How do I build a low retaining wall in a garden?

The drainage, backfill and mass that make a low retaining wall work, and the height at which this stops being a DIY job.

Difficulty: advanced
Time: 15 hr
Read: 4 min
Safety: restricted

Short answer

Up to about 600mm of retained soil, a mass wall of sleepers, blocks or dense masonry on a proper concrete foundation will do it — provided the back is drained with free-draining backfill, a geotextile and either weep holes or a land drain. Above 600mm, get it designed.

Soil behind a wall behaves like a heavy liquid, and a saturated soil behaves like a much heavier one. That is the whole problem: nearly every retaining wall that fails does so because water built up behind it, not because the wall itself was weak.

Safety

This covers retaining walls up to about 600mm of retained height only. Above that, the forces involved rise sharply and the wall must be designed by a structural engineer — an under-designed retaining wall fails suddenly and can kill. Never retain soil against an existing free-standing wall, a fence, or a wall you did not build for the purpose. Any retaining wall supporting a building, a driveway, a neighbour's land or a highway needs professional design at any height. Excavation near a boundary can engage the Party Wall etc. Act 1996 in England and Wales.

What you'll need

Sleepers, dense concrete blocks or engineering bricks
Concrete for the foundation
Clean free-draining backfill — 20mm clean stone
Geotextile membrane
Perforated land drain pipe

What has to happen behind a retaining wall

Step by step

Check the retained height honestly.Measure from the bottom of the foundation to the top of the retained soil. If it exceeds about 600mm, or the ground above slopes up, or there is any load nearby — a drive, a building, a road — stop and get it designed.
Dig a foundation below the front ground level.A concrete strip at least 150mm thick and wider than the wall, set below the level of the ground in front so the wall cannot slide forward. Down to firm subsoil.
Build the wall with mass and a slight backward lean.A retaining wall resists by weight and friction. Dense blockwork, engineering brick or stacked sleepers. Leaning the face back into the bank by around 1 in 20 improves stability considerably.
Fit a geotextile against the retained soil.It separates the soil from the drainage stone so the stone does not silt up. This is what makes the drainage last more than a couple of years.
Backfill with clean free-draining stone.At least 300mm of 20mm clean stone behind the wall, wrapped in the geotextile. Never backfill with the soil you dug out — it holds water and applies the full saturated pressure.
Drain the base.A perforated land drain in the stone at the bottom of the backfill, falling to a discharge point away from the wall, or weep holes through the wall at 1m centres at the lowest course. Both is better.
Cap the top and stop surface water.A coping to keep water out of the top of the wall, and a surface fall on the retained ground that takes water away rather than into the backfill.
Use the right fixings for sleepers.Timber sleepers stacked flat, drilled through and pinned with long steel bars or timber-connector screws, and tied back into the bank with deadmen for anything above two sleepers high.
Let the ground behind settle before finishing.Backfill in layers and compact each one lightly. Then leave it through a wet spell before laying paving or turf on top — it will settle.

Tips

Drainage matters more than the wall. A modest wall that is properly drained outperforms a massive one that is not.
Concrete crib and interlocking segmental block systems come with manufacturer design tables for low walls and take much of the guesswork out.
Never plant a tree or a large shrub immediately behind a retaining wall. Root growth applies loads the wall was never designed for.

Common mistakes

Backfilling with the excavated soil — Soil holds water, and saturated soil roughly doubles the pressure on the wall. Free-draining stone behind a geotextile is the single most important detail.
Building a retaining wall in single-skin brickwork — It has neither the mass nor the strength to resist soil pressure and it will fail. Retaining walls need mass, reinforcement, or a designed system.

If it doesn't work

Wall bulging or leaning forward

Cause: Water pressure behind it, or insufficient mass — Fix: Stop using the area, cordon it, and get it assessed. A leaning retaining wall is failing, not settling.

Water seeping through the face of the wall

Cause: Blocked or absent drainage behind — Fix: It is telling you the backfill is saturated. Excavate behind, install a geotextile, clean stone and a land drain.

Ground behind the wall sinking

Cause: Backfill settling, or fines washing into the drainage stone — Fix: Top up and compact. If it recurs, the geotextile is missing and soil is migrating into the drainage layer.

Sleeper wall bowing between fixings

Cause: Sleepers pinned but not tied back — Fix: Add deadmen — sleepers running back into the bank, fixed to the face and buried — at 1.5-2m centres.

Questions people ask

How high can I build a retaining wall myself?

Around 600mm of retained height is the usual sensible DIY limit for a mass wall with proper foundations and drainage. Above that the loads rise quickly and it needs structural design — this is one of the few garden jobs where getting it wrong is genuinely dangerous.

Do I need weep holes in a retaining wall?

You need drainage; weep holes are one way to provide it. A land drain in free-draining backfill behind the wall, discharging away from it, is more reliable. Doing both is normal practice.