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A terraced garden can turn what would otherwise be an unusable sloped wasteland into a wonderful spot for relaxation and entertainment. After all, well-designed level terraces are so much more practical, not to mention beautiful, than a steep incline. You also have the option of building in several mini-garden sections to add interest and texture to your landscape.
A terrace is defined as a flat area in a garden, often supported by a retaining wall. For larger slopes, you will need to create a series of retained flattened areas, one below the other. Terracing is the ideal solution for sloped garden areas, where planting can sometimes be challenging, if not impossible. Indeed, it is rare to find a piece of land that is absolutely flat. You will find that most blocks slope to some extent and, as in this case, it is not unusual for new homeowners to discover that, in order to create a level and functional garden or entertaining area, they need to construct a terrace. Another added benefit is that you no longer need to be concerned about erosion caused by run-off from heavy rains.
Determining slope
Here's a simple method to determine the degree of slope of your land . You will need a couple of long stakes, hammer, stringline and stringline level, spirit level and tape measure.
1) Hammer one stake at the top of your slope I (Point A) and the other a few metres down the slope (Point B). Use your spirit level to ensure that the stakes are plumb.
2) Attach a stringline between the stakes Point A Z-1 and Point B, ensuring that the line is attached at the very base of Point A. Ensure the line is level between the points.
3) Measure the vertical distance between the position of the stringline at Point B and the ground. This will tell you the amount of 'fall' you have between the two points, as well as how high your retaining wall will need to be should you wish to create a level pad between them. For larger areas, it may be easier to hire a dumpy or laser level from your local hire shop. This will help you to determine your levels quickly, easily and accurately.
Grading your land
Grading or excavating simply means manipulating (eg. cutting and filling) the contours of your land prior to building your retaining walls. Generally, the height of your walls and the number of terrace flat pads you will require will be determined by just how steep your slope is.
For example, if your run is 8m and your rise is 2.4m and you want each flat area to be 2m wide, you will need four flat pads. The rise of each flat pad will be 600mm. Of course, you can play around with this so that you have three smaller garden areas of say, 1m and one larger area for entertaining of say, 5m. The choice is yours.
Depending upon your slope and chosen landscape design, you will either need to cut soil from your land, add soil or 'fill' to your land, or both. The latter, cutting and filling , makes the best use of the soil you have cut, by using it as fill. Because you require little, if any, extra soil, the high costs of bringing in clean-fill or having excess soil taken away can be eliminated, saving you a packet.
The equipment you will need to grade your soil will depend upon how much work needs to be done. Smaller jobs can be done by hand, but a mini bobcat or dingo from your local hire shop will help make short work of a heavy task.
You will need to check with your local council about how much you can cut and fill in your area. You may find that you need to present council with engineered drawings for approval before you can start work. Councils have strict regulations on retaining walls, however, if your wall is under lm in height, approval is usually not needed.
A retaining wall should always be constructed on solid ground. Never build one on an area of filled land as it will subside, making your wall highly unstable and dangerous. All footings and piers should be in solid ground.
Retaining walls
In order to support the new vertical edges of any flat pads you have created when grading, you will need to install a retaining wall. Retaining walls can be built from numerous materials, including treated pine, bricks, rocks and concrete blocks. Other than being the most cost-effective solution, treated pine is both durable and easy to work with. While masonry products tend to be more expensive than timber, there are several DIY-friendly mortarless block systems available that are easily installed by a homeowner and look very handsome to boot.
For retaining walls to pass the test of both time and weather, you not only need to build them out of strong material, you also need to ensure that their footings are securely anchored. Never underestimate the pressure water-logged soil can place on the back of your wall. Improperly constructed retaining walls can collapse, which is why many local councils have their own building codes you must comply with when constructing them.
Drainage
No matter what type of retaining wall you choose to put in, it is essential that you install a drainage system behind it. This will ensure that water does not accumulate behind the wall, and place extra stress on it. Be on the lookout for moss or algae growth, which are surefire signs that your wall does not have adequate drainage.
Installing a drainage system is as easy as applying a coat of sealer, such as Ormanoid, to the back of masonry walls, or tacking builder's plastic onto the back side of a timber retaining wall, as a waterproof barrier or lining. Then simply lay drainage pipe, known as ag-line, in the trench behind the wall , and cover it with crushed rock or blue metal, extended al'far up the back of the wall as possible, before backfilling with soil. For added protection, wrap the ag-line and crushed rock in a blanket of geotextile fabric. This prevents the soil from filtering into the gravel and clogging it up.
Timber
Timber retaining walls are constructed from a series of vertical posts sunk into the ground that are joined by lengths of horizontal rails. Timber retaining walls are the easiest to construct as you can build them one section at a time. As a general rule of thumb, you need to ensure that your post is twice the height of your retaining wall. That is, if your wall is 600mm, your post should be 1.2m long with 600mm embedded into the ground.
When fixing horizontal slabs to the posts, make sure you use galvanised screws, coach bolts or nails. To make your fixing job much easier, and to prevent splitting, pre-drill the timbers before you drive in the screws or bolts.
Soil type
Building treated pine retaining walls in highly reactive clay soils can be costly. The best walls in these situations are masonry walls on concrete footings.
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