A retaining wall holds back soil, and soil, especially wet soil, is heavy. Most wall failures come from water pressure, not weak blocks. The drainage is as important as the wall.
What goes in a block wall
- A trench and compacted base of crushed stone
- A buried first course to stop the wall sliding
- The wall blocks, stepped back slightly as they rise
- Cap blocks, glued down
- Free-draining stone behind the wall
- A perforated drain pipe at the base, run to daylight or a soakaway
- Geotextile between the stone and the soil to keep fines out
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.
When it needs an engineer
Walls over about 1.2 m, 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. In the UK, walls next to a highway or holding up a neighbour's land need extra care and may need consent.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.