A retaining wall holds back soil, and soil, especially wet soil, is heavy. Most wall failures come from water pressure building up behind the wall, not from weak blocks. The drainage is as important as the wall itself, and on paper it's often the most underestimated part of the job.
What goes into a block wall
- A trench and a compacted base of crushed stone
- A buried first course to stop the wall sliding forward
- The wall blocks, stepped back slightly as they rise (the setback built into most block systems)
- Cap blocks, glued down with masonry adhesive
- Free-draining stone behind the wall, usually 300 mm thick for the full height
- A perforated drain pipe at the base of the stone, run to a stormwater pit or another legal point of discharge
- Geotextile between the drainage stone and the soil, so fines don't clog it
- On taller walls, geogrid layers laid back into the retained soil
Counting blocks
Courses = wall height ÷ block height, plus one buried course. Blocks per course = wall length ÷ block length. Add 5% for cuts and breakage. Caps = wall length ÷ cap length. Curves and corners need extra cuts, so allow more waste where the wall turns.
Where the ground slopes along the wall, the base steps down in block-height increments, so some sections have more courses than others. Measure each section's height separately rather than averaging.
Base and drainage quantities
Base trench: typically 600 mm wide and 150 mm deep of compacted crushed stone along the full length.
Drainage stone: wall length × (height + buried course) × drainage zone thickness. On a 10 m wall 900 mm high, that's about 3.2 m³ of 20 mm drainage aggregate. That's often more stone than base, so don't leave it off the quote.
Drain pipe: the wall length, plus the run out to its outlet.
When it needs an engineer
Walls over about 600 mm to 1 m tall, or walls that hold up a driveway, a slope, a building or a boundary, usually need engineering and often a permit. Taller block walls use geogrid layers tied back into the soil, and their design depends on soil type, slope above and loads. In Australia, the height that needs council approval or engineering varies by state and council, often somewhere from 600 mm to 1 m. Walls near a boundary, or holding up a neighbour's land or a building, need extra care.
Sleepers and other options
Treated pine and concrete sleepers and gabions are popular alternatives. Sleeper walls need galvanised steel H-posts or timber posts concreted in at regular spacing, and timber sleepers must be H4 treated for ground contact. Gabions need a level, compacted base and the right stone size for the mesh. The same drainage principle applies to all of them: give the water a way out.
Pricing tips
- Excavation and soil removal are often the biggest labour items. The excavation calculator includes swell and truck loads.
- Price access. Carrying blocks and stone through a side gate by barrow can double labour.
- Include compaction equipment, block saw hire and adhesive.
The retaining wall calculator counts blocks, caps, base, drainage stone and drain pipe together.
A worked example
A 10 m wall, 900 mm exposed, in 300 × 150 mm blocks with one buried course: 7 courses × 34 blocks = 238, plus 5% = 250 blocks. Caps: 35. Base trench 600 × 150 mm: 0.9 m³ of road base. Drainage zone 300 mm thick for 1.05 m: about 3.2 m³ of 20 mm drainage aggregate, and a 20 m coil of slotted ag pipe with a filter sock.
Building it right
- Get the first course dead level. Every course above follows it.
- Backfill and compact behind the wall as each course goes up, not all at the end.
- Keep heavy machinery away from the top of a new wall.
- Finish the top of the drainage zone with a layer of soil or a sealed surface so surface water doesn't pour straight in behind the wall.
Questions to ask before quoting
- How tall is the wall at its highest point, and what sits above it: a lawn, a slope, a drive or a building?
- Where will the drainage outlet go, and is there somewhere for the water to discharge?
- Is the wall on or near a boundary?
- What happens to the soil that's dug out?
The answers decide whether this is a straightforward garden job or one that needs a design, and they belong in the quote either way.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.