How to Build a Concrete-Block Retaining Wall on a Slope

Before You Dig

You can have the yard of your dreams, even if it slopes. A retaining wall tames a problem slope, giving you more room to work and play outside. Do-it-yourselfers with a little experience can usually complete walls up to 4x12 feet in a weekend, but it's important to recognize when it's time to call in a professional. Before you start this project—or any landscaping project—call your local One Call at 811 to have workers come out and mark buried utilities so you don't accidentally disrupt those. This federally-mandated national number was created to help protect you from unintentionally hitting underground utility lines while working on projects that require digging.

Designing a Wall

It's important to remember that retaining wall blocks can weigh 20-80 pounds, and base gravel usually is packaged in 50-pound bags, so be prepared for a workout. Most retaining walls up to 4 feet tall (including the buried blocks) can be finished without any special engineering. If your wall will be taller, or adjacent to heavy loads, such as alongside of a driveway, consult a structural engineer. A properly designed wall will save you time and money.

• DIY Retaining Wall Ideas

What You Need

Stakes, twine; a trenching spade; a long-handle, round-point shovel

Scrap lengths of rebar or wooden dowels

8-foot, 2x4 lumber

4-foot-long level

Base material, concrete blocks, sand, crushed gravel

Torpedo level, rubber mallet, construction adhesive, perforated drainpipe

Hand tamper; a gas-powered tamper is optional

Step 1: Excavate

If you have designed a straight wall, drive stakes at opposite ends of the site and attach a length of twine. The twine should line up at the front of the proposed wall, or be parallel to where you want the front of the wall. This guide will ensure that the blocks form a unified and straight front. A 3-foot-tall wall should have 4-6 inches of base material, so dig accordingly.

Step 2: Ensure a Level Base

To create a structurally sound wall, it's essential that the base material and the first layer of blocks be level. An easy way to guarantee a level base is to drive two stakes into the ground with their heads where the bottom of the first row of blocks will sit. (We used scrap lengths of rebar, but wooden dowels will also work.) To level the stakes, rest a straight piece of 2x4 lumber across the tops of the stakes. Lay a 4-foot level on the board and check the positioning of the stakes. Are they level?

Step 3: Compact the Base

A hand tamper will work fine for most walls, but for large projects, gas-powered tampers are available to rent. A 4x4 can be used as a last resort. Compact the first few inches of base, then add a few more inches. Compact this, and repeat until the tops of the stakes are just sticking out of the base material. Note: Base material—a mix of sand and crushed rock—is available at home improvement and landscape supply stores, usually in bags of 50 pounds or 0.5 cubic feet. Don not use soil, since it will settle over time, resulting an uneven or leaning wall.

Step 4: Level the Base

Using a straight 2x4, level the top of the base material so it is completely flat. This is crucial; if they're not level, you'll find yourself fighting against the base and first course of bricks throughout construction.

Step 5: Lay the Base Blocks

Start laying blocks at one end, using a full block. Use a torpedo level to check the level front to back. Tap the blocks with a rubber mallet until they are level with one another. If you need to raise a block, put sand or base material under it. Level every block on the first course. It does the most work, receiving all the pressure from the wall and determining how level the wall will be. When you finish this layer, pack native soil along the fronts of the blocks to keep them in place as you add the next courses.

Step 6: Continue Stacking Blocks

Begin the second course with a half block (start every second row with a half block). With staggered joints, the wall will have more structural integrity.

Editor's Tip: To cut, score a line around the block with a brick chisel, then tap until the block splits. If you need to make many cuts, or cut custom capstones, rent a masonry saw from a tool-rental shop.

Step 6: Continue Stacking Blocks, Part 2

Some retaining-wall blocks have locking mechanisms that allow them to be built to about 4 feet tall without extra engineering. Because we used tumbled blocks with no locking mechanisms, we used a construction adhesive on alternate courses. Remember, with locking blocks, you can remove blocks and start over if you make a mistake. But adhesive permanently bonds blocks, so work carefully.

Step 7: Backfill

As each level is added, backfill with crushed gravel and tamp it. This facilitates drainage and keeps tree and weed roots from destroying your wall. In addition to the gravel backfill, drainage that empties beyond the wall is crucial to maintaining the wall's strength. Lay a section of perforated drainpipe near the bottom of the gravel to channel water from behind the wall. The gravel backfill should be at least 8-12 inches thick.

Step 8: Add the Capstones

Although not essential, capstones give your wall a finished, professional look. Most manufacturers make capstones that blend well with the wall block, or you can use concrete pavers cut to fit, as we've done here. Apply construction adhesive to keep the capstones in place.