How to Build a Retaining Wall

A block retaining wall needs a foundation trench and a layer of crushed stone or gravel, known as paver base, for drainage and stability. Typically, you need 3 to 6 inches of compacted paver base. Check your instructions for specific paver base depths for different wall heights.

1. Dig a trench to a depth equal to the buried portion of the first row and the compacted paver base depth.
2. Dig the trench width equal to twice the front-to-back measurement of the block.
3. Compact the bottom of the trench with a tamper and ensure it’s level.
4. Pour in the paver base, spread it with a rake and tamp it to the recommended depth.
5. Ensure the compacted base is level to reduce alignment problems later.

A level, sturdy first row or course is key to keeping your retaining wall stable and aligned.

1. If you’re building with interlocking block that has a rear lip, use a drilling hammer and masonry chisel to knock the lip off the back of the blocks you use for the first row.
2. Set the first block in place at one end of the trench and use a torpedo level to check it for level side to side and front to back.
3. Tap the block with a rubber mallet or add paver base underneath to level the block as necessary.
4. Continue laying the first row, placing each block alongside the previous block and ensuring they’re even and level.
5. Finish the first row and brush off any debris.

The joints in each course need to be staggered, so you likely need a half-block to start and finish the second row. We’re using an angle grinder and a masonry cutting wheel to cut our blocks.

1. Cut a block in half to start and end the second row.
2. Apply a couple of beads of landscape construction adhesive to the end block on the first row.
3. Align a partial block with the end of the first course.
4. Make sure the block flange locks against the back of the lower block and use your mallet to tap the block firmly into the adhesive. 
5. Continue applying adhesive and installing the remaining blocks for the second row, finishing with the other partial block.

We’re using landscape adhesive to help secure each row. Your instructions may specify using adhesive only on the final course or may not recommend adhesive. Follow the instructions for your block.

Water collecting behind the retaining wall can make the structure unstable, so you need to install drainage. Depending on local codes, your site and manufacturer’s instructions, you may need to run perforated pipe at the base of the wall and backfill the wall with gravel or drainage rock, but for this low wall, we’re just using the gravel.

1. Cut landscape fabric to the length of the wall.
2. Lay the fabric behind the wall, overlapping the second row and the back edge of the trench.
3. Backfill the wall with gravel, pouring it on top of the landscape fabric to about the top of the second course and compacting it with the tamper.
4. Fold the landscape fabric over the gravel and drive in landscape pins to secure it.

For our project, the third row is our final row and we don’t need to cut any blocks:

1. Brush debris from the second course.
2. Apply construction adhesive and lay the blocks, being sure to stagger the joints and lock the flanges against the lower blocks.
3. Backfill behind the third course with the soil you dug from the trench and compact it over the drainage material.

Landscaping your retaining wall can be as simple or elaborate as you want.

• Apply a layer of mulch, as we did with our retaining wall.
• If you built your wall around a slope as we did, you can plant shrubs, ground cover, or a small tree.
• If you’re using your retaining wall to create a raised bed, fill it with raised bed soil before you begin planting.