How to Install a Block Retaining Wall Next to Existing Concrete Stairs

Use landscape spray paint to draw the course of the wall over the site.

Follow the painted line to dig the foundation with a flat-tipped shovel. Dig deep enough for a 4-inch base plus the thickness of one layer of blocks.

Spread 3 inches of angular gravel over the bottom and compact it with the flat end of a 4-by-4. The rocks will support the wall while enabling water to escape through cracks.

Sprinkle a 1-inch layer of bedding sand over the gravel to help level the blocks. Brush the surface with a wood board to make the bed smooth.

Use a utility knife to cut a strip of flexible expansion joint foam to fit vertically from the gravel foundation to the top of the wall.

Place one end of the foam strip between the back of the end block and the concrete stairs. The strip must be wedged in tight. Have a helper hold the foam up vertically against the wall so it doesn't shift.

Lay the bottom course of retaining wall blocks on top of the sand so the sides are flush. Position a bubble level over the joints and use the end of a hammer to tap them gently down until they’re completely level. Since the blocks are uniform in height, leveling the first course ensures the following courses will be level as well.

Set the next course starting with a half block against the foam strip. Alternating the ends with a half block staggers the joints for stability. Place the blocks so the lip that hangs over the bottom interlocks with the top of the first course.

Back-fill a 6-inch-wide area behind the wall with gravel; then, fill the remaining space with dirt.

Continue to build up the wall so the end blocks beside the stairs are wedged firmly against the foam strip. Back-fill the space behind the blocks with gravel and dirt after laying each course. Apply landscape adhesive over the top of the second-to-last course; then, set coping blocks on top for a clean, finished look.

Attach a tube of expansion joint sealant to a caulk gun and fill the gap in front of the foam strip with sealant from the bottom up. Use small amounts of caulk at a time or it will drip down. Layer the caulk until the gap is filled; then, run a putty knife over the surface so the caulked joint is level with the wall.