How to Build a Retaining Wall

Plan the wall and check if you need a permit

Before you dig, measure the length and finished height of the wall and check your local code. In most US jurisdictions a retaining wall over 4 feet of exposed height needs a building permit and an engineered design, and some areas set that threshold lower.

Any wall holding back a slope, a driveway, or poor soil can need engineering even under 4 feet, because of the added surcharge load. Under the National Concrete Masonry Association method, a plain gravity block wall is practical only to about 3 to 4 feet before geogrid reinforcement is required.

Choose a segmental retaining wall block with a built in lip or pin so courses lock together and resist the soil pushing from behind. To size your job first, enter the wall length and height in the retaining wall calculator to get block counts and material quantities.

Dig the trench to the right depth and width

Mark the wall line, then dig a trench deep enough to bury the first course. A common rule is trench depth equals half a block height plus 6 inches for the gravel base, which keeps at least one full course below finished grade so the soil pins the wall down.

Dig the trench about twice as wide as the block depth. That leaves room for 6 inches or more of drainage gravel behind the wall plus space to work and compact.

Strip all topsoil and any loose fill from the trench bottom, then compact the subgrade with a hand tamper or a plate compactor before any gravel goes in.

Lay and compact the gravel base

The base is the single most important part of the whole project, and a poor base is why most walls eventually lean or fail. Lay a minimum of 6 inches of compacted crushed gravel, or 4 inches for a short wall under about 18 inches, and 6 to 8 inches for taller work.

Use clean, free draining 3/4 inch crushed stone, commonly ASTM No. 57. Do not use sand, which shifts under load.

Add the gravel in 2 inch lifts and tamp each layer hard before adding the next. Then level the top course of gravel carefully, because the first block course copies whatever the base does. Aim for no more than 1/8 inch of deviation over a 4 foot span. Per NCMA guidance the leveling pad should be at least 6 inches thick.

Set a dead level first course

The first course is the most important row in the wall, because there is almost no adjusting it once you build up. Set one block at a time, leveling side to side and front to back with a torpedo level, and check each block against the last.

If the ground slopes along the run, step the base up or down by a full block so each section stays level rather than tilting the blocks. Sweep every block clean before the next course, since a single stray pebble under a block throws the whole row off.

Install drainage behind the wall

Water pressure, called hydrostatic pressure, is the leading cause of retaining wall failure, so drainage is not optional. Lay a 4 inch perforated drain pipe behind the first course, sloped to daylight or a drain so water leaves freely.

Backfill directly behind the blocks with clean 3/4 inch crushed gravel, keeping a drainage column of at least 12 inches, and ideally 18 to 24 inches, of clear stone against the back of the wall. Wrap the stone and pipe in non woven filter fabric to keep soil silt out; avoid woven fabric, which can dam water and defeat the drain.

For a wall that carries a slope or driveway above it, plan the drainage outlet before you build, because you cannot add it later.

Stack courses, backfill, and reinforce tall walls

Build up one row at a time, staggering the vertical joints like brickwork so no seam lines up over the one below. Set each block snug and sweep the course clean before stacking on it.

Backfill and compact as you go, not at the end. Add crushed gravel behind each course in 6 inch lifts and tamp each lift, but keep heavy ride on compactors at least 3 feet back from the wall so you do not shove it out of line.

Walls over about 3 to 4 feet need geogrid soil reinforcement laid between courses, typically every 16 inches of height and extending back into the fill at least 60 percent of the wall height. To count blocks, cap units, and cubic yards of gravel for your dimensions, run the numbers through the retaining wall block calculator.

Finish with caps and grade the top

For the last row, dry fit the capstones first, then glue them down with a bead of exterior construction adhesive on the course below. Caps lock the top course and give the wall a clean finish.

Grade the soil at the top so it slopes gently away from the wall, sending surface water away rather than into the backfill. That final grade protects all the drainage work underneath.

Before you buy, it helps to price the job. Our guide to retaining wall cost breaks down block, drainage, and labor so you know whether to DIY or hire out.

Frequently asked questions

References

• Retaining wall (Wikipedia)
• NCMA / CMHA: Segmental Retaining Wall design
• ASTM C1373: Dry-Cast Segmental Retaining Wall Units
• USDA NRCS: Retaining walls and drainage

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