If you’re dealing with erosion or just want to create a flat, usable space in your yard, learning how to build a retaining wall on a slope is a crucial skill. This step-by-step slope construction guide will walk you through the entire process, from planning to the final touch.
A well-built retaining wall is more than just a pile of blocks. It’s a structural solution that holds back soil, prevents erosion, and adds beauty to your landscape. With the right materials and careful work, you can tackle this project yourself and enjoy the results for years to come.
How to Build a Retaining Wall on a Slope – Step-by-Step Slope Construction Guide
This main guide breaks down the construction into clear, manageable phases. Don’t skip the planning steps—they are the foundation of a safe and lasting wall.
Phase 1: Planning and Design
Good planning prevents costly mistakes. Start by answering a few key questions.
First, check local building codes. Many areas require a permit for walls over a certain height, often 3 or 4 feet. Contact your local building department to be sure.
Next, choose your materials. Common options include:
- Concrete blocks: Interlocking systems are easiest for DIY. They’re uniform and designed for stability.
- Natural stone: Offers a classic look but requires more skill to lay.
- Timber: A cost-effective option, but has a shorter lifespan than stone or concrete.
Finally, calculate the amount of material you’ll need. Measure the total length and planned height of your wall. Your block supplier can help convert this into the number of blocks, plus base material and drainage stone.
Phase 2: Gathering Tools and Materials
Having everything on site before you start will make the job smoother. Here’s a basic list:
- Retaining wall blocks
- Landscape fabric
- Crushed gravel or road base (for the base trench)
- Drainage gravel (¾-inch crushed stone)
- Shovel, spade, and a digging bar
- Tamper (hand or mechanical)
- Level (a 4-foot level is ideal)
- Rubber mallet
- String line and stakes
- Safety glasses and gloves
Phase 3: Site Preparation and Layout
Now, it’s time to get your hands dirty. Mark the location of your wall’s front face with string and stakes. For a curved wall, use a garden hose to outline a smooth curve.
Excavate the area behind this line. You need to dig back into the slope to make room for the wall blocks and the drainage gravel behind them. A good rule is to dig a trench as wide as the block plus 12 inches for working space.
Digging the Base Trench
This is the most important step for a level wall. Dig a trench for the first course of blocks. It should be deep enough to bury the first course halfway, plus 4 to 6 inches for a gravel base.
For example, if your block is 8 inches tall, dig a trench about 8 inches deep. The bottom of the trench must be level from end to end, and it should follow the slope of the ground (step it down on a steep slope). Use your level and a long, straight board to check this constantly.
Phase 4: Building the Base Layer
Fill the trench with 4 to 6 inches of crushed gravel. Spread it evenly and then tamp it down thoroughly until it’s rock-solid. This compacted base prevents settling.
Add a layer of coarse sand or a dusting of gravel on top of the tamped base. This helps you make fine adjustments when setting your first course of blocks. It’s crucial this first course is perfectly level, both front-to-back and along its length.
Place your first block. Check it with the level. Tap it down with the rubber mallet if needed. Continue laying the first course, checking each block and ensuring they are level with each other. Your string line will guide the front face.
Phase 5: Installing Drainage
Water pressure is the main reason retaining walls fail. Proper drainage is non-negotiable. After the first course is laid, lay a length of perforated drainage pipe behind the blocks, along the entire wall.
Ensure the pipe slopes slightly (about 1 inch for every 8 feet) to one end or a center point, where it can daylight and drain away from the wall. Next, fill the space behind the blocks with ¾-inch drainage gravel, covering the pipe completely.
Before backfilling with soil, lay landscape fabric over the gravel. This fabric keeps soil from washing into and clogging the gravel drainage zone, which would render it useless.
Phase 6: Laying Subsequent Courses
Start your second course. Most interlocking blocks require a staggered pattern, like bricks. This offsets the vertical joints, which adds tremendous strength. Follow the manufacturer’s guidelines for the specific lip or pin system.
As you build up, frequently check for level and use the string line to keep the face straight. Also, check for “batter”—the slight backward lean that most walls need. Many blocks are designed with this built in.
Continue this process: lay a course, check for level and alignment, then backfill behind it with more drainage gravel as you go. Tamp the gravel gently behind each course before moving to the next one.
Phase 7: Backfilling and Finishing
Once the final course is placed and you’ve added the last of the drainage gravel, you can fold the landscape fabric over the top of the gravel. Now, backfill the remaining space with native soil, compacting it in layers.
If you’re using cap blocks, adhesive them to the top course according to the product instructions. This gives a finished look and helps hold the top row in place. Finally, you can revegetate the area above and below the wall to help with erosion control and blend the wall into your landscape.
Common Mistakes to Avoid
Even with a guide, it’s easy to make a few errors. Here’s what to watch out for:
- Poor drainage: Never skip the gravel and pipe. Soggy soil will push your wall over.
- An unlevel base: This mistake compounds with each course, leading to a crooked wall.
- Not staggering joints: This creates a weak line that can crack under pressure.
- Ignoring local codes: A failed inspection can mean tearing down your hard work.
FAQ: Your Retaining Wall Questions Answered
How deep should the base be for a retaining wall?
A solid base is critical. Dig a trench deep enough to bury the first course halfway, plus 4-6 inches for a compacted gravel base. On unstable soil, you may need to go deeper.
Do I need drainage behind a small wall?
Yes, even for short walls. Any wall that holds back soil needs a way for water to escape. At minimum, use a gravel backfill. For walls over 2 feet, always include a perforated drain pipe.
What is the easiest retaining wall block to use?
Interlocking concrete block systems are the most DIY-friendly. They have a built-in lip or pin system that automatically sets the batter and ensures a secure fit, making them simpler to keep level and stable.
Can you build a retaining wall on a steep slope?
You can, but it requires careful stepping of the base trench to follow the ground’s slope. For very steep or tall walls, it’s wise to consult an engineer to ensure the design can handle the pressure.
How do you step a retaining wall on a slope?
To step a wall up a slope, you build sections at different levels. The lower wall is built first. Then, the upper wall is set back so part of it’s base rests on the lower wall’s backfill, creating a terraced effect. Always ensure each section has it’s own proper base and drainage.