Hills and slopes are common causes of landscaping headaches, but if properly managed, they can become some of the most beautiful features on your property.
Retaining walls help stabilize and sculpt the steep grades in your landscape to create partitions, terraces, and planting beds. While short retaining walls are simple to build yourself, taller ones require careful planning and, in some cases, help from a professional landscaper.
What a Retaining Wall Does
Simply put, a retaining wall is a structure that holds back, or retains soil. It’s built where there’s a sudden change in grade, such as at the foot of a steep hill. On one side, the wall is covered by soil, and the ground surface is near or level with the top of the wall. The ground on this side might slope upward or form a flat terrace. On the other side, the ground surface is level with the bottom of the wall, just like any standard wall.
A retaining wall is usually built for one of three purposes:
- To prevent soil erosion and landslides.
- To build a terraced yard out of a steeply sloping hill.
- To form planting beds.
Retaining Wall Materials – A Look at Some Common Choices
A retaining wall can be built out of nearly any material that can be used for a standard landscaping wall, but the size of the wall you plan to build influences your options. If possible, find out how the material looks in a completed wall before you decide to buy.
Manufactured concrete blocks and stones are designed for strength, durability, and quick, easy assembly. To build with these blocks, you’ll only need to stack them and fasten them with pins or clips. Many blocks have interlocking edges that don’t require additional fasteners. Unlike natural stone, manufactured blocks are consistent in size, so you won’t need to guess at the most stable, secure way to fit them together.
While the idea of manufactured blocks might call to mind boring gray cinder blocks, today’s designs come in a variety of shapes, colors, and textures. They’re consistent in appearance, creating a wall with an attractive, professionally made look.
Because no two stones are alike, using natural stone will give you a wall that’s completely unique to your property. Stone is especially well suited to older homes built with primarily natural materials. Sandstone, limestone, and granite can create a wall with a refined, formal appearance, while fieldstones or river rock have a casual, rustic charm.
On the downside, working with natural stone is time-consuming because you’ll need to find stones that fit tightly together and will sit stably when the wall is complete. Stones can be secured with mortar, although a very short wall might not require it. The cost depends on the type of stone you use.
Standard landscape timbers are lengths of wood typically 8 feet long and between 4 to 10 inches wide. They’re most often available in brown or green. For a retaining wall, you’ll need timbers with a .40 treatment rating, which hold up to ground contact, and cut ends should be sealed with preservative to stave off rot.
A timber wall is built by stacking the timbers horizontally and fastening them with long spikes or screws. These walls complement wood homes well, especially those with a rustic look. Timbers are among the most affordable retaining wall materials and often cheaper than using manufactured blocks.
How to Build a Strong and Beautiful Retaining Wall
Unlike an ordinary wall, a retaining wall must hold up under the weight of the soil behind it and the moisture coming from that soil. A poorly built retaining wall is likely to crack, lean or bulge. Investing some time in planning will help you build a longer lasting wall.
Most short residential retaining walls are gravity walls, which rely on their own weight to hold back the soil behind them. These are best for heights of 4 feet or less. For a height of 6 to 8 feet, a cantilever wall that relies on leverage is a good choice.
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Much taller walls or those holding back especially heavy loads generally require reinforcement in the form of sheet pile (planks), anchors embedded in the ground or counterforts (buttresses).
Plan it Out
Sketch out a drawing of your wall that includes all curves, corners, junction points with other structures, steps, setbacks, and any other features. Draw in finishing touches such as lighting, built-in seating, a fountain or a fireplace you might want to incorporate into the wall. Write the dimensions on your sketch so you can estimate the number of blocks you’ll need for each section of the wall.
Decide on the wall’s height, which determines the load it can bear. A wall of 3 to 4 feet is strong enough for most moderate slopes and won’t need additional support. For a steep slope, you can create a series of terraces using several short walls. If you prefer a single tall wall, consult with a landscaper on the best way to construct it.
Build in Stages
When you’re ready to build, start by creating a stable base using a layer of compacted sand or gravel that’s at least 6 inches deep. This should be set below ground level, and the taller the wall, the deeper the base should be.
Then lay your first course of building material, whether blocks, stones or timber. Check that it’s level before laying the next course. This helps keep the wall stable.
After you’ve laid several courses of blocks or other material, start adding backfill. Backfill is the material behind the wall. For good drainage, add at least 12 inches of granular material such as gravel or pebbles between the wall and the soil. Compact the backfill to minimize settling. Add backfill as you build the wall, rather than after the wall is completed.
Depending on your soil type and climate, you may also need a perforated pipe installed within the backfill along the bottom of the wall to improve drainage. Weep holes are another option. If the wall will need extra protection from moisture, consider installing a waterproof membrane on the back of the wall.
Estimating Your Retaining Wall Costs
The cost to build a retaining wall depends on the materials you choose, the complexity of the project, and whether you plan to do it yourself or hire a landscaper.
The cost of the typical retaining wall on a residential property averages a little over $5,000. That said, in most parts of the country, you can have a simple, short wall with a low-cost material such as timbers or plain concrete blocks built for just over $1000. It could be even less if you do all the work yourself.
Your costs will increase if the site is difficult to access, the wall is tall enough to require anchors or other supports or if specific tools or materials are needed. In these situations, hiring a landscaper is the best option for getting a durable wall built in a reasonable time frame.
Certain building site conditions and other factors can raise the cost of your project. For example, very steep inclines must be carefully graded to prevent water from ponding at the top of the wall and causing damage.
Clay, silt, and other soils that retain water will place unusually high pressure on the wall. A retaining wall for this soil type might require fabric facing or other material that evenly distributes the soil’s weight. If you decide to use large boulders in your wall, your landscaper might need additional manpower to move them.
A tall, complex wall made from granite or another luxury material can cost you $12,000 or more. For a wall like this, you’ll almost always need guidance from a landscaper.
Building a retaining wall of 4 feet or less is a fairly simple job as long as you plan ahead. With low-cost material such as fieldstones or timbers, it’s also a budget-friendly way to improve your home’s landscaping. If you’re considering a taller wall or one with complex features, consult with a landscaper to determine the design that will best meet your needs.