Get the Expert Lowdown on Retaining Walls
From codes and regulations to retaining wall material options, there is a solution for every slope on your property
Steve Taylor
7 October 2015
Houzz Australia Contributor. Managing director of COS Design. Landscape, pool, alfresco and home designer based in Melbourne. I love everything about architecture, design and construction. Host of Best Gardens Australia on both the Foxtel network and Foxtel, head judge of the Victorian landscape awards of excellence, multi-award winner and registered building practitioner.
Houzz Australia Contributor. Managing director of COS Design. Landscape, pool, alfresco... More
Having visited hundreds of Australian gardens, I would say that sloping sites represent 60-70 per cent of our residential blocks, with the degree of slope varying depending on the natural topography of the land or how it has been developed. Slopes can create many challenges when designing a garden with functional and usable spaces and the costs can really blow out. However, when managed well, effective terracing and the use of retaining walls can become a valuable design feature of your landscape.
Retaining walls come in many different forms, all with varying cost implications, so let’s explore your options and present some great design tips to help you manage your sloping site.
Know your retaining wall
When we have a completely flat block we often add height and interest with raised planter boxes and feature walls, so don’t be scared of retaining walls, embrace them. You don’t always require extensive retaining walls either, as maintaining the natural slope, or a combination of the slope and more strategic use of retaining structures, can also create a great look.
When we have a completely flat block we often add height and interest with raised planter boxes and feature walls, so don’t be scared of retaining walls, embrace them. You don’t always require extensive retaining walls either, as maintaining the natural slope, or a combination of the slope and more strategic use of retaining structures, can also create a great look.
There are two aspects (or directions) to a retaining wall.
1. I call the first type of retaining wall a ‘negative wall’ and this is where the retaining wall faces away from the main view and it retains a higher ground level in the foreground. These walls are not really seen (as shown in this photo) and they can be a lower cost option.
1. I call the first type of retaining wall a ‘negative wall’ and this is where the retaining wall faces away from the main view and it retains a higher ground level in the foreground. These walls are not really seen (as shown in this photo) and they can be a lower cost option.
2. The second is a ‘positive wall’ and this is where the wall faces the main view or vista and retains a higher section of the garden in the background, as in this home. These walls need a higher level of investment as they are a permanent visual feature. So it is wise to spend as little money as possible on any negative walls and invest more in the positive walls.
Codes and regulations
With any project it is important to first be familiar with the building codes and regulations that affect you, and to make sure you comply with these when building any walls. Laws regarding engineering vary from state to state and from council to council; some areas require engineering with any wall above 600mm and other areas allow walls under 1,000mm to be exempt. You need to check your local laws. If unsure it is always safer to engage an engineer, particularly if the retaining wall will support land outside your ownership, big trees or load-bearing structures.
With any project it is important to first be familiar with the building codes and regulations that affect you, and to make sure you comply with these when building any walls. Laws regarding engineering vary from state to state and from council to council; some areas require engineering with any wall above 600mm and other areas allow walls under 1,000mm to be exempt. You need to check your local laws. If unsure it is always safer to engage an engineer, particularly if the retaining wall will support land outside your ownership, big trees or load-bearing structures.
Second to this, any retaining wall with a build cost of more than $5,000 often requires a permit, and walls over $16,000 require builders warranty insurance. Make sure you build your wall in accordance to these regulations to ensure everything is above board.
Also, you will need to find out your soil type to determine its effect on the performance of the foundations and footings. Finally, ensure there is excellent drainage and waterproofing behind every wall – the life expectancy and structural performance will be greatly improved.
Also, you will need to find out your soil type to determine its effect on the performance of the foundations and footings. Finally, ensure there is excellent drainage and waterproofing behind every wall – the life expectancy and structural performance will be greatly improved.
Retaining options
There are many options for dealing with tricky terrains; the type you choose to use will be based on a number of factors including your budget, the incline of your slope, local codes/regulations, accessibility compliance and desired design aesthetic.
There are many options for dealing with tricky terrains; the type you choose to use will be based on a number of factors including your budget, the incline of your slope, local codes/regulations, accessibility compliance and desired design aesthetic.
Use the natural slope: Walls are not always required but they will help create usable spaces on a non-user-friendly slope. Where the site allows, retaining the slope is a solution that is often overlooked; a well-defined lawn slope with the option of an isolated set of steps in the middle (as in this home) or a sloping garden bed can be a beautiful as well as cost-effective method. Utilising the slope also creates a more inviting design feature that opens up the upper or lower areas instead of separating them.
Run a blade wall: If your lawn only has minimal slope you can use a blade wall to retain part of the fall. Depending on the aspect of your slope blade walls can also be used to create a design feature and interest in your sloped lawn. Corten steel blades or small rendered masonry walls can become eye-catching features.
Build a rock bank: The most cost-effective retaining wall is a large boulder rock bank. There is less digging, no real engineering and you can create a great natural look once you scatter plants throughout. If you use spreading shrubs, such as River Wattle (Acacia cognata), or hanging/ground covers such as Dichondra argentea ‘Silver Falls’, the boulders will end up being completely covered. Be careful again and ensure the angle of the bank doesn’t become too steep without considering engineering. These walls can be used as both positive and negative walls but you can lose functional space as the battered bank requires length so they are best as positive walls.
Construct a sleeper wall: These walls can be quite popular, though I do not recommend using timber as the upright if you are building anything over 600mm high. Personally, I prefer galvanised steel uprights, as they will not rot out like timber uprights.
These walls are probably the most common due to their cost benefits and are used as negative walls most of the time. They can also be used as positive walls and, with plants draping from the top in addition to lower-level plants softening the base, the walls can be nearly invisible without spending a fortune. However, they have limited longevity as the timber will eventually rot through.
To take sleeper walls to the next level you can use concrete sleepers which have a far greater life expectancy. If you are looking for a better finish on timber sleepers, you can clad them with timber slats or Corten steel panels, but your costs can creep up to a point where it is more cost effective to build a reinforced masonry wall, so be careful not to over treat a sleeper wall.
These walls are probably the most common due to their cost benefits and are used as negative walls most of the time. They can also be used as positive walls and, with plants draping from the top in addition to lower-level plants softening the base, the walls can be nearly invisible without spending a fortune. However, they have limited longevity as the timber will eventually rot through.
To take sleeper walls to the next level you can use concrete sleepers which have a far greater life expectancy. If you are looking for a better finish on timber sleepers, you can clad them with timber slats or Corten steel panels, but your costs can creep up to a point where it is more cost effective to build a reinforced masonry wall, so be careful not to over treat a sleeper wall.
Flex with stackable masonry: These systems can be used for both negative or positive walls, although their cost can be a little high to consider for large negative walls. They work on a stabilised crushed rock or shallow concrete footing and the first layer is laid on a dry bed screed. Many companies are manufacturing these walls and they are engineered to stack on top of each other with a finished capping paver to create a nice clean look to the wall.
Stacked block walls can look nice, but for me they are a little busy and adding steps becomes very difficult due to their tapered face. What appeals to me with these walls is they are a little flexible and, therefore, a little more forgiving, so if there is a little bit of movement the effects are not too noticeable. Again, plants can help disguise these walls and minimise their impact to the aesthetics.
Stacked block walls can look nice, but for me they are a little busy and adding steps becomes very difficult due to their tapered face. What appeals to me with these walls is they are a little flexible and, therefore, a little more forgiving, so if there is a little bit of movement the effects are not too noticeable. Again, plants can help disguise these walls and minimise their impact to the aesthetics.
Splash out on a masonry wall: At the top end of retaining walls are masonry walls that require a pretty serious footing, engineering, expansion joints core-filled, reinforced cavities, extensive waterproofing and drainage. Whether it is core-filled block work, double brick, cavity brick, or any of the above with a rendered finish, these walls look fantastic and are best used as positive walls.To me it would be a waste of money to use these as a negative wall. When engineered well, masonry walls can be built a lot higher than the other options, but all the additional engineering materials and labour makes these walls very expensive.
Erect a gabion wall: If you are looking for a real point of difference to a retaining wall then you could consider Gabion walls. They are a modern take on a traditional drystone wall where steel cages are built and then selected rubble, stone or pebble are used to fill the cages. Be warned, you need to be very careful when stacking the medium into the cage, ensuring there are no air pockets or weak points where the stone can collapse, as you are only relying on the steel cage to support the structure.
The cage also needs to be considered and often chicken wire is used which is fraught with danger. I prefer a galvanised reinforcing steel mesh as a cage, for very steep or load-bearing sites you can use prefabricated interlocking cages, making them a tough and durable option. For gabion walls more than 600mm high you must consult an engineer and always ensure your base is stabilised.
Another big plus for using gabion walls is that they have a permeable nature, making them perfect for sites with drainage or soil issues where other wall options are less ideal.
The cage also needs to be considered and often chicken wire is used which is fraught with danger. I prefer a galvanised reinforcing steel mesh as a cage, for very steep or load-bearing sites you can use prefabricated interlocking cages, making them a tough and durable option. For gabion walls more than 600mm high you must consult an engineer and always ensure your base is stabilised.
Another big plus for using gabion walls is that they have a permeable nature, making them perfect for sites with drainage or soil issues where other wall options are less ideal.
Go natural with a drystone wall: These are the cream of the crop and, again, they come in many forms depending on the chosen stone. For me, there is nothing better than the craftsmanship of a stone mason creating a drystone wall with really tight gaps.
A real drystone wall requires a very broad base (one third of the height needs to be the width of the base as a minimum), so as an alternative you could build a reinforced masonry wall and clad it in drystone. Companies are now also calibrating real stone so it can be glued directly to a masonry wall to further save on time and material costs. It adds structural integrity to the wall without as much stone work. These walls are about as expensive as you can get but, when done well, they are probably the best looking.
A real drystone wall requires a very broad base (one third of the height needs to be the width of the base as a minimum), so as an alternative you could build a reinforced masonry wall and clad it in drystone. Companies are now also calibrating real stone so it can be glued directly to a masonry wall to further save on time and material costs. It adds structural integrity to the wall without as much stone work. These walls are about as expensive as you can get but, when done well, they are probably the best looking.
Creating function
One of my favourite design secrets is to use a functional garden feature as a retaining wall, or use a retaining wall as a functional garden feature – two for the price of one.
One of my favourite design secrets is to use a functional garden feature as a retaining wall, or use a retaining wall as a functional garden feature – two for the price of one.
For example, if you have a low bank of 450-500mm to retain, why not make the retaining wall double as a bench seat for a fire pit area? And, if the wall needs to be higher, you can hang the bench seat off the wall and use the wall as a back rest.
If you are building a pool on a sloping site, use the pool as the retaining wall both in a negative and a positive way. When a pool is used as a negative retainer you can create a negative edge (also known as a horizon edge or infinity edge) with the pool assuming you have a backdrop or view.
When a pool is used as a positive retaining wall you can bring the pool out of the ground 1200mm and minimise the pool fence requirements. (Check your local laws here).
So, if you are faced with a sloping site and are not sure what to do, use the above information to get you started. You can also consult with one of your local Houzz Professionals. If you do have a go at a small retaining wall yourself, remember to embrace your levels and use them to your advantage, but if you build a retaining wall, ensure you build it properly.
SHARE YOURS
We’d love to hear any tips you have, plus see a photo of your retaining wall in the comments section.
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SHARE YOURS
We’d love to hear any tips you have, plus see a photo of your retaining wall in the comments section.
MORE
Boundary Booster: Magical Ideas for Garden Walls
How to Choose the Right Garden Edging for Your Outdoor Space
All Fenced in: Find the Right Style for Your Backyard
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Retaining wall structures often alter how water drains from your site
Retaining walls often act like a dam.
Water running through your brick, block can become unsightly due to presence of salts and weaken the structure.
Please ensure you take care to properly waterproof your wall