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 12" thick for the full height
- A perforated drain pipe at the base of the stone, run out to daylight or a dry well
- 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 24" wide and 6" deep of compacted crushed stone along the full length.
Drainage stone: wall length × (height + buried course) × drainage zone thickness. On a 30 ft wall 3 ft high, that's about 2.8 m³ (3.7 cu yd) of ¾" clear stone, about 4.5 tonnes. 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 1 m (3 ft) 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. Many municipalities need a permit for retaining walls over about 1 m, or next to a property line, so check your local bylaws.
Landscape timbers and other options
Landscape timbers (or railroad ties) and gabions are popular alternatives. Timber walls need posts concreted in at regular spacing and treated lumber rated 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 30 ft wall, 3 ft exposed, in 12 × 4 inch blocks with one buried course: 10 courses × 30 blocks = 300, plus 5% = 315 blocks. Caps: 32. Base trench 24" × 6": about 0.85 m³ of Granular A, about 1.4 tonnes. Drainage zone 12" thick for 3 ft 4 in: about 2.8 m³ of ¾" clear stone, about 4.5 tonnes, and three 10 ft lengths of weeping tile plus the run to its outlet.
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 backyard 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 local codes and your inspector for your project.