How to Build an Interlocking Retaining Wall

Interlocking concrete block is a way to build a strong wall without mortar. Some such blocks are made with flanges that slip over the rear edge of the preceding course; others rely on a system of pins. With both types, you don't have to practice throwing mortar or setting block, but you will get an attractive addition to your landscape. The flanges slope the wall back into the slope for additional strength.

Interlocking-block walls don't require a footing, but some styles require you to set the first course in a trench to hold the bottom of the wall in place. Cut the slope back 12 to 15 inches from the rear of the trench to leave room for gravel backfill and a drainpipe—a must for retaining walls. Save the topsoil and use any extra as fill elsewhere in your landscape.

To lay a 3x16-foot wall, expect to spend 10 to 20 hours. You'll need to feel comfortable digging, designing a layout, leveling, and laying brick. Before you begin, call your local hotline to mark any underground utilities. 

Everything You Need to Know About Retaining Walls

Step 1: Lay Out Wall

Lay out the wall and remove the soil from the slope. Following the manufacturer's recommendations, dig and level a trench for the first course. Stake a mason's line to help align the blocks.

Step 2: Spread Landscape Fabric

Starting a foot beyond the top edge of the slope, spread landscape fabric down and over the trench. Put stones on the top to hold the fabric in place. Overlap the fabric edges by 6 inches. Tamp 3 to 4 inches of gravel into the trench and back to the bottom of the excavation.

Step 3: Lay First Course

Using the line or the edge of the trench to guide you, lay the first course in the trench. The first course blocks are set backwards for many styles. Set each block with a rubber mallet. Make sure each block is level front to back and with the adjoining blocks. Use a 4-foot level to check—shorter levels are not long enough to ensure accuracy. If you need less than a full block at the end of the wall, lay smaller blocks or trim full blocks to fit.

Editor's Tip: Precast block is trimmed the same way other masonry materials are trimmed. If you're using solid block, score the line you want to cut with a brick set and small sledgehammer. Repeated blows will split the block. Hollow-core blocks don't break cleanly. Buy half blocks and smaller blocks to fit where you need them, or cut them with a masonry blade.

Step 4: Set Remaining Courses

Set the remaining courses with the flanges tight against the back of the preceding course. Start every other course with a half or partial block so the joints are offset by at least 3 inches or the amount specified by the manufacturer's instructions. As you work, check the blocks for level. Shim the low end of a block with a small piece of cedar shingle.

Step 5: Set Up Drainpipe

After the third course of block, backfill the area between the wall and the slope with gravel, just about even with the second course. Lay in a perforated plastic drainpipe on the gravel (holes down). Let the pipe daylight beyond the edge of the wall, or run it into a French drain.

Editor's Tip: If your wall is designed with corners, the corner blocks must overlap at the joints to tie the two legs together. Cut half blocks and alternate them with full blocks as shown, following the manufacturer's instructions. Chisel off the lip of the block if it gets in the way. As you place the block, strengthen the bond with a bead of construction adhesive.

Step 6: Backfill Wall

As you build the wall, backfill gravel behind it at least every other course, covering the pipe and bringing the level of gravel up just below the top of the last course you laid.

Step 7: Tuck Fabric

Using the same techniques and starting every other row with a partial block, continue building the wall and backfilling it with gravel. When you're one or two courses below finished height, fold the landscape fabric over the gravel and tuck it behind the block.

Step 8: Tamp Soil

Shovel about 2 inches of the topsoil you removed (not the subsoil at the base of the slope) on top of the landscape fabric. Tamp the soil lightly with a garden rake and replace the sod.

Step 9: Apply Adhesive

The capstones made for block of this or a similar shape cover the V-shaped gaps between blocks in the lower courses. Prepare the next-to-last course by applying a bead of construction adhesive on the surface of each block.

Step 10: Place Capstones

Following the manufacturer's instructions place the course of capstones on the wall.

Step 11: Finish Base

If you're not building a patio or other structure below the wall, spread and tamp the topsoil you removed from the excavation. Fill in around the base of the wall and level the soil as far forward from the base as the landscape allows. Tamp the soil and replace the sod. Water the sod frequently until it is well established.

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