A poured concrete retaining wall needs a footing roughly twice the wall thickness wide, reinforcement in the tension face, and drainage behind it. Get any of those three wrong and it fails, usually within a few winters.
Retaining walls are the one piece of garden construction where amateur work regularly collapses. The loads are much higher than people expect, and the failure is sudden rather than gradual.
Understanding the forces
Soil behind a wall behaves like a very heavy, slow-moving liquid. It exerts lateral pressure that increases with depth, and that pressure is trying to rotate the wall forward about its base.
Three things add to it:
Soil weight. The basic lateral earth pressure, roughly 30 to 50% of the vertical weight of the soil depending on type. A 1.5m high wall retaining ordinary soil carries something like 8 to 12 kN per metre of length.
Water. Saturated soil is far heavier, and if water cannot drain it builds hydrostatic pressure on top of the earth pressure. This can double or triple the total load, and it’s the single most common cause of failure.
Surcharge. Anything sitting on the retained ground — a driveway, a shed, a parked car — adds directly to the pressure. A vehicle behind a wall is a substantial extra load.
When you need an engineer
Get a design for anything over 1m retained height, anything with a surcharge behind it, anything on a slope, or anything where failure would affect a neighbour or a structure.
Below 1m in ordinary ground with nothing loading the top, a well-built wall to standard proportions is usually fine. Above that the arithmetic stops being forgiving, and the cost of a design is small against the cost of rebuilding.
Footing size
The footing does most of the work. It resists both sliding and overturning, and it must extend forward of the wall — the “toe” — to give leverage against rotation.
| Retained height | Wall thickness | Footing width | Footing depth | Toe projection |
|---|---|---|---|---|
| 0.6m | 150mm | 450mm | 250mm | 150mm |
| 0.9m | 200mm | 600mm | 300mm | 200mm |
| 1.2m | 200mm | 800mm | 350mm | 250mm |
| 1.5m | 250mm | 1,000mm | 400mm | 300mm |
| Over 1.5m | Engineer designed | |||
The footing must sit below frost depth and on undisturbed bearing ground. A retaining wall on made ground or backfill will move regardless of how well the wall itself is built.
Concrete volume by wall size
| Wall length | Retained 0.9m | Retained 1.2m | Retained 1.5m |
|---|---|---|---|
| 4m | 1.44 m³ | 2.08 m³ | 3.10 m³ |
| 6m | 2.16 m³ | 3.12 m³ | 4.65 m³ |
| 8m | 2.88 m³ | 4.16 m³ | 6.20 m³ |
| 12m | 4.32 m³ | 6.24 m³ | 9.30 m³ |
Figures include both the stem and the footing at the proportions in the table above. Note how quickly this becomes a full ready-mix delivery — a 6m wall retaining 1.2m already justifies a truck rather than site mixing.
Reinforcement
Concrete is strong in compression and weak in tension. A retaining wall bends forward under load, which puts the back face into tension — so that’s where the steel goes.
Vertical bars start in the footing and run up the back of the wall, with 40 to 50mm of cover from the rear face. Horizontal bars tie them together and control shrinkage cracking.
For a 1.2m wall, typical reinforcement is 12mm vertical bars at 200mm centres with 10mm horizontals at 300mm. The vertical bars need to be cast into the footing with an L-shaped bend and a lap of at least 40 bar diameters — 480mm for 12mm bar.
Starter bars cast into the footing and bent up are the standard approach. Get them positioned accurately before the footing pour, because moving them afterwards is not possible.
Formwork
Wet concrete exerts serious pressure — roughly 24 kN per square metre per metre of depth. A 1.2m high pour pushes with about 29 kN/m² at the base, which is enough to burst inadequate formwork.
Use 18mm ply with vertical soldiers at 400 to 600mm centres, tied through with form ties at similar spacing. Brace externally from both sides.
Check the formwork is plumb and straight before pouring, and check it again halfway through — forms move under load, and correcting them at that point is possible where correcting them afterwards is not.
Pouring in lifts
Place concrete in horizontal layers of 400 to 500mm, compacting each with a poker vibrator before the next goes in. Working in lifts keeps the pressure on the formwork manageable and lets you consolidate properly.
Keep the whole pour continuous. A gap of more than 30 to 45 minutes between lifts creates a cold joint — a horizontal plane of weakness right where you least want one.
Don’t over-vibrate. Running the poker too long drives the aggregate down and brings water up, leaving a weak layer at the top of each lift.
Drainage behind the wall
This is where most retaining walls fail, and it costs very little to get right.
Behind the wall, put a 300mm wide zone of free-draining material — clean 20mm stone — running the full height. At the base of that zone, lay a perforated land drain falling to a suitable outlet.
Wrap the stone zone in geotextile so fine soil cannot migrate in and clog it. Then add weep holes through the wall at 1 to 1.5m centres, 75mm above finished ground level at the front.
There’s a fuller treatment of this in our guide on retaining wall drainage, which is worth reading before you build.
Backfilling
Backfill in compacted layers of 200mm, and use granular material rather than the clay you dug out. Clay holds water, swells, and applies exactly the pressure you’re trying to avoid.
Do not compact heavily with plant machinery close to the wall. A vibrating roller running alongside a new retaining wall can push it forward — hand compaction within a metre of the wall, machine compaction beyond that.
Wait at least seven days after pouring before backfilling at all, and 28 days before applying any surcharge load on top.
Working out the concrete
Wall volume is length × height × thickness. Footing volume is length × width × depth. Add them together.
A 6m wall retaining 1.2m, at 200mm thick with an 800 × 350mm footing: 6 × 1.2 × 0.2 = 1.44 m³ for the wall and 6 × 0.8 × 0.35 = 1.68 m³ for the footing — 3.12m³ total, plus 8% wastage.
The wall calculator handles the stem, and the footing calculator handles the base.
Common questions
How thick should a concrete retaining wall be?
150mm for up to 600mm retained, 200mm to 1.2m, 250mm to 1.5m. Above that it needs designing rather than assuming.
How deep should the footing be?
Below frost depth and on undisturbed ground — typically 350 to 450mm deep for a domestic wall, and roughly twice the wall thickness wide with a toe projecting forward.
Do I need reinforcement in a low retaining wall?
Below about 600mm a mass concrete wall can work if it’s thick enough. Above that, reinforcement in the back face is essential — that’s where the tension is.
How long before I can backfill?
Seven days minimum, and use hand compaction near the wall. Wait the full 28 days before parking anything or building on the retained ground.
Why do retaining walls lean forward?
Almost always water pressure from inadequate drainage, or a footing too narrow to resist overturning. Once a wall has started to lean it will keep going — it needs rebuilding rather than propping.