How to Build an Interlocking Concrete Block Retaining Wall in 10 Steps

Whether you’re looking to elevate your home’s curb appeal, or you’re concerned about mitigating erosion in a sloped area of your yard, installing an interlocking concrete block retaining wall may be the answer. 

We’ll explain how to build a retaining wall with interlocking concrete blocks in 10 steps, cover factors to consider before you break ground, and introduce you to our extensive collection of heavy-duty Unilock concrete pavers for your retaining wall. 

Explore retaining wall pavers at Unilock!

What is a Retaining Wall?

A retaining wall is a structure that is designed to hold back the pressure from rocks or soil that are moving downhill. 

In addition to battling gravity, a retaining wall can help to increase your property’s value and serve as an aesthetically pleasing backdrop for other outdoor features. 

What are Interlocking Concrete Wall Blocks? 

Interlocking concrete wall blocks are concrete blocks designed to fit together like a puzzle, in order to build a retaining wall. 

These concrete blocks are composed of a cement and aggregate mixture that is molded, then air-dried.  To form the wall, concrete blocks are laid side-by-side and interlocked with each other, utilizing the grooves and ridges on their edges.

Tools You’ll Need to Build an Interlocking Concrete Block Retaining Wall 

Interlocking concrete blocks allow you to build a retaining wall that’s both secure and aesthetically pleasing. Before you roll up your sleeves and get to work, you’ll need to prepare some tools and equipment, including: 

• Shovel: A tool with a broad flat blade used to dig or lift loose materials such as soil, sand or gravel. 
• Rake: A tool with a toothed bar used to distribute loose aggregate materials. 
• Gravel: A loose aggregate of rock fragments. 
• Sandy soil: A type of soil that is light, warm and dry, allowing quick water drainage. 
• Plate compactor: A machine used to compact soil, gravel or asphalt, consisting of a heavy steel plate mounted to a machine that vibrates, compressing the material. 
• Rubber mallet: A type of hammer with a soft, non-metallic head used to level blocks or pavers. 
• Circular saw: A power tool that is used to cut thick materials, such as interlocking concrete blocks or pavers. 
• Wheelbarrow: A tool that consists of a shallow tray or bucket used to transport materials such as crushed stone, gravel, or sand to the work area.  
• Level: A tool used to determine whether a surface is horizontal (level) or vertical (plumb). A level ensures that the ground and blocks are level throughout the installation process.  
• Polymeric sand: A type of fine sand used to fill the gaps between the interlocking concrete blocks or pavers and stabilize them. 
• Safety gear (P.P.E.): Personal protective clothing that includes gloves, safety glasses or goggles, and other gear designed to protect your body when using power tools. 

How to Build an Interlocking Concrete Block Retaining Wall in 10 Steps

Up for a little DIY? Here’s the step-by-step process. 

1. Design the Retaining Wall  

First, sketch out your interlocking wall on a piece of graph paper. Then, determine the height, base and depth of the retaining wall. 

• Height: The height of a non-structural retaining wall, such as a low garden wall or planters, generally do not exceed 24” in height, or 2 feet. A two-foot wall can help you prevent soil erosion on a slight slope or highlight landscape features, such as a walkway or flower bed. Structural walls, which generally exceed 2 feet, or 24” in height, must be engineered by a qualified engineering professional. Always check with local building codes before you start building a structural wall. Structural walls are generally designed to mitigate soil erosion on a steeper slope, and hold up embankments or driveways. Check out our guide on how to install pavers on a slope. 
• Base: If soil erosion is a concern, you’ll need a solid foundation of gravel. You can also use sandy soil as a base for easy drainage. If you use clay soil, it can put pressure on the wall, leading to instability and potential failure. 
• Depth: To allow for proper drainage, and to accommodate for the base material, it is recommended to make the base of the retaining wall at least 12 inches deep and 6 inches wider than the wall’s planned dimensions. 

2. Secure a Building Permit

If you’re building a retaining wall that’s over 4 feet tall, you may need to obtain a building permit, if required by the local municipality. 

This ensures your retaining wall is safe and complies with local regulations. Taller walls have a higher risk of failure, posing potential hazards to you and your property. 

3. Mark the Layout

Once you secure your building permit and are set on building a wall less than 4 feet tall, mark your layout using grade stakes and string line. 

4. Dig a Trench

Dig a trench along the area where the wall will be built. The depth of the trench must match the height of a single block, plus three inches for a gravel or sandy soil base. 

For example, if you’re using 6-inch blocks, you’ll need a trench that is nine inches deep. 

5. Lay the Base

Next, add a layer of gravel or sandy soil — at least 12 inches deep — to the bottom of the trench to provide drainage. 

Then, install a layer of geotextile fabric on top of the base to prevent soil from filtering through the gravel or sandy soil. 

6. Lay the First Interlocking Concrete Blocks

Arrange the first level of interlocking concrete blocks at the base. Starting at the center of the wall, work your way out to the sides. Make sure the right side of the concrete block is facing you. 

Use a rubber mallet to keep the concrete blocks in place, ensuring they are level. 

For straight walls, use strings that are attached to stakes to keep the edges of the blocks aligned, and a level to ensure they are even. 

Typically, interlocking concrete blocks don’t need adhesive for them to lay on top of each other, provided the wall system designed with a tongue and groove systems that locks units to the one above. 

7. Lay the Subsequent Interlocking Concrete Blocks

Start the second layer of interlocking concrete blocks at the center of the wall, laying the blocks in rows. Each row should offset by half the length of the block, creating a pattern that is reminiscent of brickwork. 

Make sure every block is pulled forward to ensure the rear edge touches the bottom blocks. 

For visual reference, the interlocking concrete block retaining wall can follow the running bond pattern. 

Unilock tip: Need more design inspiration? Check out our blog about top paver pattern trends! 

8. Cut Blocks to Fit

When you reach the edge of the retaining wall, use a circular saw to cut the blocks as needed, depending on the shape of the wall. 

9. Install Landscape Fabric (Optional)

Once all the interlocking concrete blocks are in place, you can line the back of the trench (behind the retaining wall) with landscape fabric to prevent soil and weeds from getting through the blocks. 

10. Backfill the Retaining Wall

The final step is to backfill the wall to prevent drainage issues. 

To add backfill behind your retaining wall: 

• Excavate the area behind the retaining wall. You’ll want to dig around 12 inches deep for the backfill. 
• Add layers of gravel, compacting each layer with a tamper or compactor. 
• If you’ve added landscape fabric at the back of the trench, trim it off as you reach the top of the backfill. 
• Once the gravel reaches the top of the retaining wall, use a rake to level the surface and create a slight slope away from the wall, to prevent erosion. 
• Finish the backfill with 6 inches of topsoil. 

Build your retaining wall with Unilock. Explore our pavers!

Considerations for Building a Retaining Wall With Interlocking Concrete Blocks 

Building an interlocking concrete block retaining wall requires careful planning. Working with a qualified engineer or contractor can ensure that your interlocking wall is safe and is built to survive wear and tear over time. 

1. Hire a Professional

If you plan on building a wall over 3 feet, have limited DIY experience and live in an area with surface runoff issues (think big cities with plenty of paver surfaces like roads or parking lots), consider hiring a professional, like a structural engineer. 

Building a four-foot (or higher) retaining wall poses potential safety hazards that are associated with taller structures, such as a collapsing wall, falling debris and tripping hazards. 

A structural engineer has the expertise and equipment to: 

• Design the wall: A structural engineer has a deeper understanding of soil pressure and proper drainage — components that are paramount to a stable and effective wall. 
• Utilize special equipment: A structural engineer can utilize special equipment to build your retaining wall, such as a circular saw. 
• Ensure proper drainage: A well-designed drainage system is key to prevent hydrostatic pressure, which can lead to wall failure. 

2. Prepare the Base

Properly preparing the base of your retaining wall is vital in the entire installation process. 

The foundation evenly distributes the weight of the wall across the soil, minimizing stress and preventing movement over time. 

3. Ensure Proper Drainage

Proper drainage is crucial to your retaining wall’s performance. The retaining wall should allow water to drain away from the wall and not accumulate behind it, in order to prevent instability and failure. 

4. Select the Correct Backfill Material

 Choose the correct backfill material, such as gravel, to provide proper drainage. 

5. Consider Building a Low Retaining Wall

Height: If you’re going for the DIY route, consider building a three-foot retaining wall. Taller walls (think 4 feet or more) will require building permits and a stronger base. 

Contact a Unilock Authorized Contractor to ensure your retaining wall is safe and built for optimal performance. 

6. Check Local Regulations

It’s paramount to check with your local building code to ensure that your retaining wall is built to meet safety standards.

Explore Heavy-Duty Unilock Pavers for Your Retaining Wall 

At Unilock, we have dedicated over 50 years to creating top-notch outdoor spaces across Canada and the United States. As an industry-leader in the retaining wall and paving stones industry, we manufacture high-grade products, while providing unmatched customer service. 

Our heavy-duty pavers act as interlocking concrete wall blocks, which are built to ensure your retaining wall is stable, secure and aesthetically pleasing. 

From stacked blocks that resemble natural stone to aesthetically pleasing heavy-duty wall pavers, turn your structural stronghold into a showstopping project with our retaining wall pavers. 

To build your retaining wall, explore: 

• Dura-Hold II ® – Dura-Hold II® is ideal for lower retaining walls. It gives the appearance of individual pavers in each unit, thanks to joints that cast bold shadows. 
• Pisa™ – Pisa™ is ideal for curved retaining walls. It provides rustic and timeless beauty, thanks to its rough and aged finish. 
• Sienastone™ – Sienastone™ is a large-scale wall block that offers a bold and dramatic look. Its clean and long lines make it a striking alternative to plain-looking blocks. 

For our classic retaining wall pavers, Classic Coat technology adds a factory-sealed coating that enhances the color of our Classic pavers while helping protect the surface from wear. These factory-sealed pavers help you save the time and expense of after-market sealing. 

Whether you’re looking to create a retaining wall that exudes timeless warmth or offers a modern twist, you can find your unique style with our comprehensive selection at Unilock!

Need expert help building your retaining wall? Find a Unilock Authorized Contractor