How to build a concrete block retaining wall

Engineering trigger

~4 ft and uplocal authority sets the threshold

Leveling pad

6 in compacted stonecommon SRW practice

First course buried

~10% of wall heightor one full unit, per manufacturer

Drainage zone

12 in clean stonewith perforated pipe at the base

Backfill lifts

6–8 incompact each lift

Short version

• Drainage and the first course decide whether the wall survives; everything else is assembly.
• Dry-stacked segmental units and mortared CMU are two different structures with different rules.
• Bury the bottom course and set it on compacted stone, not on soil.
• Backfill in 6 to 8 inch lifts and compact each one, keeping equipment away from the wall.
• Anything tall, sloped, or surcharged goes to a licensed engineer before you buy block.

When does a retaining wall need an engineer and a permit?

A retaining wall needs an engineer and a permit sooner than most homeowners expect, and the exact threshold is set by your local jurisdiction — not by any national rule. A commonly used trigger is a wall retaining more than about 4 feet of soil measured from the bottom of the footing or leveling pad to the top of the wall, but jurisdictions set their own numbers and their own measuring conventions.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

Height is not the only trigger. A shorter wall is treated as an engineered structure when it carries a surcharge — a driveway, parking area, pool, structure, or another wall above it — or when the retained ground slopes upward behind it. Both conditions increase the lateral load substantially, and both are common reasons a “small” wall fails.

Check with your building department before you design anything, because the answer determines what you are building. See do you need a permit to pour concrete for how to find out what applies at your address. The same call usually covers setback from property lines and any requirements about diverting surface water onto a neighbor’s land.

A failed retaining wall is not a cosmetic problem. Walls fail by rotating, sliding, or having the soil beneath them give way, and all three can happen quickly and with whatever is above the wall coming with it.

What is the difference between segmental blocks and CMU?

Segmental retaining wall units and concrete masonry units are different products that build different walls. Segmental units are solid, dry-stacked with no mortar, and rely on their weight, an interlocking lip or pin, and soil reinforcement. CMU are hollow masonry units under ASTM C90 that are mortared, grouted, and reinforced like a wall in a building.

The practical consequence is that a low segmental wall built exactly to the manufacturer’s published limits is a reasonable homeowner project, while a mortared CMU retaining wall almost never is. CMU retaining walls need a designed footing, designed reinforcement, and inspection, because a hollow unit has very little resistance to bending until it is grouted and reinforced.

CMHA publishes the design and construction guidance the segmental wall industry works from, and manufacturers publish unit-specific tables within it.

How do you build the base and set the first course?

The base and the first course are the entire job. Excavate a trench wide enough for the leveling pad plus the drainage zone, and deep enough to bury the bottom course — manufacturers commonly specify burying roughly 10% of the wall height, or one full unit, whichever is greater.

Compact the subgrade at the bottom of the trench before anything goes in, and remove any soft or organic soil rather than compacting over it. Place 6 inches of crushed stone leveling pad, screed it, and compact it in lifts with a plate compactor until it stops moving under the plate.

Set the first course on that pad and level every single unit in both directions. Check front to back and side to side on each block, and check the run with a string line and a long level. An eighth of an inch of error in course one becomes an inch by course eight, and the wall will show it.

Sweep the top of each course clean before setting the next one. A single piece of gravel under a block tilts it, and the tilt propagates upward.

Take the time here that the rest of the wall does not need. Experienced crews spend a disproportionate share of the total hours on the base and course one, and that is the correct allocation.

How do you drain a block retaining wall?

Drainage is what keeps the wall standing, because saturated soil pushes far harder than drained soil. Hydrostatic pressure behind a wall is the most commonly cited cause of retaining wall failure, and it is entirely preventable.

Build the drainage zone as you build the wall. Place a column of clean, free-draining crushed stone directly behind the units, commonly about 12 inches wide, extending up the wall as it rises. Set a perforated drain pipe at the base of that stone column, sloped to daylight at a low point or into a drainage structure — a drain that does not outlet anywhere is decoration.

Separate the stone from the soil with a geotextile filter fabric. Without it, fines migrate into the drainage stone over a few seasons and plug it, at which point the wall has no drainage and nobody can see that from outside.

Handle surface water too. Grade the ground above the wall so runoff moves away rather than over the top, and keep downspouts and irrigation from discharging into the backfill. A capped, well-graded wall with a working outlet handles ordinary rainfall indefinitely; the same wall with a plugged pipe fails in a wet spring.

How do you backfill and reinforce behind the wall?

Backfill in thin lifts and compact each one. Place 6 to 8 inches of fill at a time, compact it with a plate compactor, and repeat as the wall rises — never dump the full height of backfill and compact once from the top.

Keep compaction equipment away from the wall face. Within a few feet of the units, compact with hand equipment only, because a heavy plate or a riding compactor close to the wall pushes the courses out of alignment. That misalignment is permanent once the courses above are set.

Reinforce with geogrid where the design calls for it. Geogrid is a geosynthetic layer laid between courses and extending back into the retained soil, and it turns the soil mass itself into part of the structure. The vertical spacing, the embedment length, and the grid strength are all engineer-specified or set by the manufacturer’s design tables — they are not adjustable on site.

Keep heavy loads off the reinforced zone during construction. Parking a loaded truck on freshly built backfill applies exactly the surcharge the design was trying to avoid.

Cap the wall when the backfill is finished, and use the manufacturer’s adhesive for the cap units.

What should you check before you start digging?

Call 811 before any excavation, every time. The national call-before-you-dig service notifies utility operators to mark buried gas, electric, water, and communication lines at no cost, and hitting an unmarked line during a landscaping project is both dangerous and expensive.

Check the excavation itself for safety. OSHA’s trenching and excavation requirements apply to employees working in excavations, and the underlying hazard — soil collapse — does not care whether the person in the trench is on a payroll. Sloping, benching, or shoring applies to deep cuts regardless of the project’s size.

Confirm the property line and the setback. Retaining walls sit right at property boundaries more often than any other structure, and a wall built over the line is a wall that gets moved.

Look at where the water goes now and where it will go after. Redirecting surface runoff onto a neighboring property creates a dispute and, in many jurisdictions, a violation. If the wall changes the drainage pattern of the lot, that belongs in the plan you take to the building department.

Common mistakes

• Skipping the drainage stone and pipe — saturated backfill applies pressure the wall was never built for, and it fails from the middle outward.
• Setting the first course on soil instead of compacted stone — the base settles unevenly and the whole wall follows it out of line.
• Building past the manufacturer’s height limit without an engineer — unreinforced walls beyond their published limits rotate or slide, often years later.
• Compacting backfill with heavy equipment near the face — the courses get pushed out of alignment and cannot be corrected once the wall is above them.
• Ignoring a slope or driveway above the wall — a surcharge changes the design load, and a wall sized for level ground is undersized for it.