How to Build a CMU Retaining Wall Bench

Dig a trench 12 inches deep and 24 inches wide, using a shovel. Make the trench as long as you want your retaining wall bench.

Use a hand tamper to pack and level the bottom of the trench. Add 6 inches of small crushed gravel to the bottom of the trench, then tamp it firmly into the trench. Top the gravel with a 1-inch-thick layer of sand, then pack the sand firmly in place. The rough edges of the gravel and sand bind together to make a solid foundation.

Set the first row, or course, of 6-by-8-by-16-inch CMU blocks on top of the sand. Check that the blocks are level front to back and side to side with a carpenter’s level. If necessary, add or remove sand from underneath the blocks to level them.

Add crushed gravel in the trench behind the CMUs. Pack it with the hand tamper. This provides support for the retaining wall bench. Brush any loose gravel off the tops of the CMU blocks.

Start the second course of blocks with a 6-by-8-by-8-inch CMU block. Continue placing blocks in this course, staggering joints to give the retaining wall additional stability. Finish the row with another 6-by-8-by-8-inch CMU block. Backfill behind the course of blocks with loose gravel.

Start the third course with a full-sized block. Set the blocks in the course and backfill the trench. Start and finish the fourth course with a short block. Only add approximately 4 inches of backfill behind the fourth course.

Drive reinforcement bar, or rebar, stakes down through the openings in the tops of the CMU blocks, using a sledgehammer. The top of the rebar needs to be even or below the top of the tallest course of blocks. Embed the bottom of the rebar into the base material. The rebar holds the CMU bench in place.

Run a bead of masonry glue along the tops of the fourth course of blocks. Set capstones in a row along the top of the blocks to create the bench seating.

Finish filling the remaining open area of the trench with topsoil. Plant flowers in that area or sow it with grass seed.