Pavers & hardscape · Guide
Segmental retaining wall basics
Block walls hold back soil with geometry and drainage, not with weight alone. Both details are unforgiving.
6 min read · Updated August 2026
The buried course
The first course of a segmental wall is buried — a minimum of one-tenth of the exposed wall height, and never less than one full course plus the levelling pad. A 4-foot wall buries at least 6 inches of block below finished grade.
That course sits on 6 inches of compacted aggregate levelling pad, and it must be dead level in both directions. Every error in course one is multiplied up the wall.
Drainage is the structure
Soil behind a wall gets heavy when saturated — hydrostatic pressure is what pushes walls over, not the dry soil weight. A 12-inch column of clean 3/4 inch drainage stone behind the blocks, with a perforated drain pipe at the base daylighting to a low point, removes that pressure.
Separate the drainage stone from the retained soil with geotextile fabric, or the soil fines migrate into the stone and the drainage column silts up within a few seasons.
Block counts and quantities
Block count is straightforward once the exposed height and buried depth are set: total wall face area (including the buried course) divided by the face area of one block. A standard 12 × 8 inch face block covers 0.667 sq ft, so a 30-foot-long wall 4 feet high with a 6-inch buried course is 135 sq ft of face — about 203 blocks — plus a course of caps.
Drainage stone at 12 inches deep behind that wall is about 4.5 cubic yards, or roughly 6 tons of clean 3/4 inch stone.
When you need an engineer
Most jurisdictions require engineered design and a permit above 4 feet of exposed height, and many require it lower when there is a surcharge — a driveway, pool, slope or structure sitting above the wall.
Above about 3-4 feet, walls also need geogrid: layers of reinforcement fabric extending back into the retained soil every two or three courses, which turn the soil mass itself into part of the structure. Grid length and spacing are engineering decisions, not rules of thumb.
Terracing two short walls instead of one tall one only avoids the engineering if the horizontal separation is at least twice the height of the lower wall. Closer than that and the upper wall surcharges the lower one.
Frequently asked questions
+How high can I build a retaining wall without an engineer?
Commonly 4 feet of exposed height, but the limit is set locally and drops when anything loads the soil above the wall. Check with your building department before ordering block.
+How much drainage stone goes behind a retaining wall?
A 12-inch-wide column of clean 3/4 inch stone the full height of the wall, over a perforated pipe, wrapped away from the retained soil with geotextile fabric.
Part of a series
The complete paver patio guide
The full build sequence for a paver patio or driveway, with the quantities and depths a hardscape crew works to.