How to Install a Retaining Wall: A Step-by-Step Guide

A retaining wall serves both practical and aesthetic purposes in landscaping. It can prevent soil erosion, create level terraces on sloping terrain, and enhance the overall appearance of your outdoor space. Installing a retaining wall might seem like a daunting task, but with the right guidance and a bit of patience, it’s a manageable DIY project. In this step-by-step guide, we’ll walk you through the process of installing a retaining wall that’s not only functional but also adds beauty to your landscape.

Materials and Tools You’ll Need

Before you begin, gather the following materials and tools:

Materials:

• Retaining wall blocks or stones
• Gravel
• Landscape fabric
• Drainage pipe
• Sand or gravel for leveling
• Polymeric sand (optional)
• Wall adhesive (if required)

Tools:

• Shovel
• Wheelbarrow
• Tape measure
• Level
• Rubber mallet
• Spade
• Landscape fabric staples
• Circular saw (if cutting blocks)
• Compactor (plate or hand tamper)
• Carpenter’s pencil
• Safety gear (gloves, safety glasses)

Step 1: Plan Your Retaining Wall

Determine the Wall’s Purpose and Location

Before you begin installing a retaining wall, it’s crucial to have a clear understanding of why you need it and where it should be situated. Here’s why this step is essential:

• Purpose: Identify the primary purpose of your retaining wall. Is it to prevent soil erosion, create level terraces on a sloping landscape, or enhance the aesthetic appeal of your outdoor space? The purpose will influence the design and materials you choose.
• Location: Assess the specific location where the wall will be built. Consider the following factors:
  • Slope: Examine the slope of the terrain. A steep slope may require a taller and more robust retaining wall to hold back soil effectively. A gentler slope might only need a low-profile wall for decorative purposes.
  • Soil Type: Understand the type of soil in the area. Different soils have varying drainage properties and load-bearing capacities. Sandy soils drain quickly but may require greater stability, while clay soils retain water but can be heavier and more stable.
  • Drainage: Evaluate the existing drainage patterns. Poor drainage can lead to water buildup behind the wall, exerting additional pressure. Ensure that proper drainage measures are in place to prevent this.

Calculate the Height, Length, and Thickness

Accurately calculating the dimensions of your retaining wall is a fundamental step in the planning process. These calculations inform the materials and construction techniques you’ll use. Here’s how it works:

• Height: Measure the height from the lowest point of the slope to the desired level where the wall will end. This measurement is crucial because taller walls require stronger materials and construction methods. Walls over a certain height may also necessitate professional engineering and permits.
• Length: Determine the total length of the wall. This measurement helps you estimate the number of blocks, stones, or materials required for the project.
• Thickness: Decide on the thickness or width of the wall. Thicker walls offer more stability, especially for taller structures or when retaining heavy loads. The thickness can also affect the wall’s appearance, so consider both functional and aesthetic aspects.

Check Local Regulations and Obtain Permits

Before embarking on your retaining wall project, it’s essential to research and comply with local regulations and building codes. Here’s why this is vital:

• Compliance: Building codes and regulations exist to ensure safety and adherence to specific standards. Failing to comply can result in legal issues, fines, or even the removal of the wall.
• Permit Requirements: Different jurisdictions may have varying permit requirements based on factors like wall height, location, and purpose. Some municipalities require permits for any retaining wall, while others have specific height thresholds. Always check with your local building department to understand what permits are needed for your project.
• Professional Guidance: If your retaining wall exceeds certain height limits or structural complexity, you may need to involve a professional engineer in the design and approval process. Local regulations often dictate when engineer involvement is mandatory.

The initial steps of determining the wall’s purpose, assessing the site, calculating dimensions, and checking local regulations are critical for a successful retaining wall project. Proper planning ensures that your wall not only fulfills its intended function but also complies with safety standards and legal requirements.

Step 2: Excavate and Level the Ground

Mark the Layout of Your Wall Using Stakes and String Lines

Before you start digging the trench for your retaining wall, it’s crucial to establish a clear and precise layout. This step sets the foundation for the entire project. Here’s an elaboration on this process:

• Gather Your Materials: You’ll need wooden stakes, string lines, a tape measure, a carpenter’s level, and a rubber mallet.
• Determine the Wall’s Position: Decide where your retaining wall will begin and end. Use the stakes to mark these points on the ground. Ensure they are accurately placed, as any errors at this stage can affect the wall’s alignment and stability.
• Set Up String Lines: Attach string lines between the stakes, creating a straight and level guideline for your wall’s alignment. Use a carpenter’s level to ensure the strings are perfectly horizontal. These string lines will serve as reference points during excavation and construction.
• Check for Proper Alignment: Use the string lines to verify that the wall’s position and layout match your initial plans. Make any necessary adjustments to the stakes or string lines to achieve the desired alignment.

Excavation for a Retaining Wall

With the layout in place, it’s time to start digging the trench for your retaining wall. Here’s an elaboration on this crucial step:

• Prepare for Excavation: Ensure you have the necessary tools, including a shovel, spade, and wheelbarrow, to facilitate the excavation process.
• Follow the String Lines: Start digging along the path marked by the string lines. The trench should be wide enough to accommodate the entire width of your retaining wall blocks or stones.
• Determine Depth: The depth of your trench depends on various factors, primarily the height of your wall. As a general rule of thumb, for walls under 3 feet in height, a trench depth of 6-8 inches may suffice. For taller walls, it’s advisable to dig a deeper trench, around 10-12 inches. This depth provides stability and ensures that the first course of blocks is properly embedded below ground level.
• Maintain a Level Trench: As you dig, use a carpenter’s level to ensure that the trench remains level and uniform in depth. A level trench is essential for the proper alignment of your retaining wall.

Compact the Soil Using a Compactor to Create a Stable Base

After excavating the trench, the next crucial step is to prepare a solid and stable base. Here’s an elaboration on this process:

• Clear Debris: Remove any loose soil, rocks, or debris from the trench to ensure a clean and even base.
• Bring in the Compactor: A compactor, either a plate compactor or a hand tamper, is essential for compacting the soil. The compactor uses mechanical force to densely pack the soil, creating a stable foundation for your retaining wall.
• Compact in Layers: To achieve optimal compaction, work in thin layers. Add a few inches of soil to the trench, then use the compactor to firmly compact it. Repeat this process until you’ve compacted the entire depth of the trench.
• Check for Level: Periodically check that the base remains level as you compact the soil. Any variations in the base can affect the wall’s alignment and stability.
• Test the Base: After compacting, perform a final test to ensure the base is level and stable. Place a long wooden board or a straight piece of lumber across the trench and use a level to check for any dips or unevenness. You can also use a laser level with a grade rod to make this easier.

By meticulously marking the layout, excavating a proper trench, and compacting the base, you establish a solid foundation for your retaining wall. These steps are critical to ensure the wall’s structural integrity and longevity, as they provide a level and stable surface for the placement of retaining wall blocks or stones.

Step 3: Install a Base of Gravel

• Spread a Layer of Gravel: Begin by spreading a layer of gravel along the bottom of the trench. The thickness of this layer should align with your project’s requirements, typically between 4 to 8 inches, depending on the wall’s height.
• Even Distribution: Use a rake or a shovel to distribute the gravel evenly across the entire trench. Ensure that it reaches the desired thickness uniformly.
• Compact the Gravel: To create a stable base, compact the gravel thoroughly. This step is essential for maintaining the structural integrity of the wall.
• Compact in Layers: For optimal compaction, work in thin layers. Add a few inches of gravel at a time, then use a mechanical compactor or hand tamper to compact each layer. Make multiple passes over each section to achieve proper compaction.
• Check for Level: Periodically use a level to check that the gravel base remains level throughout the trench. A level base is critical for the proper alignment of the retaining wall.
• Smooth the Surface: After compacting the gravel, use a rake or shovel to smooth the surface. This ensures an even and stable foundation for the retaining wall blocks or stones.
• Final Check: Before proceeding to the next step, make a final check to ensure that the gravel base is both level and uniformly compacted. Address any irregularities or depressions as needed.

Step 4: Lay the First Course of Blocks – Building a Solid Foundation

Now that you’ve prepared a sturdy gravel base, it’s time to lay the first course of retaining wall blocks or stones. This foundational row sets the stage for the rest of your retaining wall. Here’s a detailed guide on how to accomplish this step:

• Place the First Row of Blocks or Stones:
  • Start by positioning the first block or stone at one end of the trench, directly on the compacted gravel base. Ensure that it’s aligned with your layout plan and that its front face is flush with the string lines set up in the previous steps.
  • Continue placing the blocks or stones one by one along the trench, making sure they are touching each other snugly. The first row should be level both from side to side and front to back. Use a carpenter’s level to verify the levelness.
  • If you encounter obstacles like irregularly shaped blocks or stones, be strategic in their placement. Ensure that the top surfaces of the blocks are even and level with one another. You may need to make slight adjustments to the gravel base under individual blocks to achieve this.
• Check for Level and Alignment:
  • Throughout the process of placing the first row, it’s crucial to frequently check for level and alignment. Use a carpenter’s level placed across the top of the blocks or stones to ensure that they are at the same height. Adjust individual blocks as needed to achieve a level first course.
  • Use string lines as reference guides to ensure that the row remains straight and aligned with your initial layout. String lines help maintain a consistent front-to-back alignment, preventing the wall from veering off course.
  • Check for level from side to side and from front to back as well
• Use a Rubber Mallet for Adjustments:
  • In some cases, you may need to make minor adjustments to the blocks’ positions to achieve the desired level and alignment. A rubber mallet is a handy tool for this purpose. Tap the blocks gently into place, ensuring they fit snugly against one another and remain level.
  • When tapping blocks with a rubber mallet, be careful not to apply excessive force, as this can damage the blocks or disrupt the gravel base. Use a soft touch and check for level frequently.
• Drainage Pipe
  • Install a drainage pipe every 2’ in vertical height. The pipe should go across the entire length of the wall and should have at least a 2 degree slope. Place an outlet every 30’ to allow water to escape through the face of the wall 
  • There should also be a final outlet on the lowest side of the wall to release the excess water not drained through the wall face
• Add Gravel Behind the Wall for Drainage:
  • Behind the first course (row) of the retaining wall, create a space filled with gravel. This gravel layer serves as a drainage zone that allows excess water to drain away from the wall. Adequate drainage is essential to relieve hydrostatic pressure, which can build up behind the wall and compromise its stability.
  • The depth of the gravel drainage zone should be at least 12”

By carefully following these steps, you’ll successfully lay the first course of retaining wall blocks or stones. This foundational row provides the necessary stability and alignment for the rest of the wall. Ensuring that this initial course is level, aligned, and properly staggered is essential for the overall structural integrity and appearance of your retaining wall.

Step 5: Build the Wall

As you move beyond the initial course of retaining wall blocks or stones, the process of stacking and interlocking them becomes crucial. This phase of construction builds the height and strength of your retaining wall. Here’s a detailed elaboration on how to proceed:

• Interlocking the Blocks Securely:
  • Place the next course of blocks or stones directly on top of the first row. Ensure that the seams or joints between blocks in the second row are offset from those in the first row. This staggering creates a stable and interlocking pattern, preventing the wall from collapsing or shifting.
  • Interlocking can involve various techniques depending on the design of your retaining wall blocks. Some blocks have built-in features that facilitate interlocking, while others may require you to insert pins or connectors to secure them together. Follow the manufacturer’s instructions for your specific block type.
• Drainage Pipe
  • Install a drainage pipe every 2’ in vertical height. Follow the above specifications for the first course installation
• Gravel Behind Every Layer for Drainage:
  • Behind all the remaining courses, continue to backfill with gravel
  • The depth of the gravel drainage zone should stay at least 12”
• Adhesive Between Layers (When Required):
  • Some retaining wall designs and materials may necessitate the use of adhesive between layers. This adhesive, typically a construction-grade adhesive or glue, enhances the bond between the blocks, increasing the wall’s stability and load-bearing capacity.
  • If your retaining wall requires adhesive, apply it according to the manufacturer’s recommendations. Usually, a bead of adhesive is applied along the top of the blocks in the lower row before placing the next row on top.
• Offset the Seams in Each Row:
  • Continuing to offset the seams or joints in each successive row is essential for both stability and aesthetics. Staggering the seams creates a “brickwork” effect, referred to as a running bonD, distributing the load more evenly and preventing vertical joints from aligning.
  • To achieve this, simply ensure that the seams or joints in the blocks of one row are positioned over the center of the blocks in the row beneath. This staggered pattern increases the wall’s resistance to lateral pressure from retained soil.
• Checking for Level and Alignment:
  • Maintain a vigilant eye on the levelness and alignment of your retaining wall as you build upward
  • If you notice any deviations from level or misalignment, make necessary adjustments by tapping the blocks gently into place with a rubber mallet
• Install Geogrid:
  • Geogrid is installed in between the courses of the retaining wall and the backfill material. Its exact placement depends on the wall’s design and engineering specifications. Generally, it is placed every 2 courses
  • Roll out the geogrid material across the top of the course and across the layer of backfill, ensuring it covers the entire width of the wall. Trim it as necessary to fit the wall’s dimensions. The geogrid should be positioned parallel to the face of the wall.
• Repeat the Process:
  • Continue stacking blocks or stones, following the same principles of interlocking, staggering seams, and checking for level and alignment with each successive row. Repeat these steps until you reach the desired height for your retaining wall.

By following these guidelines for stacking blocks, you’ll gradually build the height and strength of your retaining wall while ensuring its stability and aesthetic appeal. Proper interlocking, staggering of seams, and frequent checks for level and alignment are fundamental to the success of your retaining wall project.

Step 6: Finish and Secure

• Prepare the Surface: Ensure the top of the retaining wall is clean, level, and free of debris. Any irregularities on the wall’s surface should be addressed to ensure a proper fit for the coping stones.
• Apply Adhesive: Use a construction-grade adhesive or mortar to secure the coping stones in place. Apply the adhesive evenly along the top of the wall, where the coping stones will be positioned.
• Place the Capstones: Carefully place the coping stones on top of the adhesive, aligning them with your top course. Press them down firmly to ensure a secure bond with the wall.
• Level and Adjust: Check the levelness and alignment of each coping stone as you place it. Use a carpenter’s level to verify that the caps are even and follow the intended slope of the wall.
• Allow to Set: Use a rubber mallet to secure the adhesive and then allow the adhesive or mortar to set and cure according to the manufacturer’s recommendations. This ensures a strong and lasting bond between the coping stones and the wall. 
• Final Inspection: After the adhesive has fully set, perform a final inspection to ensure that the coping stones are securely in place, level, and provide the desired overhang for water drainage.

By capping your retaining wall with appropriate coping stones or caps, you not only enhance its appearance but also protect it from water damage, weed growth, and potential structural issues. This final step adds both functionality and aesthetic value to your landscaping project, creating a visually pleasing and structurally sound retaining wall.

Conclusion

Installing a retaining wall can transform your landscape while providing functional benefits. By following this step-by-step guide and paying attention to detail, you can create a durable and visually appealing retaining wall that enhances the beauty and functionality of your outdoor space.