2 Do’s And Don’ts Of Building Retaining Walls

Retaining walls have the important job of containing a huge amount of earth to create flat grounds for erecting structures. They’re a typical sight in sloped construction sites, enabling whole sections to be built despite the incline. Another way of using them is reshaping a property’s landscape to look more elegant.

Think of retaining walls as dams that hold back earth instead of water. Sadly, much like dams, failing retaining walls can result in a mess, if not a danger to the entire property. A noteworthy example happened last October when the retaining wall behind a house in Murrumba Downs broke, sending huge boulders into the backyard and leaving a repair bill worth AUD$350,000.

Because of this, retaining wall construction is one aspect of home improvement you don’t want to skimp on, let alone do it yourself. Consider the following dos and don’ts.

DO: Check local regulations

Construction professionals can’t stress enough the importance of getting retaining walls right the first time. In fact, most cities and towns enforce their own rules and regulations for building one. The Gold Coast City Council states that retaining walls should be:

  • No more than one metre in height (two metres if including the fence’s height)
  • More than 1.5 metres away from a building or another retaining wall
  • Only holding earth behind it, not above it
  • A separate entity from a swimming pool’s fence (if any)
  • Built away from a known sewer or stormwater line, easement, and waterfront setback
  • Compliant with structural requirements and development conditions

Any variation from these rules requires securing building approval from the local government, which can be a hassle. For starters, if building a retaining wall will entail major changes in the structure and the surrounding native area, owners must secure a development application (DA) approval. A DA approval comes before a building approval, and the entire process takes time.

DON’T: Use the wrong materials

Most builders of retaining walls Gold Coast generally offer two types: timber and concrete. Both options are ideal under specific conditions, but not knowing their benefits and drawbacks can put huge swathes of your property at risk.

Timber walls can enhance a landscape’s natural look. They use pine wood treated with copper chrome arsenic (CCA), a commonly used wood preservative. While the more affordable of the two, they need to be replaced after 15 to 20 years. Untreated timber is less ideal for a retaining wall, as they’re vulnerable to decay, fungi, and termites, among other things.

On the other hand, concrete retaining walls exhibit the look and feel of natural stone. Provided adequate maintenance, these walls can last well beyond 20 years and won’t warp or rot (decay and animals can’t eat concrete). However, they’re more expensive and require a more intricate installation plan to maximise their benefits.

DO: Install a drainage system

Remember the collapse of the retaining wall in Murrumba Downs from earlier? That was due to, as news outlets described it, the heaviest rainfall Queensland has ever had in 120 years. Parts of the state received rainfall of as much as 400 mm in one day. No retaining wall can hold back soil saturated with that much rainwater, more so without a way out for the runoff.

The retaining wall’s drainage system is located underneath the wall itself, as its construction has loosened up the soil there. More importantly, rainwater spilling at the top can degrade the topsoil and ruin the garden. A typical drainage system consists of three or four parts.

  • Drainage stones, usually crushed gravel, filling a 12-inch space behind the wall
  • Filter fabric to prevent soil particles from clogging up the gaps between the gravel
  • An outlet at the bottom of the wall face to prevent the rainwater from backing up
  • A drainage pipe with holes on as many sides as possible (optional but recommended)

Ensuring the outlet leads the rainwater to an external drainage system is also important. This can reduce the likelihood of the runoff pooling on the property and causing further problems (unless you have an underground rainwater tank).

DON’T: Build the wall too tall (unless required)

There’s a reason the city council implements a height limit for retaining walls that don’t require building approval. With the weight of the earth acting against it, the wall’s centre of gravity will shrink as it gets taller. The base won’t be able to keep the acting weight steady, causing the wall to fail even at the slightest nudge.

Instead of one huge wall, professionals always recommend building multiple smaller walls, also known as tiered retaining walls. The ratio is usually one to two—as in a one-metre retaining wall for every two metres of continuous land that needs to be walled.

Tiered wall construction can be costly and require plenty of space, but it provides more adaptable space for landscaping. To balance space needs and cost, designs can sometimes consist of a large, reinforced wall and a short unreinforced wall.

Conclusion

Retaining walls may look uncomplicated at first glance, but plenty of planning goes into building one. They’re the only things standing in the way of a mass of earth and rocks coming down and endangering anyone nearby. If built to code, they can be beautiful and long-lasting.

As mentioned earlier, skimping on retaining wall construction is ill-advised. If you want to balance cost and function, get a professional like Pride Fencescapes to help. Contact us today.