthetically pleasing
• Structural integrity
• Construction can be time consuming

Always consider drainage needs when building retaining walls

Paul warns there’s an essential rule about drainage solutions that is relevant for any form of retaining wall and must be explained to a client before work commences. “Drainage needs to be discussed first, before looking at materials,” he says, adding that water needs a pathway to drain away. “Drainage can ultimately make or break the retaining wall – no matter what material you end up going with.”

Proper drainage is also crucial to prevent possible structural failure. Effective solutions can include geotextile matting, an Agi pipe, a perforated pipe, a channel and grate, as well as gravel backfill for improved drainage and capillary-like systems which spread any back-up moisture throughout the garden

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