Planning a block retaining wall
A segmental retaining wall is built from dry-stacked concrete blocks that interlock or use pins, set on a compacted gravel base and backed by drainage stone. It is one of the most popular DIY landscape projects for walls up to about 3 or 4 ft. The calculator counts the blocks and cap units, the courses, the base gravel, the drainage stone and the drain pipe, so you can order everything at once.
How the estimate works
- Total height = exposed height + buried courses
- Courses = total height ÷ block height, rounded up
- Blocks per course = wall length ÷ block length, rounded up
- Blocks = courses × blocks per course, plus a margin for cuts
- Cap units = one per block length along the top
- Base gravel: a trench 24 in wide and 6 in deep of compacted crushed stone
- Drainage stone: a zone 12 in wide behind the wall, the full height
Worked example
A homeowner wants a wall 30 ft long and 3 ft high above grade, using standard blocks with a 16 × 6 in face and one buried course.
- Total height: 36 + 6 = 42 in, or 7 courses.
- Blocks per course: 360 ÷ 16 = 22.5, so 23.
- Blocks: 7 × 23 = 161, plus 5% = 170 blocks, or about $765 at $4.50 each.
- Caps: 23.
- Base gravel: 30 × 2 × 0.5 ft = 30 cubic feet, about 1.1 cubic yards.
- Drainage stone: 30 × 1 × 3.5 ft = 105 cubic feet, about 3.9 cubic yards, plus 30 ft of perforated drain pipe.
Building it right
- Dig and compact. Excavate for the base and the buried course, then compact the soil and lay 6 in of crushed stone in thin, compacted lifts.
- Level the first course carefully. Every error is multiplied as the wall rises.
- Set back each course. Most blocks have a lip that creates a slight backward lean, which helps the wall resist the soil.
- Drain the water. Clean crushed stone behind the blocks and a perforated pipe at the base, routed to daylight, relieve water pressure, the main cause of bulging and collapse. Wrap the stone in filter fabric to keep soil out.
- Glue the caps with a concrete adhesive.
When to call an engineer
Most US building codes require a permit and an engineered design for walls over 4 ft, measured from the bottom of the footing, and for walls that hold back a slope, a driveway, a pool or a building. Taller walls usually need geogrid reinforcement layers extending into the backfill. Check with your local building department and call 811 before you dig to locate buried utilities.