Everything you need to know before building a retaining wall

Retaining walls are useful and can create beautiful gardenscapes with depth and interest, but the project of constructing one that is structurally sound and has longevity might be a bit more difficult than you think.

What is a retaining wall?

Let’s start with the basic definition. A retaining wall creates an artificial slope in your backyard or garden. Retaining walls are used for both practical and aesthetic purposes, being used to protect dug-out house pads, paths or driveways, or can create visually-pleasing separation for your garden beds.

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If you are looking to build a boundary retaining wall, you may have to get your neighbour involved in the process. It will be the benefitting owner who is responsible for the cost and construction of the wall. Sometimes it may not be clear who is the benefitting neighbour, and occasionally there may be opportunity for two neighbours to split the cost proportionately. 

A completed retaining wall project by the team at Core4. Image source: Core4

Preparing for construction

Building a retaining wall can be quite a disruptive and expensive project, depending on the environment. The area will need to be excavated prior to building the wall. Depending on the size of the job, this might be a task for a shovel and bit of elbow grease, or an excavator may need to be brought in for the job. In the case of the latter, you should consider how the 2 tonne machine will get through to the jobsite and the damage that may be caused along the way. 

Cross your Ts

Some paperwork may need to be completed ahead of building the wall. Here are a few things you should check on prior to construction.

  • Retaining walls taller than 1m must be engineered. You can later use the engineer’s plans to obtain a quote for the works.
  • Prior to going ahead, check with your local council’s requirements, as you will need to acquire a permit in some cases. For most councils, this will depend on the height of the wall or the amount of earth that must be moved. The application process is usually quite simple, and if you have questions or require assistance, you can call the council directly.
  • You may additionally require a building permit, in the case of larger walls. 

Materials

You will have a choice of a few materials when looking to construct your retaining wall. There are pros and cons of each material, but at the end of the day it will come down to balancing your budget and longevity of materials. We recommend stretching your budget if you can, in order to build a longer lasting wall. As the works can be quite disruptive, the best option is to ensure the wall lasts the longest time possible.

The two basic options are treated pine and concrete. Treated pine sleepers are the more affordable option, and can last up to 10 or 15 years if installed and maintained correctly. Concrete sleepers cost approximately 15% more, but will last up to twice as long, holding tight for 40 years if installed and maintained correctly. 

There are other options available, like block, brick, and stone, however these may need to be waterproofed and engineered and usually come at a premium cost. If you are seeking design over function, and have a bigger budget to play with, you may like to consider these options.

Drainage, drainage, drainage

We find the most common cause of failure in retaining walls is inadequate drainage. While every good contractor will know to install drainage when constructing a retaining wall, it greatly benefits the resident to understand how and why drainage works and is so important.

The job of a retaining wall is to hold up a certain amount of soil. If proper draining is not installed behind the wall rain or water from a hose or other sources can build up and add additional pressure to the wall. A retaining wall is designed and built to withstand a certain amount of pressure from the soil behind it, and if additional pressure is applied by vast amounts of water, the wall may begin to bulge or lean. 

It is imperative that drainage is installed during construction of the wall. Drainage must be installed behind the wall at a low level, in order to avoid water pressure building up. Drainage should additionally be considered at top of the wall and, where possible, surface water should be diverted away from the wall.

Engaging a contractor

Even if your wall is under 1m high and won’t require an engineer, it is important that you engage a qualified contractor to undertake the construction. It can be difficult to know how to choose the best person for the job, which is why we have a blog post to step you through the process. Don’t forget that all domestic jobs over $3 000 require a written contract and Home Warranty Insurance in accordance with the QBCC.

Maintenance Tips

We are big proponents of prevention rather than cure. We also believe that an untrained eye is very capable of undertaking an inspection of a retaining wall. A quick visual inspection once a year can make all of the difference. The things you should look out for include:

  • Cracks in the exposed face of the wall
  • Signs of wood rot (if made from timber), or rust (if made from concrete and steel)
  • Bulging, or leaning towards the lower side of the wall
  • Sunken soil and gravel along the top of the wall (small amounts of settlement is normal, but should be topped up to assist drainage)
  • Piles of debris at the base of the wall – this is often wash out from behind the wall, which could indicate a drainage issue
  • Trees and weeds growing from the wall or along the top of the wall – tree roots are a major contributor to failure

We have a fully qualified team available for retaining wall replacement in Brisbane, in addition other body corporate maintenance tasks. Call our friendly team on 1300 267 333 or contact us to talk about your requirements.

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