How To Build A Retaining Wall On A Slope

Building a retaining wall on a slope can be a tricky task depending on the steepness of the slope.

When done right, it can significantly improve the usability and aesthetics of your landscape.

Retaining walls help in preventing soil erosion, managing water runoff, and creating usable flat areas on sloped land.

In this guide, we will introduce you to the different materials you can use when building retaining walls on slopes.

Mainly, we will focus on using interlocking concrete blocks, a popular choice due to their strength, durability, and ease of installation.

What Material is Used to Build Retaining Walls?

Retaining walls can be built from a variety of materials, each offering different benefits in terms of aesthetics, durability, cost, and suitability for specific types of landscapes or engineering requirements.

Here are some commonly used materials for building retaining walls:

  • Wood
  • Concrete blocks
  • Poured concrete
  • Bricks
  • Natural stones
  • Gabion walls
  • Stone veneers
  • Interlocking concrete blocks

When building a retaining wall on a sloped terrain, the best material choice often depends on the slope's severity, the height of the wall, the soil conditions, and the desired aesthetics.

However, certain materials are generally better suited for sloped conditions due to their flexibility, strength, and ability to accommodate ground movement and drainage.

Some of the strongest materials you can use for a sloped retaining wall includes interlocking concrete blocks, poured concrete, and natural stones.

In this article, we will focus on using interlocking concrete blocks. These blocks do not require mortar and lock together to create a sturdy wall. They are easy to install and can be used for walls of various heights.

How To Build A Retaining Wall Using Interlocking Concrete Blocks

Building a retaining wall on a slope using interlocking concrete blocks can help control erosion, manage water runoff, and create functional garden spaces.

This project requires careful planning and execution due to the additional challenges presented by the sloped terrain. Here's a step-by-step guide tailored for slopes:

1. Planning and Design

Before you begin, it's crucial to assess your slope's specific conditions. How high should the wall be? What is the length of the entire wall?

Analyze the slope to determine the best location for the wall. Consider the natural flow of water and the stability of the soil.

Design your wall, considering its curve and height to match the slope's contour. Use stakes and string to outline the wall's path.

Contact your local building department for any required permits or specific regulations regarding retaining walls on slopes.

Measure the length and height of your planned wall to estimate the number of blocks, as well as the amount of base material and backfill needed.

For stability, it's generally best to keep the wall under four feet tall.

2. Prepare the Site

Start by clearing the area where your wall will be built.

Remove any debris, roots, or plants. For sloped terrains, site preparation may involve some grading to create a level starting point.

Mark the layout of the wall using stakes and string. This step ensures your wall follows the desired path and helps in visualizing the project.

3. Dig a Trench for Foundation

The foundation is critical for the stability of the wall. Since you're working on a slope, the trench must be level, so take your time with this step, using a level tool to check your work.

For walls that are at least 4 feet high, dig a trench that is about 24 inches wide and 6 inches deep. There should be an additional 1 inch depth for every 1 foot of wall height.

If the wall is less than 4 fee tall, the depth of the trench can be around 4 inches and the width about 18 inches.

Next is to level the base. Use a hand tamper or plate compactor to compact the soil in the trench.

Add a base layer of crushed stone or gravel, ensuring it's level across both the width and the slope.

This may require more material on the lower side of the slope. Compact the gravel thoroughly to create a solid, level foundation.

4. Place the Base Blocks 

Begin at the lowest point of the slope. Place the first block and use a level to ensure it's flat. This first block is critical as it sets the alignment for the rest of the wall.

Install your first row of blocks on top of your packed gravel base. Lay one block at a time, leveling all sides before moving onto the next block.

5. Rough Gravel

With your first row of blocks set and leveled, you will notice space between your trench wall and your blocks.

This space will be filled with rough gravel for drainage. This gravel will be loose compared to your gravel base. 

An elderly man making forms for the retaining wall

After each row, backfill the area behind the wall with crushed stone or gravel to ensure proper drainage. Compact the backfill before adding the next row.

6. Install Remaining Block Layers

Continue placing blocks, ensuring they interlock according to the manufacturer's instructions.

Check for level and alignment frequently, especially as you work up the slope.

Make sure there is a snug fit between blocks and no rocks or dirt between layers. Keep adding drainage gravel behind your rows of blocks as you go.

To match the slope, you may need to step the blocks back slightly with each row.

This involves starting each new row a bit behind the one below it, creating a stepped effect that follows the slope's contour.

Use a masonry saw or chisel and hammer for any cuts needed to fit the blocks, especially at the ends or to match the slope.

7. Secure Top Row Blocks

Secure capstones on the top row with construction adhesive for a finished look and added stability.

To do this, you can flip each block in the top layer, apply your adhesive, and set the block back down. Do not let dirt or rocks get in between.

Here's more information: Should You Glue Retaining Wall Blocks?

8. Backfill

After your final row is laid and sealed, you will fill the gap behind these blocks.

This time, you will use soil instead of your drainage gravel. You should be left with a clean look, ready to grow grass and other plants. 

The video below provides these steps in a detailed video, including some extra tips and tricks. 

How Do You Build A Retaining Wall On A Sloped Yard?

By using the steps provided, you can build a retaining wall on a sloped yard. There are key points to pay special attention to when working on a slope. 

When digging your trench, you must create a stepped trench when building in a sloped yard.

This step will save you from digging deeper higher in your slope while keeping your blocks level. 

When you are layering your foundation gravel and the first layer of blocks, you will complete these steps in one section of your trench before moving up the steps. The block at each step-up must be completely buried.

Watch the video below for a visual of a stepped trench. 

Read more in this blog post: How To Integrate A Vertical Garden Into A Retaining Wall

How Deep Should A Trench Be For A Retaining Wall?

As mentioned above, your trench needs to be deep enough to house 4-6 inches of gravel base and cover at least 1/2 the height of your block.

The exact depth of the trench does vary based on what size blocks you are using and how high you plan to build.

A good rule of thumb to follow is that your trench should be deep enough for your gravel base plus 1 inch for every 8 inches of wall height you planned for.

Read more in this blog post: 23 Vertical Gardening Tips That Will Take Your Green Wall To The Next Level

Which Blocks To Use For A Retaining Wall?

There are several retaining wall blocks to go through. Which blocks are best? Think about how big you plan to build your wall,  as this can help you decide. 

If you plan for a larger wall up to 3 feet high, consider using large concrete garden wall blocks.

These blocks are on the cheaper side and easy to work with. Maybe you are planning for a shorter wall.

If your wall is under 2 feet high, Pavestones are a great option. These lightweight stones are easy to move and can be found in just about any home improvement shop. 

A retaining wall with plants on the back

How Many Blocks Do I Need For A Retaining Wall?

To make things easier, many companies have created retaining wall block calculators.

These calculators will do the work for you when figuring how many blocks you will need for your project, but you need some information.

Decide on an ideal height and length of your wall, and have the dimensions of the blocks you intend to use. Plug these numbers in and let the calculator do the rest.

Do All Retaining Walls Need Drainage?

All retaining walls need drainage. The involvement of drainage depends on the material of the wall blocks.

If you have a short wall with natural joints for water to drain, the rough gravel placed behind the rows of blocks should be enough to keep water flowing. 

A weehole on the retaining wall on the garden

Blocks that are not porous whatsoever, not allowing any water flow, will need a little help.

When building a wall with these blocks, you will need to include a drainage system in your plans, no matter the wall height. 

What Is The Strongest Type Of Retaining Wall?

Retaining walls have been made from various materials, including stone, timber, concrete, and brick.

Which building material will be the strongest, leaving you with a sturdy, long-lasting wall?

The strongest type of retaining wall is concrete or stone. These materials are durable, strong, and the best option to support your landscape.

When to Call the Experts

Building a retaining wall on a slope with interlocking concrete blocks requires attention to detail and careful execution.

The steps may be more complex due to the need to accommodate the terrain's gradient.

For large or complex projects, consulting with a professional engineer or landscape architect is advisable to ensure the wall's stability and compliance with local regulations.

Looking for more wall ideas? Have a look at this blog post: How To Create A Vertical Grass Wall (7 Easy Steps)

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A retaining wall made out of decorative rocks, How To Build A Retaining Wall On A Slope

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