Retaining walls

1. 1

   Assess the height and load

   Anything over 1 m, or retaining ground carrying a driveway or building, needs design by a structural engineer.

2. 2

   Dig the foundation

   Excavate a strip footing to firm subsoil, typically 150 to 300 mm of concrete, wide enough to spread the load.

3. 3

   Build the wall

   Lay blockwork or sleepers on the footing, keeping the wall plumb and tying courses as the design requires.

4. 4

   Waterproof the back face

   Tank the retained face so water in the soil cannot soak through and stain or freeze in the wall.

5. 5

   Build the drainage layer

   Fill behind with clean free-draining stone wrapped in geotextile, with a perforated land drain at the base running to an outfall.

6. 6

   Fit weep holes

   Set weep holes at 1 m centres in the bottom course so trapped water can escape rather than pushing the wall over.

7. 7

   Backfill in layers

   Backfill in 200 mm layers, compacting lightly, and never with a heavy machine tight against a new wall.

Retaining is the one landscaping structure where getting it wrong can hurt someone, so the regulatory line is drawn low and enforced seriously.

• Retaining walls over 1 m generally need structural design and may need building control approval where they retain ground next to a building or footpath. ^[1]
• Walls on or near a boundary are also covered by the Party Wall etc. Act 1996 in some cases. ^[2]
• Where the retained ground supports a highway or public footpath, the highway authority's approval is needed regardless of height. ^[1]
• Any wall retaining more than about 1.2 m, or any height next to a building, road or slope surcharge, should be designed by a structural engineer to Eurocode 7 principles, insurers ask for the calculation when a wall fails. ^[1]

Retaining walls almost never fail from the material being too weak; they fail from water and from the wrong base. These are the details that matter.

• Before excavating the wall trench or driving any soldier piles, CAT & Genny sweep (HSE HSG47). Retaining walls often run along boundaries where services hide. Full method in the Safety guide.
• Always provide drainage behind the wall, 100 mm perforated land drain, clean stone backfill, geotextile separator.
• Sleeper walls in oak or treated softwood: 200×100 sleepers max 600 mm high stacked, 1000 mm with steel posts driven into concrete pads.
• Allow a batter (lean back) of 1:10 on dry-stone or gabion walls.
• Surcharge from a driveway above doubles the lateral loads, never DIY a retaining wall holding back a vehicle.
• On clay subgrades the active earth pressure roughly doubles when saturated, design walls over 600 mm with a structural engineer, not from a YouTube video.
• Steep sites benefit from terracing into two lower walls instead of one tall wall, halves the loads, no structural design needed.
• Concrete block (hollow dense block, filled): footing width roughly half the retained height, 600 mm deep in reasonable ground, with vertical bars in the cores over 900 mm retained.
• Weep holes every 1–1.5 m along the base course are the cheap insurance; even with a land drain, give the water a visible exit.
• Geogrid reinforcement extends the reinforced zone to roughly 0.7× the wall height; on segmental block systems it is what turns a pretty facing into a structure.

The right system depends on retained height first, aesthetics second. Use this as the honest hierarchy before falling in love with a finish.

• Up to 600 mm: sleeper walls and dry-stone are DIY-sensible. Sleepers laid flat as stretchers with posts at 1.2 m centres; dry-stone needs the 1:10 batter.
• 600 mm to 1 m: dense concrete block (filled and barred), gabion baskets, or engineered sleeper walls with steel posts. Drainage becomes non-negotiable here.
• 1 m to 1.5 m: segmental block systems with geogrid, or reinforced masonry designed by an engineer. Party wall and building control questions start here.
• Over 1.5 m, or any height retaining a driveway or building: engineered solution only. Reinforced concrete, crib or anchored systems, designed and insured.
• Gabions deserve a mention on their own: cheap, permeable, tolerant of settlement, and the 1:10 face batter is built into the basket layout.

• No drainage behind the wall: hydrostatic pressure, not soil, topples most failed walls. A winter of saturated clay behind an undrained wall can double the design load.
• Footing poured on topsoil: strip to firm subsoil. A wall on 100 mm of made ground creeps forward within two seasons.
• Backfilling with the excavated clay: it holds water against the wall exactly where you do not want it. Clean stone or gravel to 300 mm behind the face, clay above the drain only.
• Stacking sleepers vertically as posts without depth: a post needs about as much in the ground as the retained height above it, set in concrete.
• Forgetting the surcharge: a car parked 300 mm behind the wall adds load the wall was never given. Fence or kerb the edge, or engineer for the vehicle.

Installed 2026 ranges per square metre of wall face, which is the way retaining walls are priced, not per metre run.

• Timber sleepers: roughly £150 to £250 per m2 of face, including posts, concrete and drainage stone.
• Concrete block with a rendered or faced finish: roughly £250 to £400 per m2 of face on a designed strip footing.
• Gabion baskets: roughly £100 to £200 per m2 of face, cheaper where the fill stone is local.
• Interlocking concrete retaining blocks: roughly £180 to £320 per m2 of face, quick to build and forgiving on curves.
• Structural engineer's design and calculations for anything over about 1 m or carrying a surcharge: roughly £400 to £900.
• Footings, drainage stone and pipe usually add 20 to 30 percent on top of the wall itself, and are the last thing to cut.