How to Build a Concrete Block Retaining Wall

Concrete block retaining walls are ideal for holding back the soil after digging into a slope for a pathway, patio, or landscaping project. Concrete block retaining walls are generally the same as freestanding block walls but with a few crucial differences.

A retaining wall must provide a way to release the water that builds up in the slope behind it. Without a pressure-relief system, the weight of the water in the soil would crack or even buckle the wall. Weep holes, lengths of ¾-inch pipe inserted along the top of the first course, escort some of the water out. The other part of the system is a plastic drainpipe covered with gravel. As water accumulates behind the wall, it percolates through the gravel into the drainpipe, which carries it off safely.

Concrete block retaining walls must be stronger than freestanding walls. Insert rebar in the footing when you pour it; this should be done every three blocks or at intervals specified by your local codes. As the last step, fill the cores around the rebar with mortar from the bottom to the top.

Expect to spend 20-36 hours building a 3x10-foot concrete block retaining wall. Before you begin, you'll need to design the layout and pour the footing.

Step 1: Build and Prep Footing

Lay out the wall and excavate the slope and the footing trench. Build the footing and insert rebar into the wet concrete at intervals corresponding to the cores in the blocks. Prepare the footing, spread mortar, and build leads, sliding the block over the rebar as needed.

Step 2: Insert Weep Holes

As you lay the second course of the concrete block retaining wall, insert weep holes (¾-inch pipe) into the mortar every third block. Chip off a small recess in the block and trowel a mortar bed for the pipe, sloping it slightly toward the front. Then, chip out a recess on the next block, butter the ears, and set the block.

Step 3: Scrape Off Excess Mortar

Stop occasionally to check the mortar. When it begins to set up slightly, scrape off the excess with an upswing of the edge of the trowel. Do not plug the weep holes with excess mortar as you remove it.

Step 4: Lay Landscape Fabric

After you've laid at least two courses, spread landscape fabric on the soil behind the base of the concrete block retaining wall, temporarily laying the excess over the grass on the slope. Lay rocks on the fabric to hold it while you work. Backfill the wall with gravel between the rear of the wall and the slope, laying perforated drainpipe on the gravel bed level with the top of the first course. Face the holes down.

Step 5: Cover Drainpipe

Shovel more gravel on top of the drainpipe. Backfill with additional gravel as you add courses to the wall. Fold the landscape fabric over the top of the gravel fill, and backfill with soil. Replace the sod.

Step 6: Fill with Mortar

When you reach the finished height of the concrete-block retaining wall, fill those cores containing rebar to the top with mortar. Mix the fill mortar slightly wetter than what you'd use for joints. Smooth the top with a trowel. Spread additional mortar, and lay cap block to finish the wall. Source