Planning a Small Retaining Wall: Base, Batter and Drainage

A retaining wall holds back soil that would rather be somewhere else. Soil is heavy, and wet soil is heavier and pushes harder. A wall that looks solid on the day it is finished can lean, bulge and fall within a few years if the forces behind it were not allowed for.

The good news is that for low garden walls built from interlocking concrete blocks, the method is well established and forgiving, as long as you do not skip the hidden parts.

First, the limits

This guide is about low walls: up to about 3 feet (0.9 m) of exposed height, holding back level garden soil. That is the range most block makers rate their garden wall blocks for without reinforcement.

Stop and get professional advice if any of these apply:

• The wall will be more than about 4 feet (1.2 m) tall, measured from the bottom of the base to the top. In many places that height requires a permit and an engineered design.
• The ground above the wall slopes upward, which adds load.
• There is a driveway, parking area, building, pool or fence close to the top of the wall.
• The wall is one of several terraces stacked close together, which act as a single tall wall.
• The soil is soft, wet, or fill.
• The wall is near a property line, a slope, or a watercourse.

Check your block maker’s installation guide for the height limit of the specific block. It is usually printed on the pallet tag or the maker’s website.

What holds a block wall up

Four things work together.

Weight. The blocks are heavy, and their weight resists the soil pushing on them. This is called a gravity wall.

Setback, also called batter. Each course sits a little farther back than the one below, so the wall leans into the slope. Most blocks have a lip or pins that set this automatically, usually about ¾ inch per course. A wall that leans back is far harder to push over than a vertical one.

A buried first course. The bottom row of blocks sits below ground level in front of the wall. The soil in front stops the base of the wall from sliding forward.

Drainage. Water is the real enemy. Saturated soil is much heavier than dry soil, and trapped water adds its own pressure. A zone of clean gravel behind the wall, with a pipe at the bottom, lets water escape before it can build up.

Leave out any one of these and the others are overloaded.

The base trench

The wall is only as level and as stable as its base.

• Depth: enough for 6 inches (15 cm) of compacted gravel, plus the buried part of the wall. For a low wall, bury one full course. Some makers specify 1 inch of burial for every 8 inches of wall height, with a minimum of about 6 inches.
• Width: about twice the depth of the block from front to back. A block 10 inches deep needs a trench about 20 inches wide. This leaves room for base in front of and behind the block.
• Material: crusher run or paver base, the kind with fine particles that packs hard. Not round gravel, and not sand.
• Compaction: in layers, with a plate compactor or a hand tamper, until it is firm underfoot.
• Level: dead level from side to side and along the length. Spend more time here than anywhere else.

On a sloping site, the trench is cut in level steps, each one a whole block height, so that every course stays horizontal.

The first course

Every block in the first course must be level front to back and side to side, and in line with its neighbors. Check each one with a level as you set it, and tap it into place with a rubber mallet. Add or scrape away a little base material as needed.

It is slow. It is also the only slow part. Once the first course is right, the rest stack quickly, because each block locates on the one below.

Drainage

As the wall goes up, fill behind it in stages.

• Drainage zone: at least 12 inches (30 cm) of clean, angular crushed stone directly behind the blocks. “Clean” means washed, with no fine particles. Three-quarter inch crushed stone is typical. It should also fill any hollow cores in the blocks.
• Pipe: a perforated drain pipe at the bottom of the drainage zone, sloped slightly to run out at one or both ends of the wall, or through the wall face at intervals.
• Fabric: landscape fabric between the drainage stone and the soil behind it, and folded over the top of the stone before the last few inches of soil go on. It keeps soil from washing into the stone and clogging it.
• Backfill in layers: add stone and soil a course or two at a time, and compact as you go. Do not stack the whole wall and then fill behind it.

Do not backfill with the heavy clay you dug out, if you can avoid it. Clay holds water against the wall.

Estimating the materials

Take a wall 24 feet long and 2 feet 6 inches tall, built from blocks 16 inches long, 6 inches high and 10 inches deep.

Blocks.

• Per course: 24 × 12 ÷ 16 = 18 blocks
• Courses above ground: 30 ÷ 6 = 5
• Plus one buried course: 6 courses
• Total: 18 × 6 = 108, plus 5% = 114 blocks

Caps. 18, plus 5% = 19 caps, if the caps are the same length as the blocks.

Base gravel. Trench 20 inches wide, 6 inches of gravel, 24 feet long.

• 24 × 1.67 × 0.5 = 20 cu ft compacted
• With 15% for compaction: 23 cu ft, about 1.2 tons

Drainage stone. 12 inches thick, the full exposed height, 24 feet long.

• 24 × 1 × 2.5 = 60 cu ft, about 3 tons

Pipe and fabric. 24 feet of perforated pipe, and fabric about 4 feet wide for the same length.

Notice the stone. For this modest wall, the gravel weighs more than four tons. People budget for the blocks and are surprised by the stone.

Corners, curves and ends

Curves. Most garden wall blocks are tapered so that they can form curves. An outside curve, bulging away from the slope, tightens with each course as the wall steps back. An inside curve opens up. Set out the first course with a generous radius and let the upper courses follow.

Corners. Outside corners need corner blocks or blocks cut and glued, alternating direction on each course so the joints overlap. Inside corners are simpler: one wall runs through and the other butts against it, swapping on alternate courses.

Ends. A wall that simply stops looks unfinished and lets soil spill around it. Either turn the end back into the slope, or step the wall down course by course until it meets the ground.

Joints. Stagger them. Each block should sit across the joint between two blocks below, like brickwork.

Caps

Caps finish the top and stop water getting into the cores. Glue them down with a landscape block adhesive made for the purpose, on clean, dry blocks. Let them overhang the face by an inch or so for a shadow line.

Mistakes that bring walls down

• No buried course. The base of the wall slides forward.
• Base on topsoil or loose fill. It settles unevenly and the wall cracks or tips.
• No drainage stone, or no outlet for the pipe. Water pressure builds behind the wall after heavy rain.
• Backfilling with clay. It holds water exactly where you do not want it.
• Stacking vertically. If you defeat the setback, by removing the lips or pulling blocks forward, the wall loses much of its resistance.
• Building too tall for the block. Garden wall blocks have a rated height. Past it, the wall needs reinforcement and a design.
• Planting a tree at the top. Roots and wind load pry walls apart over time. Keep trees back by at least the height of the wall.

Build the hidden parts properly and the visible wall takes care of itself. The block count and the gravel for your own dimensions are a minute’s work in the calculator.

Written and checked by Bertha
Last updated October 4, 2026 · How we work out our numbers