Sloped yards can be beautiful, but they often come with the challenge of erosion. Rain washes away soil, creating unsightly ruts and potentially undermining your landscaping. A well-built retaining wall is a fantastic solution. Not only does it prevent erosion, but it also adds visual appeal and can create usable space in your yard. This guide provides a comprehensive overview of how to build a retaining wall on a slope, covering everything from planning and preparation to construction and finishing touches.
Understanding Retaining Walls and Their Purpose
Before diving into the construction process, it's important to understand what a retaining wall is and what it does. A retaining wall is a structure designed to hold back soil or other materials. It counteracts the force of gravity pushing the soil downward and outward.
Types of Retaining Walls
There are several types of retaining walls, each suited to different situations:
- Gravity Walls: These walls rely on their own weight to resist the pressure of the soil. They are typically made of concrete, stone, or large retaining wall blocks.
- Cantilever Walls: These walls have a footing that extends horizontally beneath the soil they are retaining. The weight of the soil on the footing helps to stabilize the wall. These are often made of reinforced concrete.
- Sheet Pile Walls: These walls are made of interlocking sheets of steel, vinyl, or wood that are driven into the ground. They are best suited for temporary support or in areas with soft soil.
- Buttressed Walls: These walls have vertical supports (buttresses) on the back side of the wall that provide additional stability. They are typically made of reinforced concrete.
- Anchored Walls: These walls use cables or rods anchored into the soil behind the wall to provide extra support. They are useful for very tall walls or in areas with unstable soil.
For most residential projects, gravity walls constructed with retaining wall blocks are the most practical and DIY-friendly option. This guide will primarily focus on this type of retaining wall.
When Do You Need a Retaining Wall?
A retaining wall is necessary when you want to:
- Prevent soil erosion on a slope.
- Create level areas in a sloped yard for patios, gardens, or play areas.
- Improve drainage by redirecting water flow.
- Add visual interest and definition to your landscape.
Planning and Preparation: Setting the Foundation for Success
Careful planning is crucial for a successful retaining wall project. This stage involves assessing the site, determining the wall's dimensions, and obtaining necessary permits.
Assessing the Slope and Soil
Start by carefully assessing the slope and soil conditions. Consider the following:
- Slope Angle: A steeper slope will exert more pressure on the wall, requiring a more robust design.
- Soil Type: Sandy soil drains well but provides less support than clay soil. Clay soil retains water, increasing the pressure on the wall. Conduct a soil test to determine the composition of your soil. You can purchase a DIY soil test kit or send a sample to a lab for professional analysis.
- Water Drainage: Poor drainage can lead to hydrostatic pressure, which can damage or even collapse the wall. Identify any existing drainage issues and plan for proper drainage in your wall design.
- Vegetation: Remove any vegetation, including roots, from the area where the wall will be built. Roots can interfere with the wall's foundation and drainage.
Determining Wall Height and Length
The height of your retaining wall will depend on the amount of soil you need to retain. Consider these factors:
- Desired Level Area: How much level space do you want to create?
- Slope Angle: A steeper slope will require a taller wall to achieve the same level area.
- Building Codes: Local building codes may have restrictions on the maximum height of retaining walls without requiring engineering approval.
Measure the length of the area you want to retain. This will determine the length of your wall. Remember to add extra length at each end for a more aesthetically pleasing and structurally sound finish (see "Finishing Touches" section).
Obtaining Permits and Checking Building Codes
Before you start building, check with your local building department to determine if you need a permit. Most jurisdictions require permits for retaining walls over a certain height (typically 3-4 feet). Building codes also specify requirements for drainage, foundation depth, and other aspects of wall construction. Ignoring these regulations can result in fines or even the need to rebuild your wall.
Actionable Advice: Contact your local building department early in the planning process. They can provide you with the specific requirements for your area and help you avoid costly mistakes.
Gathering Materials and Tools
Here's a list of materials and tools you'll need for building a retaining wall with blocks:
- Retaining Wall Blocks: Choose blocks that are appropriate for the height and style of your wall.
- Base Material (Gravel or Crushed Stone): This provides a stable and well-draining foundation for the wall.
- Drainage Gravel: Used behind the wall to promote drainage and reduce hydrostatic pressure.
- Drainage Pipe (Perforated): Installed behind the wall to collect and redirect water.
- Landscape Fabric: Prevents soil from clogging the drainage system.
- Construction Adhesive: Used to secure the blocks together.
- Shovel
- Level (4-foot and torpedo level)
- Tamper or Plate Compactor: To compact the base material.
- Measuring Tape
- String Line and Stakes: To ensure the wall is straight.
- Safety Glasses
- Gloves
- Circular Saw with Masonry Blade (Optional): For cutting blocks.
- Hammer and Chisel (Optional): For shaping blocks.
Building the Foundation: The Key to a Stable Wall
A solid foundation is essential for the long-term stability of your retaining wall. A poorly constructed foundation is the most common cause of retaining wall failure.
Excavating the Base
Excavate a trench along the planned wall alignment. The depth of the trench should be determined by the height of your wall and local building codes. A general rule of thumb is to bury at least one-tenth of the wall's height below grade. For example, a 3-foot wall should have at least 4 inches of the bottom row buried. The width of the trench should be at least twice the width of the retaining wall blocks.
Ensure the base of the trench is level along its length. Use a level and adjust the excavation as needed. A slightly sloped base that follows the overall slope is acceptable and can help with drainage, as long as each block is individually leveled.
Installing the Base Material
Pour a layer of base material (gravel or crushed stone) into the trench. The thickness of the base layer should be at least 6 inches. Compact the base material thoroughly using a tamper or plate compactor. This creates a solid and level foundation for the first row of blocks.
Tip: Compacting the base material in layers (2-3 inches at a time) will achieve better compaction than trying to compact the entire layer at once.
Laying the First Course of Blocks
The first course of blocks is the most critical. It must be perfectly level and properly aligned. Place the blocks on the compacted base material, ensuring they are tightly butted together. Use a level to check each block and adjust as needed. You can use a rubber mallet to gently tap the blocks into place.
Use a string line and stakes to ensure the wall is straight. Run the string line along the back of the blocks and adjust the blocks as needed to align with the string line.
Constructing the Wall: Layer by Layer
Once the foundation is complete, you can begin constructing the wall, layer by layer.
Staggering the Joints
Stagger the joints between the blocks in each course. This is similar to laying bricks. Staggering the joints increases the strength and stability of the wall. The amount of stagger will depend on the size and shape of the blocks you are using.
Using Construction Adhesive
Apply construction adhesive to the top of each block before placing the next course. This helps to secure the blocks together and prevent them from shifting. Use a high-quality construction adhesive that is specifically designed for masonry.
Installing Drainage
Proper drainage is essential to prevent hydrostatic pressure from building up behind the wall. Follow these steps to install drainage:
- Backfill with Drainage Gravel: Behind each course of blocks, backfill with drainage gravel. The gravel should extend at least 1 foot behind the wall.
- Install Drainage Pipe: Place a perforated drainage pipe at the base of the wall, behind the first course of blocks. The pipe should slope slightly to allow water to drain away from the wall. Connect the drainage pipe to a suitable outlet, such as a daylight drain or a storm sewer.
- Wrap with Landscape Fabric: Wrap the drainage gravel with landscape fabric to prevent soil from clogging the drainage system. Overlap the fabric edges to ensure complete coverage.
Practical Example: Imagine you're building a wall in an area with heavy rainfall. Investing in a robust drainage system, including a larger diameter pipe and a wider gravel backfill, is crucial to prevent water buildup and potential wall failure.
Backfilling and Compacting
As you build the wall, backfill behind it with soil. Compact the soil in layers (6-8 inches at a time) using a tamper or plate compactor. This will help to stabilize the wall and prevent settling. Be careful not to compact the soil too close to the wall, as this can put undue pressure on the blocks.
Caution: Avoid using heavy machinery to backfill behind the wall, as this can damage the blocks and drainage system. Use hand tools to backfill in tight spaces.
Finishing Touches: Aesthetics and Longevity
The final stage involves adding finishing touches to improve the appearance and longevity of your retaining wall.
Capping the Wall
Install capstones on the top of the wall to give it a finished look. Capstones are typically larger and wider than the wall blocks. Secure the capstones to the wall using construction adhesive. Overhang the capstones slightly to create a drip edge that will help to protect the wall from water damage.
Extending the Wall Ends
Consider extending the wall slightly at each end, curving it gently into the landscape. This creates a more natural and aesthetically pleasing appearance. It also provides additional stability to the wall.
Planting and Landscaping
Plant flowers, shrubs, or ground cover along the top and base of the wall to soften its appearance and integrate it into the landscape. Choose plants that are appropriate for the soil and climate conditions. Avoid planting trees too close to the wall, as their roots can damage the structure.
Sealing the Wall (Optional)
Apply a masonry sealer to the wall to protect it from water damage and staining. Choose a sealer that is specifically designed for retaining wall blocks. Follow the manufacturer's instructions for application.
Maintenance and Long-Term Care
With proper care, your retaining wall can last for many years. Here are some tips for maintaining your wall:
- Inspect the wall regularly: Look for signs of cracking, bulging, or settling. Address any problems promptly.
- Clean the wall: Remove dirt, debris, and algae from the wall using a garden hose or pressure washer.
- Maintain drainage: Ensure the drainage system is functioning properly. Clear any obstructions from the drainage pipe and repair any damage to the landscape fabric.
- Repair cracks: Fill any cracks in the blocks or mortar with a masonry repair compound.
- Re-seal the wall: Re-apply masonry sealer every few years to protect the wall from water damage.
Conclusion
Building a retaining wall on a slope is a challenging but rewarding project. By following these guidelines, you can create a functional and aesthetically pleasing structure that will enhance your landscape and prevent erosion for years to come. Remember to plan carefully, build a solid foundation, and pay attention to drainage. With a little patience and effort, you can successfully build a retaining wall that you can be proud of.
Next Steps:
- Assess your slope and soil conditions.
- Determine the desired height and length of your wall.
- Check local building codes and obtain necessary permits.
- Gather your materials and tools.
- Start building!